From: Hugh Dickins Date: Mon, 29 Oct 2007 21:37:20 +0000 (-0700) Subject: fix tmpfs BUG and AOP_WRITEPAGE_ACTIVATE X-Git-Tag: v2.6.24-rc2~84 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=487e9bf25cbae11b131d6a14bdbb3a6a77380837;p=linux-2.6 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 (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 Cc: Erez Zadok Cc: Reviewed-by: Pekka Enberg Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/mm/shmem.c b/mm/shmem.c index 404e53bb21..253d205914 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -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;