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292 lines
13 KiB
ReStructuredText
292 lines
13 KiB
ReStructuredText
DataFlowSanitizer Design Document
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=================================
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This document sets out the design for DataFlowSanitizer, a general
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dynamic data flow analysis. Unlike other Sanitizer tools, this tool is
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not designed to detect a specific class of bugs on its own. Instead,
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it provides a generic dynamic data flow analysis framework to be used
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by clients to help detect application-specific issues within their
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own code.
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DataFlowSanitizer is a program instrumentation which can associate
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a number of taint labels with any data stored in any memory region
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accessible by the program. The analysis is dynamic, which means that
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it operates on a running program, and tracks how the labels propagate
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through that program.
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Use Cases
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---------
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This instrumentation can be used as a tool to help monitor how data
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flows from a program's inputs (sources) to its outputs (sinks).
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This has applications from a privacy/security perspective in that
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one can audit how a sensitive data item is used within a program and
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ensure it isn't exiting the program anywhere it shouldn't be.
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Interface
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---------
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A number of functions are provided which will attach taint labels to
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memory regions and extract the set of labels associated with a
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specific memory region. These functions are declared in the header
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file ``sanitizer/dfsan_interface.h``.
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.. code-block:: c
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/// Sets the label for each address in [addr,addr+size) to \c label.
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void dfsan_set_label(dfsan_label label, void *addr, size_t size);
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/// Sets the label for each address in [addr,addr+size) to the union of the
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/// current label for that address and \c label.
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void dfsan_add_label(dfsan_label label, void *addr, size_t size);
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/// Retrieves the label associated with the given data.
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///
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/// The type of 'data' is arbitrary. The function accepts a value of any type,
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/// which can be truncated or extended (implicitly or explicitly) as necessary.
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/// The truncation/extension operations will preserve the label of the original
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/// value.
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dfsan_label dfsan_get_label(long data);
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/// Retrieves the label associated with the data at the given address.
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dfsan_label dfsan_read_label(const void *addr, size_t size);
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/// Returns whether the given label contains the label elem.
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int dfsan_has_label(dfsan_label label, dfsan_label elem);
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/// Computes the union of \c l1 and \c l2, resulting in a union label.
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dfsan_label dfsan_union(dfsan_label l1, dfsan_label l2);
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/// Flushes the DFSan shadow, i.e. forgets about all labels currently associated
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/// with the application memory. Use this call to start over the taint tracking
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/// within the same process.
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///
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/// Note: If another thread is working with tainted data during the flush, that
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/// taint could still be written to shadow after the flush.
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void dfsan_flush(void);
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The following functions are provided to check origin tracking status and results.
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.. code-block:: c
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/// Retrieves the immediate origin associated with the given data. The returned
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/// origin may point to another origin.
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///
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/// The type of 'data' is arbitrary. The function accepts a value of any type,
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/// which can be truncated or extended (implicitly or explicitly) as necessary.
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/// The truncation/extension operations will preserve the label of the original
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/// value.
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dfsan_origin dfsan_get_origin(long data);
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/// Retrieves the very first origin associated with the data at the given
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/// address.
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dfsan_origin dfsan_get_init_origin(const void *addr);
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/// Prints the origin trace of the label at the address `addr` to stderr. It also
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/// prints description at the beginning of the trace. If origin tracking is not
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/// on, or the address is not labeled, it prints nothing.
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void dfsan_print_origin_trace(const void *addr, const char *description);
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/// Prints the origin trace of the label at the address `addr` to a pre-allocated
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/// output buffer. If origin tracking is not on, or the address is`
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/// not labeled, it prints nothing.
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///
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/// `addr` is the tainted memory address whose origin we are printing.
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/// `description` is a description printed at the beginning of the trace.
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/// `out_buf` is the output buffer to write the results to. `out_buf_size` is
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/// the size of `out_buf`. The function returns the number of symbols that
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/// should have been written to `out_buf` (not including trailing null byte '\0').
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/// Thus, the string is truncated iff return value is not less than `out_buf_size`.
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size_t dfsan_sprint_origin_trace(const void *addr, const char *description,
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char *out_buf, size_t out_buf_size);
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/// Returns the value of `-dfsan-track-origins`.
