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, 53 insertions, 23 deletions

diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 492728f9e021..1a5edec485f1 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -468,11 +468,60 @@ 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
@@ -491,18 +540,10 @@ 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;
@@ -680,8 +721,6 @@ 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)
@@ -690,15 +729,7 @@ 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);
@@ -708,7 +739,6 @@ 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