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85f4f12d51
Reviewing Tobin's patches for getting pointers out early before
entropy has been established, I noticed that there's a lone smp_mb() in
the code. As with most lone memory barriers, this one appears to be
incorrectly used.
We currently basically have this:
get_random_bytes(&ptr_key, sizeof(ptr_key));
/*
* have_filled_random_ptr_key==true is dependent on get_random_bytes().
* ptr_to_id() needs to see have_filled_random_ptr_key==true
* after get_random_bytes() returns.
*/
smp_mb();
WRITE_ONCE(have_filled_random_ptr_key, true);
And later we have:
if (unlikely(!have_filled_random_ptr_key))
return string(buf, end, "(ptrval)", spec);
/* Missing memory barrier here. */
hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
As the CPU can perform speculative loads, we could have a situation
with the following:
CPU0 CPU1
---- ----
load ptr_key = 0
store ptr_key = random
smp_mb()
store have_filled_random_ptr_key
load have_filled_random_ptr_key = true
BAD BAD BAD! (you're so bad!)
Because nothing prevents CPU1 from loading ptr_key before loading
have_filled_random_ptr_key.
But this race is very unlikely, but we can't keep an incorrect smp_mb() in
place. Instead, replace the have_filled_random_ptr_key with a static_branch
not_filled_random_ptr_key, that is initialized to true and changed to false
when we get enough entropy. If the update happens in early boot, the
static_key is updated immediately, otherwise it will have to wait till
entropy is filled and this happens in an interrupt handler which can't
enable a static_key, as that requires a preemptible context. In that case, a
work_queue is used to enable it, as entropy already took too long to
establish in the first place waiting a little more shouldn't hurt anything.
The benefit of using the static key is that the unlikely branch in
vsprintf() now becomes a nop.
Link: http://lkml.kernel.org/r/20180515100558.21df515e@gandalf.local.home
Cc: stable@vger.kernel.org
Fixes:
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|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| firmware | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.