Re: [PATCH] arm64: bti: Set PROT_BTI on all BTI executables mapped by the kernel

From: Catalin Marinas <catalin.marinas@arm.com>
To: Mark Brown <broonie@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	libc-alpha@sourceware.org, Kees Cook <keescook@chromium.org>,
	Szabolcs Nagy <szabolcs.nagy@arm.com>,
	Jeremy Linton <jeremy.linton@arm.com>,
	Will Deacon <will@kernel.org>, Dave Martin <dave.martin@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: bti: Set PROT_BTI on all BTI executables mapped by the kernel
Date: Fri, 5 Feb 2021 17:51:29 +0000	[thread overview]
Message-ID: <20210205175128.GB12697@gaia> (raw)
In-Reply-To: <20210205173837.39315-1-broonie@kernel.org>

On Fri, Feb 05, 2021 at 05:38:37PM +0000, Mark Brown wrote:
> Currently for dynamically linked executables the kernel only enables
> PROT_BTI for the interpreter, the interpreter is responsible for
> enabling it for everything else including the main executable.
> Unfortunately this interacts poorly with systemd's
> MemoryDenyWriteExecute feature which uses a seccomp filter to prevent
> setting PROT_EXEC on already mapped memory via mprotect(), it lacks the
> context to detect that PROT_EXEC is already set and so refuses to allow
> the mprotect() on the main executable which the kernel has already
> mapped.
> 
> Since we don't want to force users to choose between having MDWX and BTI
> as these are othogonal features have the kernel enable PROT_BTI for all
> the ELF objects it loads, not just the dynamic linker.  This means that
> if there is a problem with BTI it will be harder to disable at the
> executable level but we currently have no conditional support for this
> in any libc anyway so that would be new development.  Ideally we would
> have interfaces that allowed us to more clearly specify what is enabled
> and disabled by a given syscall but this would be a far more difficult
> change to deploy.
> 
> Reported-by: Jeremy Linton <jeremy.linton@arm.com>
> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
> Cc: Dave Martin <dave.martin@arm.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: libc-alpha@sourceware.org

Thanks Mark for putting the patch together. You may want to add a
reference to some of the discussions around the ABI, one of them:

Link: https://lore.kernel.org/r/20201207200338.GB24625@arm.com/

(so we can keep Szabolcs accountable if something breaks ;))

For this patch:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

I wouldn't merge it as a fix yet but I'm ok with getting in 5.12 if Will
is ok. It would give us some time to revert.

-- 
Catalin

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