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int dfsan_get_track_origins(void);
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The following functions are provided to register hooks called by custom wrappers.
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.. code-block:: c
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/// Sets a callback to be invoked on calls to `write`. The callback is invoked
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/// before the write is done. The write is not guaranteed to succeed when the
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/// callback executes. Pass in NULL to remove any callback.
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typedef void (*dfsan_write_callback_t)(int fd, const void *buf, size_t count);
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void dfsan_set_write_callback(dfsan_write_callback_t labeled_write_callback);
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/// Callbacks to be invoked on calls to `memcmp` or `strncmp`.
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void dfsan_weak_hook_memcmp(void *caller_pc, const void *s1, const void *s2,
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size_t n, dfsan_label s1_label,
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dfsan_label s2_label, dfsan_label n_label);
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void dfsan_weak_hook_strncmp(void *caller_pc, const char *s1, const char *s2,
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size_t n, dfsan_label s1_label,
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dfsan_label s2_label, dfsan_label n_label);
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Taint label representation
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--------------------------
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We use an 8-bit unsigned integer for the representation of a
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label. The label identifier 0 is special, and means that the data item
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is unlabelled. This is optimizing for low CPU and code size overhead
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of the instrumentation. When a label union operation is requested at a
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join point (any arithmetic or logical operation with two or more
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operands, such as addition), we can simply OR the two labels in O(1).
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Users are responsible for managing the 8 integer labels (i.e., keeping
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track of what labels they have used so far, picking one that is yet
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unused, etc).
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Origin tracking trace representation
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------------------------------------
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An origin tracking trace is a list of chains. Each chain has a stack trace
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where the DFSan runtime records a label propagation, and a pointer to its
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previous chain. The very first chain does not point to any chain.
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Every four 4-bytes aligned application bytes share a 4-byte origin trace ID. A
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4-byte origin trace ID contains a 4-bit depth and a 28-bit hash ID of a chain.
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A chain ID is calculated as a hash from a chain structure. A chain structure
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contains a stack ID and the previous chain ID. The chain head has a zero
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previous chain ID. A stack ID is a hash from a stack trace. The 4-bit depth
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limits the maximal length of a path. The environment variable ``origin_history_size``
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can set the depth limit. Non-positive values mean unlimited. Its default value
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is 16. When reaching the limit, origin tracking ignores following propagation
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chains.
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The first chain of a trace starts by `dfsan_set_label` with non-zero labels. A
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new chain is appended at the end of a trace at stores or memory transfers when
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``-dfsan-track-origins`` is 1. Memory transfers include LLVM memory transfer
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instructions, glibc memcpy and memmove. When ``-dfsan-track-origins`` is 2, a
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new chain is also appended at loads.
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Other instructions do not create new chains, but simply propagate origin trace
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IDs. If an instruction has more than one operands with non-zero labels, the origin
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treace ID of the last operand with non-zero label is propagated to the result of
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the instruction.
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Memory layout and label management
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----------------------------------
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The following is the memory layout for Linux/x86\_64:
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+---------------+---------------+--------------------+
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| Start | End | Use |
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+===============+===============+====================+
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| 0x700000000000|0x800000000000 | application 3 |
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+---------------+---------------+--------------------+
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| 0x610000000000|0x700000000000 | unused |
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+---------------+---------------+--------------------+
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| 0x600000000000|0x610000000000 | origin 1 |
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+---------------+---------------+--------------------+
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| 0x510000000000|0x600000000000 | application 2 |
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+---------------+---------------+--------------------+
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| 0x500000000000|0x510000000000 | shadow 1 |
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+---------------+---------------+--------------------+
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| 0x400000000000|0x500000000000 | unused |
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+---------------+---------------+--------------------+
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| 0x300000000000|0x400000000000 | origin 3 |
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+---------------+---------------+--------------------+
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| 0x200000000000|0x300000000000 | shadow 3 |
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+---------------+---------------+--------------------+
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| 0x110000000000|0x200000000000 | origin 2 |
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+---------------+---------------+--------------------+
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| 0x100000000000|0x110000000000 | unused |
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+---------------+---------------+--------------------+
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| 0x010000000000|0x100000000000 | shadow 2 |
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+---------------+---------------+--------------------+
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| 0x000000000000|0x010000000000 | application 1 |
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+---------------+---------------+--------------------+
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Each byte of application memory corresponds to a single byte of shadow
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memory, which is used to store its taint label. We map memory, shadow, and
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origin regions to each other with these masks and offsets:
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* shadow_addr = memory_addr ^ 0x500000000000
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* origin_addr = shadow_addr + 0x100000000000
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As for LLVM SSA registers, we have not found it necessary to associate a label
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with each byte or bit of data, as some other tools do. Instead, labels are
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associated directly with registers. Loads will result in a union of
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all shadow labels corresponding to bytes loaded, and stores will
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result in a copy of the label of the stored value to the shadow of all
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bytes stored to.
