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

[Helge Deller <deller@gmx.de>]

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.

> 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...

> 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
>