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Commit 006360ec authored by Vinayak Menon's avatar Vinayak Menon Committed by Greg Kroah-Hartman
Browse files

mm/page_io.c: do not free shared swap slots 

[ Upstream commit 5df373e95689b9519b8557da7c5bd0db0856d776 ]

The following race is observed due to which a processes faulting on a
swap entry, finds the page neither in swapcache nor swap.  This causes
zram to give a zero filled page that gets mapped to the process,
resulting in a user space crash later.

Consider parent and child processes Pa and Pb sharing the same swap slot
with swap_count 2.  Swap is on zram with SWP_SYNCHRONOUS_IO set.
Virtual address 'VA' of Pa and Pb points to the shared swap entry.

Pa                                       Pb

fault on VA                              fault on VA
do_swap_page                             do_swap_page
lookup_swap_cache fails                  lookup_swap_cache fails
                                         Pb scheduled out
swapin_readahead (deletes zram entry)
swap_free (makes swap_count 1)
                                         Pb scheduled in
                                         swap_readpage (swap_count == 1)
                                         Takes SWP_SYNCHRONOUS_IO path
                                         zram enrty absent
                                         zram gives a zero filled page

Fix this by making sure that swap slot is freed only when swap count
drops down to one.

Link: http://lkml.kernel.org/r/1571743294-14285-1-git-send-email-vinmenon@codeaurora.org


Fixes: aa8d22a1 ("mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference")
Signed-off-by: default avatarVinayak Menon <vinmenon@codeaurora.org>
Suggested-by: default avatarMinchan Kim <minchan@google.com>
Acked-by: default avatarMinchan Kim <minchan@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent 16a300fb

mm/page_io.c

+4 −3
Original line number Diff line number Diff line
@@ -76,6 +76,7 @@ static void swap_slot_free_notify(struct page *page) 
{

	struct swap_info_struct *sis;

	struct gendisk *disk;

	swp_entry_t entry;



	/*

	 * There is no guarantee that the page is in swap cache - the software
@@ -107,11 +108,11 @@ static void swap_slot_free_notify(struct page *page) 
	 * we again wish to reclaim it.

	 */

	disk = sis->bdev->bd_disk;

	if (disk->fops->swap_slot_free_notify) {

		swp_entry_t entry;

	entry.val = page_private(page);

	if (disk->fops->swap_slot_free_notify &&

			__swap_count(sis, entry) == 1) {

		unsigned long offset;



		entry.val = page_private(page);

		offset = swp_offset(entry);



		SetPageDirty(page);