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Propagating labels through arguments
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------------------------------------
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In order to propagate labels through function arguments and return values,
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DataFlowSanitizer changes the ABI of each function in the translation unit.
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There are currently two supported ABIs:
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* Args -- Argument and return value labels are passed through additional
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arguments and by modifying the return type.
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* TLS -- Argument and return value labels are passed through TLS variables
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``__dfsan_arg_tls`` and ``__dfsan_retval_tls``.
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The main advantage of the TLS ABI is that it is more tolerant of ABI mismatches
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(TLS storage is not shared with any other form of storage, whereas extra
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arguments may be stored in registers which under the native ABI are not used
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for parameter passing and thus could contain arbitrary values). On the other
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hand the args ABI is more efficient and allows ABI mismatches to be more easily
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identified by checking for nonzero labels in nominally unlabelled programs.
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Implementing the ABI list
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-------------------------
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The `ABI list <DataFlowSanitizer.html#abi-list>`_ provides a list of functions
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which conform to the native ABI, each of which is callable from an instrumented
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program. This is implemented by replacing each reference to a native ABI
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function with a reference to a function which uses the instrumented ABI.
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Such functions are automatically-generated wrappers for the native functions.
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For example, given the ABI list example provided in the user manual, the
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following wrappers will be generated under the args ABI:
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.. code-block:: llvm
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define linkonce_odr { i8*, i16 } @"dfsw$malloc"(i64 %0, i16 %1) {
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entry:
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%2 = call i8* @malloc(i64 %0)
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%3 = insertvalue { i8*, i16 } undef, i8* %2, 0
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%4 = insertvalue { i8*, i16 } %3, i16 0, 1
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ret { i8*, i16 } %4
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}
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define linkonce_odr { i32, i16 } @"dfsw$tolower"(i32 %0, i16 %1) {
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entry:
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%2 = call i32 @tolower(i32 %0)
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%3 = insertvalue { i32, i16 } undef, i32 %2, 0
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%4 = insertvalue { i32, i16 } %3, i16 %1, 1
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ret { i32, i16 } %4
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}
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define linkonce_odr { i8*, i16 } @"dfsw$memcpy"(i8* %0, i8* %1, i64 %2, i16 %3, i16 %4, i16 %5) {
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entry:
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%labelreturn = alloca i16
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%6 = call i8* @__dfsw_memcpy(i8* %0, i8* %1, i64 %2, i16 %3, i16 %4, i16 %5, i16* %labelreturn)
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%7 = load i16* %labelreturn
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%8 = insertvalue { i8*, i16 } undef, i8* %6, 0
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%9 = insertvalue { i8*, i16 } %8, i16 %7, 1
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ret { i8*, i16 } %9
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}
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As an optimization, direct calls to native ABI functions will call the
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native ABI function directly and the pass will compute the appropriate label
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internally. This has the advantage of reducing the number of union operations
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required when the return value label is known to be zero (i.e. ``discard``
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functions, or ``functional`` functions with known unlabelled arguments).
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Checking ABI Consistency
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------------------------
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DFSan changes the ABI of each function in the module. This makes it possible
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for a function with the native ABI to be called with the instrumented ABI,
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or vice versa, thus possibly invoking undefined behavior. A simple way
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of statically detecting instances of this problem is to append the suffix
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".dfsan" to the name of each instrumented-ABI function.
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This will not catch every such problem; in particular function pointers passed
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across the instrumented-native barrier cannot be used on the other side.
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These problems could potentially be caught dynamically.
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