[PATCH 2/4] lib/scatterlist: Avoid potential scatterlist entry overflow

[Tvrtko Ursulin]

From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Since the scatterlist length field is an unsigned int, make
sure that sg_alloc_table_from_pages does not overflow it while
coallescing pages to a single entry.

v2: Drop reference to future use. Use UINT_MAX.
v3: max_segment must be page aligned.
v4: Do not rely on compiler to optimise out the rounddown.
    (Joonas Lahtinen)
v5: Simplified loops and use post-increments rather than
    pre-increments. Use PAGE_MASK and fix comment typo.
    (Andy Shevchenko)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> (v2)
Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
Cc: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
---
 include/linux/scatterlist.h |  6 ++++++
 lib/scatterlist.c           | 31 ++++++++++++++++++++-----------
 2 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 205aefb4ed93..6dd2ddbc6230 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -21,6 +21,12 @@ struct scatterlist {
 };
 
 /*
+ * Since the above length field is an unsigned int, below we define the maximum
+ * length in bytes that can be stored in one scatterlist entry.
+ */
+#define SCATTERLIST_MAX_SEGMENT (UINT_MAX & PAGE_MASK)
+
+/*
  * These macros should be used after a dma_map_sg call has been done
  * to get bus addresses of each of the SG entries and their lengths.
  * You should only work with the number of sg entries dma_map_sg
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index dee0c5004e2f..7b2e74da2c44 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -394,17 +394,22 @@ int sg_alloc_table_from_pages(struct sg_table *sgt,
 	unsigned int offset, unsigned long size,
 	gfp_t gfp_mask)
 {
-	unsigned int chunks;
-	unsigned int i;
-	unsigned int cur_page;
+	const unsigned int max_segment = SCATTERLIST_MAX_SEGMENT;
+	unsigned int chunks, cur_page, seg_len, i;
 	int ret;
 	struct scatterlist *s;
 
 	/* compute number of contiguous chunks */
 	chunks = 1;
-	for (i = 1; i < n_pages; ++i)
-		if (page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1)
-			++chunks;
+	seg_len = 0;
+	for (i = 1; i < n_pages; i++) {
+		seg_len += PAGE_SIZE;
+		if (seg_len >= max_segment ||
+		    page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1) {
+			chunks++;
+			seg_len = 0;
+		}
+	}
 
 	ret = sg_alloc_table(sgt, chunks, gfp_mask);
 	if (unlikely(ret))
@@ -413,17 +418,21 @@ int sg_alloc_table_from_pages(struct sg_table *sgt,
 	/* merging chunks and putting them into the scatterlist */
 	cur_page = 0;
 	for_each_sg(sgt->sgl, s, sgt->orig_nents, i) {
-		unsigned long chunk_size;
-		unsigned int j;
+		unsigned int j, chunk_size;
 
 		/* look for the end of the current chunk */
-		for (j = cur_page + 1; j < n_pages; ++j)
-			if (page_to_pfn(pages[j]) !=
+		seg_len = 0;
+		for (j = cur_page + 1; j < n_pages; j++) {
+			seg_len += PAGE_SIZE;
+			if (seg_len >= max_segment ||
+			    page_to_pfn(pages[j]) !=
 			    page_to_pfn(pages[j - 1]) + 1)
 				break;
+		}
 
 		chunk_size = ((j - cur_page) << PAGE_SHIFT) - offset;
-		sg_set_page(s, pages[cur_page], min(size, chunk_size), offset);
+		sg_set_page(s, pages[cur_page],
+			    min_t(unsigned long, size, chunk_size), offset);
 		size -= chunk_size;
 		offset = 0;
 		cur_page = j;
-- 
2.9.4