[PATCH 4.20 114/127] iomap: dont search past page end in iomap_is_partially_uptodate

From: Greg Kroah-Hartman
Date: Thu Jan 24 2019 - 14:44:45 EST

4.20-stable review patch. If anyone has any objections, please let me know.


[ Upstream commit 3cc31fa65d85610574c0f6a474e89f4c419923d5 ]

iomap_is_partially_uptodate() is intended to check wither blocks within
the selected range of a not-uptodate page are uptodate; if the range we
care about is up to date, it's an optimization.

However, the iomap implementation continues to check all blocks up to
from+count, which is beyond the page, and can even be well beyond the
iop->uptodate bitmap.

I think the worst that will happen is that we may eventually find a zero
bit and return "not partially uptodate" when it would have otherwise
returned true, and skip the optimization. Still, it's clearly an invalid
memory access that must be fixed.

So: fix this by limiting the search to within the page as is done in the
non-iomap variant, block_is_partially_uptodate().

Zorro noticed thiswhen KASAN went off for 512 byte blocks on a 64k
page system:

BUG: KASAN: slab-out-of-bounds in iomap_is_partially_uptodate+0x1a0/0x1e0
Read of size 8 at addr ffff800120c3a318 by task fsstress/22337

Reported-by: Zorro Lang <zlang@xxxxxxxxxx>
Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxxx>
Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
fs/iomap.c | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/iomap.c b/fs/iomap.c
index d6bc98ae8d35..ce837d962d47 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -492,16 +492,29 @@ done:

+ * iomap_is_partially_uptodate checks whether blocks within a page are
+ * uptodate or not.
+ *
+ * Returns true if all blocks which correspond to a file portion
+ * we want to read within the page are uptodate.
+ */
iomap_is_partially_uptodate(struct page *page, unsigned long from,
unsigned long count)
struct iomap_page *iop = to_iomap_page(page);
struct inode *inode = page->mapping->host;
- unsigned first = from >> inode->i_blkbits;
- unsigned last = (from + count - 1) >> inode->i_blkbits;
+ unsigned len, first, last;
unsigned i;

+ /* Limit range to one page */
+ len = min_t(unsigned, PAGE_SIZE - from, count);
+ /* First and last blocks in range within page */
+ first = from >> inode->i_blkbits;
+ last = (from + len - 1) >> inode->i_blkbits;
if (iop) {
for (i = first; i <= last; i++)
if (!test_bit(i, iop->uptodate))