Re: [PATCH 0/2] ARM: prctl: Reject PR_SET_MDWE where not supported

[Zev Weiss <zev@bewilderbeest.net>]

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