[PATCH 6/6] userfaultfd: build __VMA_UFFD_FLAGS from config-gated masks

[Kiryl Shutsemau (Meta)]

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.

Fixes: 9ea35a25d51b ("mm: introduce VMA flags bitmap type")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Sashiko AI review <sashiko-bot@xxxxxxxxxx>
Suggested-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-8
---
 include/linux/mm.h            | 39 +++++++++++++++++++++++++++++++++++
 include/linux/userfaultfd_k.h |  4 ++--
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0f2612a70fb1..485df9c2dbdd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -496,6 +496,21 @@ enum {
 #else
 #define VM_UFFD_MINOR	VM_NONE
 #endif
+
+/*
+ * vma_flags_t masks for the userfaultfd VMA flags. VMA_UFFD_MINOR is gated on
+ * the same config as VM_UFFD_MINOR -- which implies 64BIT, where the bit fits
+ * -- so an out-of-range bit is never fed to mk_vma_flags() on a build whose
+ * bitmap cannot hold it.
+ */
+#define VMA_UFFD_MISSING	mk_vma_flags(VMA_UFFD_MISSING_BIT)
+#define VMA_UFFD_WP		mk_vma_flags(VMA_UFFD_WP_BIT)
+#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
+#define VMA_UFFD_MINOR		mk_vma_flags(VMA_UFFD_MINOR_BIT)
+#else
+#define VMA_UFFD_MINOR		EMPTY_VMA_FLAGS
+#endif
+
 #ifdef CONFIG_64BIT
 #define VM_ALLOW_ANY_UNCACHED	INIT_VM_FLAG(ALLOW_ANY_UNCACHED)
 #define VM_SEALED		INIT_VM_FLAG(SEALED)
@@ -1238,6 +1253,30 @@ static __always_inline void vma_flags_set_mask(vma_flags_t *flags,
 #define vma_flags_set(flags, ...) \
 	vma_flags_set_mask(flags, mk_vma_flags(__VA_ARGS__))
 
+static __always_inline vma_flags_t __mk_vma_flags_from_masks(size_t count,
+		const vma_flags_t *masks)
+{
+	vma_flags_t flags = EMPTY_VMA_FLAGS;
+	size_t i;
+
+	for (i = 0; i < count; i++)
+		vma_flags_set_mask(&flags, masks[i]);
+	return flags;
+}
+
+/*
+ * Combine pre-computed vma_flags_t masks into one value, e.g.:
+ *
+ * vma_flags_t flags = mk_vma_flags_from_masks(VMA_UFFD_WP, VMA_UFFD_MINOR);
+ *
+ * Unlike mk_vma_flags(), which takes bit numbers, this takes whole masks --
+ * each of which may be EMPTY_VMA_FLAGS when its feature is unavailable -- so a
+ * bit that does not exist on the current build is never materialised.
+ */
+#define mk_vma_flags_from_masks(...)					\
+	__mk_vma_flags_from_masks(COUNT_ARGS(__VA_ARGS__),		\
+		(const vma_flags_t []){__VA_ARGS__})
+
 /* Clear all of the to-clear flags in flags, non-atomically. */
 static __always_inline void vma_flags_clear_mask(vma_flags_t *flags,
 		vma_flags_t to_clear)
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 3ec8e1071673..68edac4dcd78 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -23,8 +23,8 @@
 /* The set of all possible UFFD-related VM flags. */
 #define __VM_UFFD_FLAGS (VM_UFFD_MISSING | VM_UFFD_WP | VM_UFFD_MINOR)
 
-#define __VMA_UFFD_FLAGS mk_vma_flags(VMA_UFFD_MISSING_BIT, VMA_UFFD_WP_BIT, \
-				      VMA_UFFD_MINOR_BIT)
+#define __VMA_UFFD_FLAGS mk_vma_flags_from_masks(VMA_UFFD_MISSING, VMA_UFFD_WP, \
+						 VMA_UFFD_MINOR)
 
 /*
  * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining
-- 
2.54.0