Commit 487e9bf2 authored Oct 29, 2007 by Hugh Dickins Committed by Linus Torvalds Oct 30, 2007

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: Hugh Dickins <hugh@veritas.com>
Cc: Erez Zadok <ezk@cs.sunysb.edu>
Cc: <stable@kernel.org>
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>

parent 8bca44bb

mm/shmem.c

+15 −0

Original line number	Diff line number	Diff line
		@@ -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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