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

[Russell King (Oracle)]

Hi,

Where is patch 1 of this series? It doesn't seem to have been Cc'd to
linux-arm-kernel. Therefore, this can't be reviewed.

Thanks.

On Wed, Feb 07, 2024 at 05:26:18PM -0800, 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.
> 
> 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]).
> 
> 
> Thanks,
> Zev
> 
> [0] https://lore.kernel.org/all/2023112456-linked-nape-bf19@gregkh/
> 
> Zev Weiss (2):
>   prctl: Generalize PR_SET_MDWE support check to be per-arch
>   ARM: prctl: Reject PR_SET_MDWE on pre-ARMv6
> 
>  arch/arm/include/asm/mman.h    | 14 ++++++++++++++
>  arch/parisc/include/asm/mman.h | 14 ++++++++++++++
>  include/linux/mman.h           |  8 ++++++++
>  kernel/sys.c                   |  7 +++++--
>  4 files changed, 41 insertions(+), 2 deletions(-)
>  create mode 100644 arch/arm/include/asm/mman.h
>  create mode 100644 arch/parisc/include/asm/mman.h
> 
> -- 
> 2.43.0
> 
> 
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> 

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