iomap: don't search past page end in iomap_is_partially_uptodate

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@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

1 files changed, 15 insertions, 2 deletions

diff --git a/fs/iomap.c b/fs/iomap.c
index 5bc172f3dfe8..fba8fdb734e0 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -499,16 +499,29 @@ done:
 }
 EXPORT_SYMBOL_GPL(iomap_readpages);
 
+/*
+ * 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.
+ */
 int
 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))