Vulnerability CVE-2024-42096
Published: 2024-07-29

Description:
In the Linux kernel, the following vulnerability has been resolved:

x86: stop playing stack games in profile_pc()

The 'profile_pc()' function is used for timer-based profiling, which
isn't really all that relevant any more to begin with, but it also ends
up making assumptions based on the stack layout that aren't necessarily
valid.

Basically, the code tries to account the time spent in spinlocks to the
caller rather than the spinlock, and while I support that as a concept,
it's not worth the code complexity or the KASAN warnings when no serious
profiling is done using timers anyway these days.

And the code really does depend on stack layout that is only true in the
simplest of cases. We've lost the comment at some point (I think when
the 32-bit and 64-bit code was unified), but it used to say:

Assume the lock function has either no stack frame or a copy
of eflags from PUSHF.

which explains why it just blindly loads a word or two straight off the
stack pointer and then takes a minimal look at the values to just check
if they might be eflags or the return pc:

Eflags always has bits 22 and up cleared unlike kernel addresses

but that basic stack layout assumption assumes that there isn't any lock
debugging etc going on that would complicate the code and cause a stack
frame.

It causes KASAN unhappiness reported for years by syzkaller [1] and
others [2].

With no real practical reason for this any more, just remove the code.

Just for historical interest, here's some background commits relating to
this code from 2006:

0cb91a229364 ("i386: Account spinlocks to the caller during profiling for !FP kernels")
31679f38d886 ("Simplify profile_pc on x86-64")

and a code unification from 2009:

ef4512882dbe ("x86: time_32/64.c unify profile_pc")

but the basics of this thing actually goes back to before the git tree.

 References:
https://git.kernel.org/stable/c/65ebdde16e7f5da99dbf8a548fb635837d78384e
https://git.kernel.org/stable/c/27c3be840911b15a3f24ed623f86153c825b6b29
https://git.kernel.org/stable/c/49c09ca35a5f521d7fa18caf62fdf378f15e8aa4
https://git.kernel.org/stable/c/2d07fea561d64357fb7b3f3751e653bf20306d77
https://git.kernel.org/stable/c/161cef818545ecf980f0e2ebaf8ba7326ce53c2b
https://git.kernel.org/stable/c/16222beb9f8e5ceb0beeb5cbe54bef16df501a92
https://git.kernel.org/stable/c/a3b65c8cbc139bfce9541bc81c1bb766e5ba3f68
https://git.kernel.org/stable/c/093d9603b60093a9aaae942db56107f6432a5dca
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