ublk: clear VM_MAYWRITE on read-only ublk char device mmap

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ublk_ch_mmap() rejects mmap requests with VM_WRITE set, but never
clears VM_MAYWRITE on the resulting read-only mapping. This allows
a userspace daemon to mmap the per-queue command buffer PROT_READ,
then upgrade it to PROT_WRITE via mprotect(), since VM_MAYWRITE was
never cleared.

The command buffer holds struct ublksrv_io_desc entries that are
kernel-written ABI; a writable mapping lets an unprivileged daemon
process corrupt fields such as addr, op_flags, nr_sectors, and
start_sector.

Same bug class as the drm/panthor and drm/vc4 VM_MAYWRITE fixes, and
the 2026-08-13 ptp/vmclock fix (a5edadbae5).

Verified via mprotect() PoC: before the fix, a PROT_READ mapping can
be upgraded to PROT_READ|PROT_WRITE and a write into the command
buffer corrupts io_desc fields (confirmed under KASAN). After the
fix, mprotect() returns -EACCES.

Fixes: 3fee8d7599e1 ("ublk_drv: add io_uring based userspace block driver")
Cc: stable@vger.kernel.org
Signed-off-by: Kanishka De Silva <kpskanna1915@gmail.com>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260830070133.559-1-kpskanna1915@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

...

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Kanishka De Silva 2026-08-30 12:31:33 +05:30 • committed by Jens Axboe

parent e52349a5ea

commit 6e2b571b0a

1 changed files with 6 additions and 0 deletions

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drivers/block/ublk_drv.c

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