linux-kernel - Re: [PATCH -fixes] riscv: Do not use fortify in early code

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Message-ID: <CAJD_bPLcKX+U1k60mgsB_==qrQE+jnLMXSotq3rMrH_o+FtOQQ@mail.gmail.com>
Date: Wed, 16 Oct 2024 01:26:02 -0400
From: Jason Montleon <jmontleo@...hat.com>
To: Jessica Clarke <jrtc27@...c27.com>
Cc: Alexandre Ghiti <alexghiti@...osinc.com>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Heiko Stuebner <heiko@...ech.de>, Björn Töpel <bjorn@...osinc.com>, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH -fixes] riscv: Do not use fortify in early code

On Tue, Oct 15, 2024 at 6:05 PM Jessica Clarke <jrtc27@...c27.com> wrote:
>
> On 9 Oct 2024, at 08:27, Alexandre Ghiti <alexghiti@...osinc.com> wrote:
> >
> > Early code designates the code executed when the MMU is not yet enabled,
> > and this comes with some limitations (see
> > Documentation/arch/riscv/boot.rst, section "Pre-MMU execution").
> >
> > FORTIFY_SOURCE must be disabled then since it can trigger kernel panics
> > as reported in [1].
> >
> > Reported-by: Jason Montleon <jmontleo@...hat.com>
> > Closes: https://lore.kernel.org/linux-riscv/CAJD_bPJes4QhmXY5f63GHV9B9HFkSCoaZjk-qCT2NGS7Q9HODg@mail.gmail.com/ [1]
> > Fixes: a35707c3d850 ("riscv: add memory-type errata for T-Head")
> > Fixes: 26e7aacb83df ("riscv: Allow to downgrade paging mode from the command line")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
>
> Is the problem in [1] not just that the early boot path uses memcpy on
> the result of ALT_OLD_PTR, which is a wildly out-of-bounds pointer from
> the compiler’s perspective? If so, it would seem better to use
> unsafe_memcpy for that one call site rather than use the big
> __NO_FORTIFY hammer, surely?
>

I can add that replacing memcpy with unsafe_memcpy did also work for
me. Once it was narrowed down, this is what I originally did in order
to boot.

Jason

> Presumably the non-early path is just as bad to the compiler, but works
> because patch_text_nosync isn’t instrumented, so that would just align
> the two.
>
> Getting the implementation to not be silent on failure during early
> boot would also be a good idea, but it’s surely better to have
> FORTIFY_SOURCE enabled with no output for positives than disable the
> checking in the first place and risk uncaught corruption.
>
> Jess
>