Fix off-by-one error in iov_iter_advance()

The iov_iter_advance() function would look at the iov->iov_len entry
even though it might have iterated over the whole array, and iov was
pointing past the end.  This would cause DEBUG_PAGEALLOC to trigger a
kernel page fault if the allocation was at the end of a page, and the
next page was unallocated.

The quick fix is to just change the order of the tests: check that there
is any iovec data left before we check the iov entry itself.

Thanks to Alexey Dobriyan for finding this case, and testing the fix.

Reported-and-tested-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@kernel.org> [2.6.25.x, 2.6.26.x]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index 42bbc69..d97d1ad 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1879,7 +1879,7 @@ void iov_iter_advance(struct iov_iter *i, size_t bytes)
                 * The !iov->iov_len check ensures we skip over unlikely
                 * zero-length segments (without overruning the iovec).
                 */
-               while (bytes || unlikely(!iov->iov_len && i->count)) {
+               while (bytes || unlikely(i->count && !iov->iov_len)) {
                        int copy;
 
                        copy = min(bytes, iov->iov_len - base);