lib/scatterlist: Avoid potential scatterlist entry overflow

Since the scatterlist length field is an unsigned int, make
sure that sg_alloc_table_from_pages does not overflow it while
coalescing 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)
v6: Commit spelling fix.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170803091312.22875-1-tvrtko.ursulin@linux.intel.com

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 205aefb4ed93990d7c8729da83461e8c0fdc95e2..6dd2ddbc62301f545888e4983da41ab5ba33b07c 100644 (file)
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -20,6 +20,12 @@ struct scatterlist {
 #endif
 };
 
+/*
+ * 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.

diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index dee0c5004e2f209ddb720ffe84f24fb1c5f3002d..7b2e74da2c44ba4103d5fe7ff8b3d66ac3ea0444 100644 (file)
--- 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;