cve-2024-42096
Vulnerability from cvelistv5
Published
2024-07-29 17:39
Modified
2024-12-19 09:12
Severity ?
EPSS score ?
Summary
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
Impacted products
| Vendor | Product | Version | |||||
|---|---|---|---|---|---|---|---|
| ▼ | Linux | Linux |
Version: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 Version: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 Version: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 Version: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 Version: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 Version: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 Version: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 Version: 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 |
||||
|
|||||||
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Perdimos el comentario en alg\u00fan momento (creo que cuando se unific\u00f3 el c\u00f3digo de 32 y 64 bits), pero sol\u00eda decir: Supongamos que la funci\u00f3n de bloqueo no tiene marco de pila o tiene una copia de eflags de PUSHF. lo que explica por qu\u00e9 simplemente carga ciegamente una o dos palabras directamente desde el puntero de la pila y luego echa un vistazo m\u00ednimo a los valores para comprobar si podr\u00edan ser eflags o la PC de retorno: Eflags siempre tiene los bits 22 y superiores limpios, a diferencia de las direcciones del kernel, pero Esa suposici\u00f3n b\u00e1sica del dise\u00f1o de la pila supone que no hay ninguna depuraci\u00f3n de bloqueo, etc., que complicar\u00eda el c\u00f3digo y provocar\u00eda un marco de pila. Provoca la infelicidad de KASAN que Syzkaller [1] y otros [2] han informado durante a\u00f1os. Sin ninguna raz\u00f3n pr\u00e1ctica real para esto, simplemente elimine el c\u00f3digo. 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