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authorHugh Dickins <hugh@veritas.com>2007-10-29 14:37:20 -0700
committerGreg Kroah-Hartman <gregkh@suse.de>2007-11-16 08:12:44 -0800
commitbba9d994eb41060c8a6e09207f659cf4e26e9384 (patch)
tree5a2d147cecc0a42972b789b6ffecbf9a070d79bf
parent59ddd4607313e7ee20b9ad36cd3b00f83068189a (diff)
fix tmpfs BUG and AOP_WRITEPAGE_ACTIVATE
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 <hugh@veritas.com> Cc: Erez Zadok <ezk@cs.sunysb.edu> Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
-rw-r--r--mm/shmem.c15
1 files changed, 15 insertions, 0 deletions
diff --git a/mm/shmem.c b/mm/shmem.c
index fcd19d323f9f..95558e498fae 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -916,6 +916,21 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
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;