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

On Fri, Feb 05, 2021 at 05:51:29PM +0000, Catalin Marinas wrote:

On Fri, Feb 05, 2021 at 05:38:37PM +0000, Mark Brown wrote:

quoted

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 <redacted>
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 <redacted>
Cc: Dave Martin <dave.martin@arm.com>
Cc: Kees Cook <redacted>
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/ (local)

(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.

I'd like an Ack from Szabolcs before we queue this.

Will

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