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Dmitry Vyukov ba18afea77 docs: describe syzbot bisection

Update #501

2019-03-17 11:47:33 +01:00

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syzbot

syzbot system continuously fuzzes main Linux kernel branches and automatically reports found bugs to kernel mailing lists. syzbot dashboard shows current statuses of bugs. All syzbot-reported bugs are also CCed to syzkaller-bugs mailing list. Direct all questions to syzkaller@googlegroups.com.

Bug status tracking

syzbot needs to know when a bug is fixed in order to (1) verify that it is in fact fixed and (2) be able to report other similarly-looking crashes (while a bug is considered open all similarly-looking crashes are merged into the existing bug). To understand when a bug is fixed syzbot needs to know what commit fixes the bug; once syzbot knows the commit it will track when the commit reaches all kernel builds on all tracked branches. Only when the commit reaches all builds, the bug is considered closed (new similarly-looking crashes create a new bug).

Communication with syzbot

If you fix a bug reported by syzbot, please add the provided Reported-by tag to the commit (Reported-and-tested-by and Tested-by tags with the syzbot+HASH address are recognized as well). You can also communicate with syzbot by replying to its emails. The commands are:

to attach a fixing commit to the bug (if you forgot to add Reported-by tag):

#syz fix: exact-commit-title

It's enough that the commit is merged into any tree or you are reasonably sure about its final title, in particular, you don't need to wait for the commit to be merged into upstream tree. syzbot only needs to know the title by which it will appear in tested trees. In case of an error or a title change, you can override the commit simply by sending another #syz fix command.

to mark the bug as a duplicate of another syzbot bug:

#syz dup: exact-subject-of-another-report

to undo a previous dup command and turn it into an independent bug again:

#syz undup

to mark the bug as a one-off invalid report (e.g. induced by a previous memory corruption):

#syz invalid

Note: if the crash happens again, it will cause creation of a new bug report.

Note: all commands must start from beginning of the line.

Note: please keep syzkaller-bugs@googlegroups.com mailing list in CC. It serves as a history of what happened with each bug report.

Testing patches

syzbot can test patches for bugs with reproducers. This can be used for testing of fix patches, or just for debugging (i.e. adding additional checks to code and testing with them), or to check if the bug still happens. To test on a particular git tree and branch reply with:

#syz test: git://repo/address.git branch

or alternatively, to test on exact commit reply with:

#syz test: git://repo/address.git commit-hash

If you also provide a patch with the email, syzbot will apply it on top of the tree before testing. The patch can be provided either inline in email text or as a text attachment (which is more reliable if your email client messes with whitespaces).

If you don't provide a patch, syzbot will test the tree as is. This is useful if this is your own tree which already contains the patch, or to check if the bug is already fixed by some recent commit.

After sending an email you should get a reply email with results within an hour.

Note: you may send the request only to syzbot email address, as patches sent to some mailing lists (e.g. netdev, netfilter-devel) will trigger patchwork.

Note: see below for testing KMSAN bugs.

Bisection

syzbot bisects bugs with reproducers to find commit that introduced the bug. syzbot starts with the commit on which the bug was discovered, ensures that it can reproduce the bug and then goes back release-by-release to find the first release where kernel does not crash. Once such release is found, syzbot starts bisection on that range. syzbot has limitation of how far back in time it can go (currently v4.1), going back in time is very hard because of incompatible compiler/linker/asm/perl/make/libc/etc, kernel build/boot breakages and large amounts of bugs.

The predicate for bisection is binary (crash/doesn't crash), syzbot does not look at the exact crash and does not try to differentiate them. This is intentional because lots of bugs can manifest in different ways (sometimes 50+ different ways). For each revision syzbot repeats testing 10 times and a single crash marks revision as bad (lots of bugs are due to races and are hard to trigger).

During bisection syzbot uses different compilers depending on kernel revision (a single compiler can't build all revisions). These compilers are available here. Exact compiler used to test a particular revision is specified in the bisection log.

Bisection is best-effort and may not find the right commit for multiple reasons, including:

hard to reproduce bugs that trigger with very low probability
bug being introduced before the tool that reliably detects it (LOCKDEP, KASAN, FAULT_INJECTION, WARNING, etc);
such bugs may be bisection to the addition/improvement of the tool
kernel build/boot errors that force skipping revisions
some kernel configs are disabled as bisection goes back in time because they build/boot break release tags;
bugs in these subsystems may be bisected to release tags
reproducers triggering multiple kernel bugs at once
unrelated kernel bugs that break even simple programs

A single incorrect decision during bisection leads to an incorrect result, so please treat the results with understanding. You may consult the provided bisection log to see how/why syzbot has arrived to a particular commit. Suggestions and patches that improve bisection quality for common cases are welcome.

syzkaller reproducers

syzbot aims at providing stand-alone C reproducers for all reported bugs. However, sometimes it can't extract a reproducer at all, or can only extract a syzkaller reproducer. syzkaller reproducers are programs in a special syzkaller notation and they can be executed on the target system with a little bit more effort. See this for instructions.

A syskaller program can also give you an idea as to what syscalls with what arguments were executed (note that some calls can actually be executed in parallel).

A syzkaller program can be converted to an almost equivalent C source using syz-prog2c utility. syz-prog2c has lots of flags in common with syz-execprog, e.g. -threaded/-collide which control if the syscalls are executed sequentially or in parallel. An example invocation:

syz-prog2c -prog repro.syz.txt -enable=all -threaded -collide -repeat -procs=8 -sandbox=namespace -segv -tmpdir -waitrepeat

However, note that if syzbot did not provide a C reproducer, it wasn't able to trigger the bug using the C program (though, it can be just because the bug is triggered by a subtle race condition).

