-stable review patch. If anyone has any objections, please let us know.
------------------
From: Hugh Dickins <[email protected]>
patch 487e9bf25cbae11b131d6a14bdbb3a6a77380837 in mainline.
It's possible to provoke unionfs (not yet in mainline, though in mm and
some distros) to hit shmem_writepage's BUG_ON(page_mapped(page)). I expect
it's possible to provoke the 2.6.23 ecryptfs in the same way (but the
2.6.24 ecryptfs no longer calls lower level's ->writepage).
This came to light with the recent find that AOP_WRITEPAGE_ACTIVATE could
leak from tmpfs via write_cache_pages and unionfs to userspace. There's
already a fix (e423003028183df54f039dfda8b58c49e78c89d7 - writeback: don't
propagate AOP_WRITEPAGE_ACTIVATE) in the tree for that, and it's okay so
far as it goes; but insufficient because it doesn't address the underlying
issue, that shmem_writepage expects to be called only by vmscan (relying on
backing_dev_info capabilities to prevent the normal writeback path from
ever approaching it).
That's an increasingly fragile assumption, and ramdisk_writepage (the other
source of AOP_WRITEPAGE_ACTIVATEs) is already careful to check
wbc->for_reclaim before returning it. Make the same check in
shmem_writepage, thereby sidestepping the page_mapped BUG also.
Signed-off-by: Hugh Dickins <[email protected]>
Cc: Erez Zadok <[email protected]>
Reviewed-by: Pekka Enberg <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
mm/shmem.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -916,6 +916,21 @@ static int shmem_writepage(struct page *
struct inode *inode;
BUG_ON(!PageLocked(page));
+ /*
+ * shmem_backing_dev_info's capabilities prevent regular writeback or
+ * sync from ever calling shmem_writepage; but a stacking filesystem
+ * may use the ->writepage of its underlying filesystem, in which case
+ * we want to do nothing when that underlying filesystem is tmpfs
+ * (writing out to swap is useful as a response to memory pressure, but
+ * of no use to stabilize the data) - just redirty the page, unlock it
+ * and claim success in this case. AOP_WRITEPAGE_ACTIVATE, and the
+ * page_mapped check below, must be avoided unless we're in reclaim.
+ */
+ if (!wbc->for_reclaim) {
+ set_page_dirty(page);
+ unlock_page(page);
+ return 0;
+ }
BUG_ON(page_mapped(page));
mapping = page->mapping;
--
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
