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

From: Kees Cook <kees@kernel.org>
To: Alexandre Ghiti <alex@ghiti.fr>
Cc: "Jessica Clarke" <jrtc27@jrtc27.com>,
	"Alexandre Ghiti" <alexghiti@rivosinc.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Heiko Stuebner" <heiko@sntech.de>,
	"Björn Töpel" <bjorn@rivosinc.com>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	"Jason Montleon" <jmontleo@redhat.com>,
	stable@vger.kernel.org
Subject: Re: [PATCH -fixes] riscv: Do not use fortify in early code
Date: Wed, 16 Oct 2024 09:56:30 -0700	[thread overview]
Message-ID: <202410160951.E825F7A5@keescook> (raw)
In-Reply-To: <3fe1e610-c863-4fbf-85cb-6e83ba7684af@ghiti.fr>

On Wed, Oct 16, 2024 at 01:26:24PM +0200, Alexandre Ghiti wrote:
> On 16/10/2024 00:04, Jessica Clarke wrote:
> > 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?
> 
> Not sure why fortify complains here, and I have just seen that I forgot to
> cc Kees (done now).

I haven't had time to investigate this -- something is confusing the
compiler about the object size. It's likely that it has decided that
"char *" is literally pointing to a single byte. (Instead of being
unable to determine the origin of the pointer and being forced to return
SIZE_MAX for the object size -- "unknown" size.) In other cases, we've
been able to convert "char *ptr" to "char ptr[]" and that tells the
compiler it's an array of unknown size. That didn't look very possible
here.

> [...]
> And I believe that enabling fortify and using the unsafe_*() variants is
> error-prone since we'd have to make sure that all the "fortified" functions
> used in that code use the unsafe_*() variants.
> 
> So to me, it's way easier in terms of maintenance to just disabling fortify.

I would agree: there's no way to report a fortify failure, so best to
turn it off here.

-- 
Kees Cook

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