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From: Zev Weiss <zev@bewilderbeest.net>
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Subject: [PATCH 0/2] ARM: prctl: Reject PR_SET_MDWE where not supported
Date: Wed,  7 Feb 2024 17:26:18 -0800
Message-ID: <20240208012620.32604-4-zev@bewilderbeest.net>
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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