page_pool: Fix PP_MAGIC_MASK to avoid crashing on some 32-bit arches

Helge reported that the introduction of PP_MAGIC_MASK let to crashes on
boot on his 32-bit parisc machine. The cause of this is the mask is set
too wide, so the page_pool_page_is_pp() incurs false positives which
crashes the machine.

Just disabling the check in page_pool_is_pp() will lead to the page_pool
code itself malfunctioning; so instead of doing this, this patch changes
the define for PP_DMA_INDEX_BITS to avoid mistaking arbitrary kernel
pointers for page_pool-tagged pages.

The fix relies on the kernel pointers that alias with the pp_magic field
always being above PAGE_OFFSET. With this assumption, we can use the
lowest bit of the value of PAGE_OFFSET as the upper bound of the
PP_DMA_INDEX_MASK, which should avoid the false positives.

Because we cannot rely on PAGE_OFFSET always being a compile-time
constant, nor on it always being >0, we fall back to disabling the
dma_index storage when there are not enough bits available. This leaves
us in the situation we were in before the patch in the Fixes tag, but
only on a subset of architecture configurations. This seems to be the
best we can do until the transition to page types in complete for
page_pool pages.

v2:
- Make sure there's at least 8 bits available and that the PAGE_OFFSET
  bit calculation doesn't wrap

Link: https://lore.kernel.org/all/aMNJMFa5fDalFmtn@p100/
Fixes: ee62ce7a1d90 ("page_pool: Track DMA-mapped pages and unmap them when destroying the pool")
Cc: stable@vger.kernel.org # 6.15+
Tested-by: Helge Deller <deller@gmx.de>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Tested-by: Helge Deller <deller@gmx.de>
Link: https://patch.msgid.link/20250930114331.675412-1-toke@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 13 insertions, 9 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1ae97a0b8ec7..0905eb6b55ec 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4159,14 +4159,13 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
  * since this value becomes part of PP_SIGNATURE; meaning we can just use the
  * space between the PP_SIGNATURE value (without POISON_POINTER_DELTA), and the
  * lowest bits of POISON_POINTER_DELTA. On arches where POISON_POINTER_DELTA is
- * 0, we make sure that we leave the two topmost bits empty, as that guarantees
- * we won't mistake a valid kernel pointer for a value we set, regardless of the
- * VMSPLIT setting.
+ * 0, we use the lowest bit of PAGE_OFFSET as the boundary if that value is
+ * known at compile-time.
  *
- * Altogether, this means that the number of bits available is constrained by
- * the size of an unsigned long (at the upper end, subtracting two bits per the
- * above), and the definition of PP_SIGNATURE (with or without
- * POISON_POINTER_DELTA).
+ * If the value of PAGE_OFFSET is not known at compile time, or if it is too
+ * small to leave at least 8 bits available above PP_SIGNATURE, we define the
+ * number of bits to be 0, which turns off the DMA index tracking altogether
+ * (see page_pool_register_dma_index()).
  */
 #define PP_DMA_INDEX_SHIFT (1 + __fls(PP_SIGNATURE - POISON_POINTER_DELTA))
 #if POISON_POINTER_DELTA > 0
@@ -4175,8 +4174,13 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
  */
 #define PP_DMA_INDEX_BITS MIN(32, __ffs(POISON_POINTER_DELTA) - PP_DMA_INDEX_SHIFT)
 #else
-/* Always leave out the topmost two; see above. */
-#define PP_DMA_INDEX_BITS MIN(32, BITS_PER_LONG - PP_DMA_INDEX_SHIFT - 2)
+/* Use the lowest bit of PAGE_OFFSET if there's at least 8 bits available; see above */
+#define PP_DMA_INDEX_MIN_OFFSET (1 << (PP_DMA_INDEX_SHIFT + 8))
+#define PP_DMA_INDEX_BITS ((__builtin_constant_p(PAGE_OFFSET) && \
+			    PAGE_OFFSET >= PP_DMA_INDEX_MIN_OFFSET && \
+			    !(PAGE_OFFSET & (PP_DMA_INDEX_MIN_OFFSET - 1))) ? \
+			      MIN(32, __ffs(PAGE_OFFSET) - PP_DMA_INDEX_SHIFT) : 0)
+
 #endif
 
 #define PP_DMA_INDEX_MASK GENMASK(PP_DMA_INDEX_BITS + PP_DMA_INDEX_SHIFT - 1, \