linux-kernel - Re: [syzbot] [kernel?] KASAN: slab-use-after-free Write in flush_tlb

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Message-ID: <874ivuldog.ffs@tglx>
Date: Wed, 02 Jul 2025 19:12:31 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Jann Horn <jannh@...gle.com>, Rik van Riel <riel@...riel.com>,
 yury.norov@...il.com
Cc: syzbot <syzbot+084b6e5bc1016723a9c4@...kaller.appspotmail.com>,
 bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
 linux-kernel@...r.kernel.org, luto@...nel.org, mingo@...hat.com,
 neeraj.upadhyay@...nel.org, paulmck@...nel.org, peterz@...radead.org,
 syzkaller-bugs@...glegroups.com, x86@...nel.org, kernel-team
 <kernel-team@...a.com>, David Hildenbrand <david@...hat.com>
Subject: Re: [syzbot] [kernel?] KASAN: slab-use-after-free Write in
 flush_tlb_func

On Wed, Jul 02 2025 at 19:00, Jann Horn wrote:
> On Wed, Jul 2, 2025 at 6:53 PM Jann Horn <jannh@...gle.com> wrote:
>> TLB flushes via IPIs on x86 are always synchronous, right?
>> flush_tlb_func is only referenced from native_flush_tlb_multi() in
>> calls to on_each_cpu_mask() (with wait=true) or
>> on_each_cpu_cond_mask() (with wait=1).
>> So I think this is not an issue, unless you're claiming that we call
>> native_flush_tlb_multi() with an already-freed info->mm?
>>
>> And I think the bisected commit really is the buggy one: It looks at
>> "nr_cpus", which tracks *how many CPUs we have to IPI*, but assumes
>> that "nr_cpus" tracks *how many CPUs we posted work to*. Those numbers
>> are not the same: If we post work to a CPU that already had IPI work
>> pending, we just add a list entry without sending another IPI.
>
> Or in other words: After that blamed commit, if CPU 1 posts a TLB
> flush to CPU 3, and then CPU 2 also quickly posts a TLB flush to CPU
> 3, then CPU 2 will erroneously not wait for the TLB flush to complete
> before reporting flush completion, which AFAICS means we can get both
> stale TLB entries and (less often) UAF.

Right you are. Well analyzed and I missed it when taking the lot.

> I think the correct version of that commit would be to revert that
> commit and instead just move the "run_remote = true;" line down, below
> the cond_func() check.

I remove it from the relevant tip branch

Thanks,

        tglx