[PATCH v4 1/4] lib/scatterlist: Fix mapping iterator when sg->offset is greater than PAGE_SIZE

[Christophe Leroy]

All mapping iterator logic is based on the assumption that sg->offset
is always lower than PAGE_SIZE.

But there are situations where sg->offset is such that the SG item
is on the second page. In that case sg_copy_to_buffer() fails
properly copying the data into the buffer. One of the reason is
that the data will be outside the kmapped area used to access that
data.

This patch fixes the issue by adjusting the mapping iterator
offset and pgoffset fields such that offset is always lower than
PAGE_SIZE.

Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
Fixes: 4225fc8555a9 ("lib/scatterlist: use page iterator in the mapping iterator")
Cc: stable at vger.kernel.org
---
 lib/scatterlist.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 739dc9fe2c55..39f00659898f 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -678,7 +678,7 @@ static bool sg_miter_get_next_page(struct sg_mapping_iter *miter)
 {
 	if (!miter->__remaining) {
 		struct scatterlist *sg;
-		unsigned long pgoffset;
+		unsigned long pgoffset, offset;
 
 		if (!__sg_page_iter_next(&miter->piter))
 			return false;
@@ -686,7 +686,12 @@ static bool sg_miter_get_next_page(struct sg_mapping_iter *miter)
 		sg = miter->piter.sg;
 		pgoffset = miter->piter.sg_pgoffset;
 
-		miter->__offset = pgoffset ? 0 : sg->offset;
+		offset = pgoffset ? 0 : sg->offset;
+		while (offset >= PAGE_SIZE) {
+			miter->piter.sg_pgoffset = ++pgoffset;
+			offset -= PAGE_SIZE;
+		}
+		miter->__offset = offset;
 		miter->__remaining = sg->offset + sg->length -
 				(pgoffset << PAGE_SHIFT) - miter->__offset;
 		miter->__remaining = min_t(unsigned long, miter->__remaining,
-- 
2.13.3