userfaultfd: build __VMA_UFFD_FLAGS from config-gated masks

The VMA flags bitmap is a single word today: NUM_VMA_FLAG_BITS is
BITS_PER_LONG, so on 32-bit vma_flags_t holds only 32 bits.  (The bitmap
type exists so this can grow past BITS_PER_LONG later; until it does,
anything declared above the first word is out of range on 32-bit.) The bit
enum nevertheless declares some bits unconditionally above BITS_PER_LONG
-- VMA_UFFD_MINOR_BIT is 41, with VM_UFFD_MINOR == VM_NONE on 32-bit so no
VMA actually carries the bit.

__VMA_UFFD_FLAGS feeds VMA_UFFD_MINOR_BIT to mk_vma_flags()
unconditionally.  On 32-bit that becomes __set_bit(41, &one_long), a write
one word past the end of the single-word bitmap.  The compiler folds the
out-of-bounds store with wraparound (1UL << (41 % 32) == bit 9) into the
first word; bit 9 is already in __VMA_UFFD_FLAGS so the mask happens to
come out right today, but it is an out-of-bounds write all the same, and
any high-numbered bit whose mod-BITS_PER_LONG position is otherwise unused
would silently OR an extra bit into the mask.

Rather than feed bit numbers that may not exist on the current build to
mk_vma_flags(), build the mask from whole per-mode masks that collapse to
EMPTY_VMA_FLAGS when their feature is unavailable.  Add
mk_vma_flags_from_masks() for that, and define VMA_UFFD_MISSING / _WP /
_MINOR alongside the VM_UFFD_* flags, gating VMA_UFFD_MINOR on the same
config as VM_UFFD_MINOR (which implies 64BIT, where bit 41 fits).  An
out-of-range bit is then never materialised, on any arch, and the in-range
fast path stays a compile-time constant.

Link: https://lore.kernel.org/20260529172331.356655-7-kas@kernel.org
Fixes: 9ea35a25d51b ("mm: introduce VMA flags bitmap type")
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reported-by: Sashiko AI review <sashiko-bot@kernel.org>
Suggested-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Assisted-by: Claude:claude-opus-4-8
Cc: David Hildenbrand <david@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions