From: Vinayak Menon vinmenon@codeaurora.org Subject: mm/page_io.c: do not free shared swap slots
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@codeauro... Fixes: aa8d22a11da9 ("mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference") Signed-off-by: Vinayak Menon vinmenon@codeaurora.org Suggested-by: Minchan Kim minchan@google.com Acked-by: Minchan Kim minchan@kernel.org Cc: Michal Hocko mhocko@suse.com Cc: Hugh Dickins hughd@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/page_io.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
--- a/mm/page_io.c~mm-do-not-free-shared-swap-slots +++ a/mm/page_io.c @@ -73,6 +73,7 @@ static void swap_slot_free_notify(struct { 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 @@ -104,11 +105,10 @@ static void swap_slot_free_notify(struct * 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(entry) == 1) { unsigned long offset;
- entry.val = page_private(page); offset = swp_offset(entry);
SetPageDirty(page); _