• Hugh Dickins's avatar
    fix tmpfs BUG and AOP_WRITEPAGE_ACTIVATE · bba9d994
    Hugh Dickins authored
    patch 487e9bf2 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 (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>
    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>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@suse.de>
    bba9d994
shmem.c 65 KB