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 no 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.

Link: https://lore.kernel.org/all/aMNJMFa5fDalFmtn@p100/
Fixes: ee62ce7a1d90 ("page_pool: Track DMA-mapped pages and unmap them when destroying the pool")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
Sorry for the delay on getting this out. I have only compile-tested it,
since I don't have any hardware that triggers the original bug. Helge, I'm
hoping you can take it for a spin?

 include/linux/mm.h   | 18 +++++------
 net/core/page_pool.c | 76 ++++++++++++++++++++++++++++++--------------
 2 files changed, 62 insertions(+), 32 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1ae97a0b8ec7..28541cb40f69 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 some 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 +4174,9 @@ 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)
+/* Constrain to the lowest bit of PAGE_OFFSET if known; see above. */
+#define PP_DMA_INDEX_BITS ((__builtin_constant_p(PAGE_OFFSET) && PAGE_OFFSET > PP_SIGNATURE) ? \
+			      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, \
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 36a98f2bcac3..e224d2145eed 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -472,11 +472,60 @@ page_pool_dma_sync_for_device(const struct page_pool *pool,
 	}
 }
 
+static int page_pool_register_dma_index(struct page_pool *pool,
+					netmem_ref netmem, gfp_t gfp)
+{
+	int err = 0;
+	u32 id;
+
+	if (unlikely(!PP_DMA_INDEX_BITS))
+		goto out;
+
+	if (in_softirq())
+		err = xa_alloc(&pool->dma_mapped, &id, netmem_to_page(netmem),
+			       PP_DMA_INDEX_LIMIT, gfp);
+	else
+		err = xa_alloc_bh(&pool->dma_mapped, &id, netmem_to_page(netmem),
+				  PP_DMA_INDEX_LIMIT, gfp);
+	if (err) {
+		WARN_ONCE(err != -ENOMEM, "couldn't track DMA mapping, please report to netdev@");
+		goto out;
+	}
+
+	netmem_set_dma_index(netmem, id);
+out:
+	return err;
+}
+
+static int page_pool_release_dma_index(struct page_pool *pool,
+				       netmem_ref netmem)
+{
+	struct page *old, *page = netmem_to_page(netmem);
+	unsigned long id;
+
+	if (unlikely(!PP_DMA_INDEX_BITS))
+		return 0;
+
+	id = netmem_get_dma_index(netmem);
+	if (!id)
+		return -1;
+
+	if (in_softirq())
+		old = xa_cmpxchg(&pool->dma_mapped, id, page, NULL, 0);
+	else
+		old = xa_cmpxchg_bh(&pool->dma_mapped, id, page, NULL, 0);
+	if (old != page)
+		return -1;
+
+	netmem_set_dma_index(netmem, 0);
+
+	return 0;
+}
+
 static bool page_pool_dma_map(struct page_pool *pool, netmem_ref netmem, gfp_t gfp)
 {
 	dma_addr_t dma;
 	int err;
-	u32 id;
 
 	/* Setup DMA mapping: use 'struct page' area for storing DMA-addr
 	 * since dma_addr_t can be either 32 or 64 bits and does not always fit
@@ -495,18 +544,10 @@ static bool page_pool_dma_map(struct page_pool *pool, netmem_ref netmem, gfp_t g
 		goto unmap_failed;
 	}
 
-	if (in_softirq())
-		err = xa_alloc(&pool->dma_mapped, &id, netmem_to_page(netmem),
-			       PP_DMA_INDEX_LIMIT, gfp);
-	else
-		err = xa_alloc_bh(&pool->dma_mapped, &id, netmem_to_page(netmem),
-				  PP_DMA_INDEX_LIMIT, gfp);
-	if (err) {
-		WARN_ONCE(err != -ENOMEM, "couldn't track DMA mapping, please report to netdev@");
+	err = page_pool_register_dma_index(pool, netmem, gfp);
+	if (err)
 		goto unset_failed;
-	}
 
-	netmem_set_dma_index(netmem, id);
 	page_pool_dma_sync_for_device(pool, netmem, pool->p.max_len);
 
 	return true;
@@ -684,8 +725,6 @@ void page_pool_clear_pp_info(netmem_ref netmem)
 static __always_inline void __page_pool_release_netmem_dma(struct page_pool *pool,
 							   netmem_ref netmem)
 {
-	struct page *old, *page = netmem_to_page(netmem);
-	unsigned long id;
 	dma_addr_t dma;
 
 	if (!pool->dma_map)
@@ -694,15 +733,7 @@ static __always_inline void __page_pool_release_netmem_dma(struct page_pool *poo
 		 */
 		return;
 
-	id = netmem_get_dma_index(netmem);
-	if (!id)
-		return;
-
-	if (in_softirq())
-		old = xa_cmpxchg(&pool->dma_mapped, id, page, NULL, 0);
-	else
-		old = xa_cmpxchg_bh(&pool->dma_mapped, id, page, NULL, 0);
-	if (old != page)
+	if (page_pool_release_dma_index(pool, netmem))
 		return;
 
 	dma = page_pool_get_dma_addr_netmem(netmem);
@@ -712,7 +743,6 @@ static __always_inline void __page_pool_release_netmem_dma(struct page_pool *poo
 			     PAGE_SIZE << pool->p.order, pool->p.dma_dir,
 			     DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_WEAK_ORDERING);
 	page_pool_set_dma_addr_netmem(netmem, 0);
-	netmem_set_dma_index(netmem, 0);
 }
 
 /* Disconnects a page (from a page_pool).  API users can have a need
-- 
2.51.0