Crash does not reproduce?

If the provided reproducer does not work for you, most likely it is related to the fact that you have slightly different setup than syzbot. syzbot has obtained the provided crash report on the provided reproducer on a freshly-booted machine, so the reproducer worked for it somehow.

Note: if the report contains userspace arch: i386, then the program needs to be built with -m32 flag.

syzbot uses GCE VMs for testing, but usually it is not important.

If the reproducer exits quickly, try to run it several times, or in a loop. There can be some races involved.

Exact compilers used by syzbot can be found here:

A qemu-suitable Debian/wheezy image can be found here (1GB, compression somehow breaks it), root ssh key for it is here (do chmod 0600 on it). A reference qemu command line to run it is as follows:

qemu-system-x86_64 -smp 2 -m 4G -enable-kvm -cpu host \
    -net nic -net user,hostfwd=tcp::10022-:22 \
    -kernel arch/x86/boot/bzImage -nographic \
    -device virtio-scsi-pci,id=scsi \
    -device scsi-hd,bus=scsi.0,drive=d0 \
    -drive file=wheezy.img,format=raw,if=none,id=d0 \
    -append "root=/dev/sda console=ttyS0 earlyprintk=serial rodata=n \
      oops=panic panic_on_warn=1 panic=86400 kvm-intel.nested=1 \      
      security=apparmor ima_policy=tcb workqueue.watchdog_thresh=140 \
      nf-conntrack-ftp.ports=20000 nf-conntrack-tftp.ports=20000 \
      nf-conntrack-sip.ports=20000 nf-conntrack-irc.ports=20000 \
      nf-conntrack-sane.ports=20000 vivid.n_devs=16 \
      vivid.multiplanar=1,2,1,2,1,2,1,2,1,2,1,2,1,2,1,2 \
      spec_store_bypass_disable=prctl nopcid"

And then you can ssh into it using:

ssh -p 10022 -i wheezy.id_rsa root@localhost

No reproducer at all?

Reproducers are best-effort. syzbot always tries to create reproducers, and once it has one it adds it to the bug. If there is no reproducer referenced in a bug, a reproducer does not exist. There are multiple reasons why syzbot can fail to create a reproducer: some crashes are caused by subtle races and are very hard to reproduce in general; some crashes are caused by global accumulated state in kernel (e.g. lockdep reports); some crashes are caused by non-reproducible coincidences (e.g. an integer 0x12345 happened to reference an existing IPC object) and there is long tail of other reasons.

Moderation queue

Bugs with reproducers are automatically reported to kernel mailing lists. Bugs without reproducers are first staged in moderation queue to filter out invalid, unactionable or duplicate reports. Staged bugs are shown on dashboard in moderation section and mailed to syzkaller-upstream-moderation mailing list. Staged bugs accept all commands supported for reported bugs (fix, dup, invalid) with a restriction that bugs reported upstream can't be dup-ed onto bugs in moderation queue. Additionally, staged bugs accept upstream command:

#syz upstream

which sends the bug to kernel mailing lists.

KMSAN bugs

KMSAN is a dynamic, compiler-based tool (similar to KASAN) that detects uses of uninitialized values. As compared to (now deleted) KMEMCHECK which simply detected loads of non-stored-to memory, KMSAN tracks precise propagation of uninitialized values through memory and registers and only flags actual eventual uses of uninitialized values. For example, KMSAN will detect a branch on or a copy_to_user() of values that transitively come from uninitialized memory created by heap/stack allocations. This ensures /theoretical/ absense of both false positives and false negatives (with some implementation limitations of course). Note that KMSAN requires clang compiler.

KMSAN is not upstream yet, though, we want to upstream it later. For now, it lives in github.com/google/kmsan and is based on a reasonably fresh upstream tree. As the result, any patch testing requests for KMSAN bugs need to go to KMSAN tree (https://github.com/google/kmsan.git repo, master branch). A standard way for triggering the test with KMSAN tree is to send an email to syzbot+HASH address containing the following line:

#syz test: https://github.com/google/kmsan.git master

and attach/inline your test patch in the same email.

Report explanation. The first call trace points to the use of the uninit value (which is usually a branching or copying it to userspace). Then there are 0 or more "Uninit was stored to memory at:" stacks which denote how the unint value travelled through memory. Finally there is a "Uninit was created at:" section which points either to a heap allocation or a stack variable which is the original source of uninitialized-ness.

No custom patches

While syzbot can test patches that fix bugs, it does not support applying custom patches during fuzzing. It always tests vanilla unmodified git trees. There are several reasons for this:

custom patches may not apply tomorrow
custom patches may not apply to all of the tested git trees
it's hard to communicate exact state of the code with bug reports (not just hash anymore)
line numbers won't match in reports (which always brings suspecion as to the quality of reports)
custom patches can also introduce bugs, and even if they don't a developer may (rightfully) question validity of and may not want to spend time on reports obtained with a number of out-of-tree patches
order of patch application generatelly matters, and at some point patches need to be removed, there is nobody to manage this

We've experimented with application of custom patches in the past and it lead to unrecoverable mess. If you want syzbot to pick up patches sooner, ask tree maintainers for priority handling.

However, syzbot kernel config always includes CONFIG_DEBUG_AID_FOR_SYZBOT=y setting, which is not normally present in kernel. What was used for particularly elusive bugs in the past is temporary merging some additional debugging code into linux-next under this config setting (e.g. more debug checks and/or debug output) and waiting for new crash reports from syzbot.

Kernel configs

Kernel configs, sysctls and command line arguments that syzbot uses are available in /dashboard/config.

Is syzbot code available?

Yes, it is here.

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