linux-kernel - Re: WARNING: CPU: 0 PID: 0 at arch/x86/include/asm/kfence.h:46 kfence

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Date:   Thu, 17 Nov 2022 06:34:15 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Marco Elver <elver@...gle.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     kasan-dev <kasan-dev@...glegroups.com>, X86 ML <x86@...nel.org>,
        open list <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, regressions@...ts.linux.dev,
        lkft-triage@...ts.linaro.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: WARNING: CPU: 0 PID: 0 at arch/x86/include/asm/kfence.h:46
 kfence_protect

On 11/17/22 05:58, Marco Elver wrote:
> [    0.663761] WARNING: CPU: 0 PID: 0 at arch/x86/include/asm/kfence.h:46 kfence_protect+0x7b/0x120
> [    0.664033] WARNING: CPU: 0 PID: 0 at mm/kfence/core.c:234 kfence_protect+0x7d/0x120
> [    0.664465] kfence: kfence_init failed

Any chance you could add some debugging and figure out what actually
made kfence call over?  Was it the pte or the level?

        if (WARN_ON(!pte || level != PG_LEVEL_4K))
                return false;

I can see how the thing you bisected to might lead to a page table not
being split, which could mess with the 'level' check.

Also, is there a reason this code is mucking with the page tables
directly?  It seems, uh, rather wonky.  This, for instance:

>         if (protect)
>                 set_pte(pte, __pte(pte_val(*pte) & ~_PAGE_PRESENT));
>         else
>                 set_pte(pte, __pte(pte_val(*pte) | _PAGE_PRESENT));
> 
>         /*
>          * Flush this CPU's TLB, assuming whoever did the allocation/free is
>          * likely to continue running on this CPU.
>          */
>         preempt_disable();
>         flush_tlb_one_kernel(addr);
>         preempt_enable();

Seems rather broken.  I assume the preempt_disable() is there to get rid
of some warnings.  But, there is nothing I can see to *keep* the CPU
that did the free from being different from the one where the TLB flush
is performed until the preempt_disable().  That makes the
flush_tlb_one_kernel() mostly useless.

Is there a reason this code isn't using the existing page table
manipulation functions and tries to code its own?  What prevents it from
using something like the attached patch?
View attachment "kfence.patch" of type "text/x-patch" (1265 bytes)