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Date: Wed, 7 Feb 2024 23:42:27 -0800
From: Zev Weiss <zev@bewilderbeest.net>
To: Helge Deller <deller@gmx.de>
Cc: linux-parisc@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	Florent Revest <revest@chromium.org>,
	"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
	"Mike Rapoport (IBM)" <rppt@kernel.org>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	"Borislav Petkov (AMD)" <bp@alien8.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Yang Shi <yang@os.amperecomputing.com>,
	Stefan Roesch <shr@devkernel.io>, Oleg Nesterov <oleg@redhat.com>,
	David Hildenbrand <david@redhat.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Ondrej Mosnacek <omosnace@redhat.com>,
	Miguel Ojeda <ojeda@kernel.org>, openbmc@lists.ozlabs.org,
	linux-kernel@vger.kernel.org, Russell King <linux@armlinux.org.uk>,
	Sam James <sam@gentoo.org>, stable@vger.kernel.org
Subject: Re: [PATCH 0/2] ARM: prctl: Reject PR_SET_MDWE where not supported
Message-ID: <aee52007-b805-40a0-976b-eee52c98099c@hatter.bewilderbeest.net>
References: <20240208012620.32604-4-zev@bewilderbeest.net>
 <385d72eb-2443-42e5-858d-0cc083a29a26@gmx.de>
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In-Reply-To: <385d72eb-2443-42e5-858d-0cc083a29a26@gmx.de>

Hi Helge,

Thanks for taking a look!

On Wed, Feb 07, 2024 at 11:02:24PM PST, Helge Deller wrote:
>Hi Zev,
>
>On 2/8/24 02:26, Zev Weiss wrote:
>>Hello,
>>
>>I noticed after a recent kernel update that my ARM926 system started
>>segfaulting on any execve() after calling prctl(PR_SET_MDWE).  After
>>some investigation it appears that ARMv5 is incapable of providing the
>>appropriate protections for MDWE, since any readable memory is also
>>implicitly executable.
>>
>>(Note that I'm not an expert in either ARM arch details or the mm
>>subsystem, so please bear with me if I've botched something in the
>>above analysis.)
>>
>>The prctl_set_mdwe() function already had some special-case logic
>>added disabling it on PARISC (commit 793838138c15, "prctl: Disable
>>prctl(PR_SET_MDWE) on parisc"); this patch series (1) generalizes that
>>check to use an arch_*() function, and (2) adds a corresponding
>>override for ARM to disable MDWE on pre-ARMv6 CPUs.
>
>Instead of splitting it out to a new function in mman.h,
>I'd prefer having it as config option, e.g. ARCH_HAS_NO_MDWE_SUPPORT (?)
>which could be checked instead.
>For parisc we still want to allow mdwe in the future, we just have
>to wait until most user-space programs have updated to the latest
>binaries which don't need an executable stack any longer.
>

I considered that, but it seems that ARM kernels at least may not know 
the answer to that question at compile-time -- see patch 2, where the 
ARM implementation does a runtime check on cpu_architecture().

>>With the series applied, prctl(PR_SET_MDWE) is rejected on ARMv5 and
>>subsequent execve() calls (as well as mmap(PROT_READ|PROT_WRITE)) can
>>succeed instead of unconditionally failing; on ARMv6 the prctl works
>>as it did previously.
>>
>>Since this was effectively a userspace-breaking change in v6.3 (with
>>newer MDWE-aware userspace on older pre-MDWE kernels the prctl would
>>simply fail safely) I've CCed -stable for v6.3+, though since the
>>patches depend on the PARISC one above it will only apply cleanly on
>>the linux-6.6.y and linux-6.7.y branches, since at least at time of
>>writing the 6.3 through 6.5 branches don't have that patch backported
>>(due to further missing dependencies [0]).
>>[0] https://lore.kernel.org/all/2023112456-linked-nape-bf19@gregkh/
>
>I think you don't need to worry about that, since stable kernel series
>for 6.3 up to 6.5 were stopped...
>

Ah, hadn't realized that -- thanks for the tip.


Zev