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

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Message-ID: <Zw6X9KQT0-z7r7SY@J2N7QTR9R3.cambridge.arm.com>
Date: Tue, 15 Oct 2024 17:27:32 +0100
From: Mark Rutland <mark.rutland@....com>
To: Ard Biesheuvel <ardb@...nel.org>
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, 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.

Mark.