Commit 487e9bf2 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds

fix tmpfs BUG and AOP_WRITEPAGE_ACTIVATE

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 (e4230030 - 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: default avatarHugh Dickins <hugh@veritas.com>
Cc: Erez Zadok <ezk@cs.sunysb.edu>
Cc: <stable@kernel.org>
Reviewed-by: default avatarPekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8bca44bb
......@@ -915,6 +915,21 @@ 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;
......
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