Subject: CVE-2024-42096: x86: stop playing stack games in profile_pc()

From: Greg Kroah-Hartman <[email-masked]>

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.

The Linux kernel CVE team has assigned CVE-2024-42096 to this issue.

Affected and fixed versions
===========================

	Fixed in 4.19.317 with commit 65ebdde16e7f
	Fixed in 5.4.279 with commit 27c3be840911
	Fixed in 5.10.221 with commit 49c09ca35a5f
	Fixed in 5.15.162 with commit 2d07fea561d6
	Fixed in 6.1.97 with commit 161cef818545
	Fixed in 6.6.37 with commit 16222beb9f8e
	Fixed in 6.9.8 with commit a3b65c8cbc13
	Fixed in 6.10 with commit 093d9603b600

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-42096
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	arch/x86/kernel/time.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	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