[PATCH v8 1/2] dma-buf: Fix silent overflow for phys vec to sgt

[David Hu <dhu@x6u.co>]

From: David Hu <xuehaohu@google.com>

In case MMIO size is bigger than 4G and peer2peer DMA goes
through host bridge, we trigger a code path that assigns the
total linked IOVA (which is greater than 4G) to mapped_len.

Previously, `mapped_len` was declared as 32-bit `unsigned int`.
When accumulating `size_t` lengths, this leads to a silent wrap-around.
This truncation causes truncated lengths to be passed to functions
like `fill_sg_entry()`.

Fix this by changing `mapped_len` to `size_t` (64-bit). While
at it, fix similar potential overflow issues in `calc_sg_nents`
by using `check_add_overflow()` for `nents` and using
`unsigned int` for the loop iterator in `fill_sg_entry` to match.

Fixes: 3aa31a8bb11e ("dma-buf: provide phys_vec to scatter-gather mapping routine")
Cc: stable@vger.kernel.org
Cc: iommu@lists.linux.dev
Reviewed-by: Pranjal Shrivastava <praan@google.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: David Hu <xuehaohu@google.com>
---
Changes in v7:
 - Added a missing blank line after local variable declaration in
   `calc_sg_nents()` (Leon).
 - Collected Reviewed-by from Leon Romanovsky.

Changes in v6:
 - Used `check_add_overflow()` in `calc_sg_nents()` for safer
   accumulation (Leon).
 - Dropped explicit `!nents` check and added a comment noting that
   `sg_alloc_table` handles `nents == 0` (Leon).
 - Collected Reviewed-by from Kevin Tian.

Changes in v5:
 - Removed WARN_ON_ONCE from calc_sg_nents() to avoid log noise (Jason).
 - Added explicit check for `!nents` in dma_buf_phys_vec_to_sgt() to
   cleanly return -EINVAL on overflow (Jason).

Changes in v4:
 - Added WARN_ON_ONCE() to the nents overflow check to prevent silent
   failures (Claude Bot).

Changes in v3:
 - Removed leftover sentence fragment from the commit message.
 - Kept `nents = 0` initialization (previously stated as removed in the
   v2 changelog) as it is strictly required for the `+=` accumulation
   loop in `calc_sg_nents()`.

Changes in v2:
 - Fixed 'IVOA' -> 'IOVA' typo and expanded commit message (Claude Bot).
 - Added Reverse Xmas tree formatting (Pranjal).
 - Folded in extra bounds checking for calc_sg_nents() (Pranjal).
 - Folded in type consistency fix for fill_sg_entry() (Pranjal).
 - Collected Reviewed-by from Pranjal Shrivastava.

 drivers/dma-buf/dma-buf-mapping.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c
index 794acff2546a..80f6ab2f4809 100644
--- a/drivers/dma-buf/dma-buf-mapping.c
+++ b/drivers/dma-buf/dma-buf-mapping.c
@@ -5,12 +5,13 @@
  */
 #include <linux/dma-buf-mapping.h>
 #include <linux/dma-resv.h>
+#include <linux/overflow.h>
 
 static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length,
 					 dma_addr_t addr)
 {
 	unsigned int len, nents;
-	int i;
+	unsigned int i;
 
 	nents = DIV_ROUND_UP(length, UINT_MAX);
 	for (i = 0; i < nents; i++) {
@@ -40,8 +41,12 @@ static unsigned int calc_sg_nents(struct dma_iova_state *state,
 	size_t i;
 
 	if (!state || !dma_use_iova(state)) {
-		for (i = 0; i < nr_ranges; i++)
-			nents += DIV_ROUND_UP(phys_vec[i].len, UINT_MAX);
+		for (i = 0; i < nr_ranges; i++) {
+			unsigned int added = DIV_ROUND_UP(phys_vec[i].len, UINT_MAX);
+
+			if (check_add_overflow(nents, added, &nents))
+				return 0;
+		}
 	} else {
 		/*
 		 * In IOVA case, there is only one SG entry which spans
@@ -95,9 +100,10 @@ struct sg_table *dma_buf_phys_vec_to_sgt(struct dma_buf_attachment *attach,
 					 size_t nr_ranges, size_t size,
 					 enum dma_data_direction dir)
 {
-	unsigned int nents, mapped_len = 0;
 	struct dma_buf_dma *dma;
 	struct scatterlist *sgl;
+	size_t mapped_len = 0;
+	unsigned int nents;
 	dma_addr_t addr;
 	size_t i;
 	int ret;
@@ -133,6 +139,8 @@ struct sg_table *dma_buf_phys_vec_to_sgt(struct dma_buf_attachment *attach,
 	}
 
 	nents = calc_sg_nents(dma->state, phys_vec, nr_ranges, size);
+
+	/* sg_alloc_table will cleanly fail and return -EINVAL if nents == 0 */
 	ret = sg_alloc_table(&dma->sgt, nents, GFP_KERNEL | __GFP_ZERO);
 	if (ret)
 		goto err_free_state;
-- 
2.55.0.897.gb25b4bd76c-goog