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From:   Evgenii Stepanov <eugenis@google.com>
Date:   Tue, 25 Jan 2022 08:59:07 -0800
Message-ID: <CAFKCwrjbn3e2w-LikMY5bOvUKbUBi7_4iwBOD7KQUn8QHPwbng@mail.gmail.com>
Subject: Re: [PATCH] arm64: extable: fix null deref in load_unaligned_zeropad.
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Jisheng Zhang <jszhang@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Jan 25, 2022 at 7:50 AM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Tue, Jan 25, 2022 at 03:23:20PM +0000, Mark Rutland wrote:
> > On Fri, Jan 21, 2022 at 06:34:47PM -0800, Evgenii Stepanov wrote:
> > > ex_handler_load_unaligned_zeropad extracts the source and data register
> > > numbers from the wrong field of the exception table.
> >
> > Ouch. Did you find this by inspection, or did this show up in testing?
> >
> > Sorry about this.
> >
> > I think we should be a little more explicit as to exactly what goes wrong. How
> > about:
> >
> > | In ex_handler_load_unaligned_zeropad() we erroneously extract the data and
> > | addr register indices from ex->type rather than ex->data. As ex->type will
> > | contain EX_TYPE_LOAD_UNALIGNED_ZEROPAD (i.e. 4):
> > |
> > | * We'll always treat X0 as the address register, since EX_DATA_REG_ADDR is
> > |   extracted from bits [9:5]. Thus, we may attempt to dereference an arbitrary
> > |   address as X0 may hold an arbitary value.
> > |
> > | * We'll always treat X4 as the data register, since EX_DATA_REG_DATA is
> > |   extracted from bits [4:0]. Thus we will corrupt X4 and cause arbitrary
> > |   behaviour within load_unaligned_zeropad() and its caller.
> > |
> > | Fix this by extracting both values from ex->data as originally intended.
> >
> > > Fixes: 753b3236
> >
> > That should be expanded, e.g.
> >
> >   Fixes: 753b32368705c396 ("arm64: extable: add load_unaligned_zeropad() handler")
> >
> > With those changes:
> >
> > Reviewed-by: Mark Rutland <mark.rutland@arm.com>
>
> Looking again, sicne this isn't jsut a null-deref, can we also rework the
> title, something like:
>
> | arm64: extable: fix load_unaligned_zeropad() reg indices

That's a much better commit message, thank you! I'll upload v2 shortly.

This was found by updating to a newer QEMU that correctly delivers MTE
faults from unaligned memory accesses, and triggers this bug reliably
during Android boot. I'll add a stack trace to v2.

>
> Thanks,
> Mark.