linux-kernel - Re: Crash on armv7-a using KASAN

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Message-ID: <CAMj1kXGGmsWs2XpM7zLURjKp67Uz2ePi1pSV1=rPCMgviLVUgw@mail.gmail.com>
Date: Tue, 15 Oct 2024 19:28:06 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Linus Walleij <linus.walleij@...aro.org>, 
	Clement LE GOFFIC <clement.legoffic@...s.st.com>, Russell King <linux@...linux.org.uk>, 
	"Russell King (Oracle)" <rmk+kernel@...linux.org.uk>, Kees Cook <kees@...nel.org>, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, Mark Brown <broonie@...nel.org>, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-stm32@...md-mailman.stormreply.com, 
	Antonio Borneo <antonio.borneo@...s.st.com>
Subject: Re: Crash on armv7-a using KASAN

On Tue, 15 Oct 2024 at 18:27, Mark Rutland <mark.rutland@....com> wrote:
>
> On Tue, Oct 15, 2024 at 06:07:00PM +0200, Ard Biesheuvel wrote:
> > On Tue, 15 Oct 2024 at 17:26, Mark Rutland <mark.rutland@....com> wrote:
> > > Looking some more, I don't see how VMAP_STACK guarantees that the
> > > old/active stack is mapped in the new mm when switching from the old mm
> > > to the new mm (which happens before __switch_to()).
> > >
> > > Either I'm missing something, or we have a latent bug. Maybe we have
> > > some explicit copying/prefaulting elsewhere I'm missing?
> >
> > We bump the vmalloc_seq counter for that. Given that the top-level
> > page table can only gain entries covering the kernel space, this
> > should be sufficient for the old task's stack to be mapped in the new
> > task's page tables.
>
> Ah, yep -- I had missed that. Thanks for the pointer!
>
> From a superficial look, it sounds like it should be possible to extend
> that to also handle the KASAN shadow of the vmalloc area (which
> __check_vmalloc_seq() currently doesn't copy), but I'm not sure of
> exactly when we initialise the shadow for a vmalloc allocation relative
> to updating vmalloc_seq.
>

Indeed. It appears both vmalloc_seq() and arch_sync_kernel_mappings()
need to take the vmalloc shadow into account specifically. And we may
also need the dummy read from the stack's shadow in __switch_to - I am
pretty sure I added that for a reason.