[PATCH] btrfs: fix out of bounds array access while reading extent buffer

[Young Xiao]

There is a corner case that slips through the checkers in functions
reading extent buffer, ie.

if (start < eb->len) and (start + len > eb->len), then:
the checkers in read_extent_buffer_to_user(), and memcmp_extent_buffer()
WARN_ON(start > eb->len) and WARN_ON(start + len > eb->start + eb->len),
both are OK in this corner case, but it'd actually try to access the eb->pages
out of bounds because of (start + len > eb->len).

This is adding proper checks in order to avoid invalid memory access,
ie. 'general protection fault', before it's too late.

See commit f716abd55d1e ("Btrfs: fix out of bounds array access while
reading extent buffer") for details.

Signed-off-by: Young Xiao <92siuyang@xxxxxxxxx>
---
 fs/btrfs/extent_io.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index db337e5..dcf3b2e 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -5476,8 +5476,12 @@ int read_extent_buffer_to_user(const struct extent_buffer *eb,
 	unsigned long i = (start_offset + start) >> PAGE_SHIFT;
 	int ret = 0;
 
-	WARN_ON(start > eb->len);
-	WARN_ON(start + len > eb->start + eb->len);
+	if (start + len > eb->len) {
+		WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, wanted %lu %lu\n",
+		     eb->start, eb->len, start, len);
+		memset(dst, 0, len);
+		return;
+	}
 
 	offset = offset_in_page(start_offset + start);
 
@@ -5554,8 +5558,12 @@ int memcmp_extent_buffer(const struct extent_buffer *eb, const void *ptrv,
 	unsigned long i = (start_offset + start) >> PAGE_SHIFT;
 	int ret = 0;
 
-	WARN_ON(start > eb->len);
-	WARN_ON(start + len > eb->start + eb->len);
+	if (start + len > eb->len) {
+		WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, wanted %lu %lu\n",
+		     eb->start, eb->len, start, len);
+		memset(ptr, 0, len);
+		return;
+	}
 
 	offset = offset_in_page(start_offset + start);
 
-- 
2.7.4