Techwizz/kernel-fxtec-pro1x
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity

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

Browse source 
[ 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: aa8d22a11d ("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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Vinayak Menon 2019-11-15 17:35:00 -08:00 committed by Greg Kroah-Hartman
parent 16a300fb1d
commit 006360ec33
1 changed files with 4 additions and 3 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

7
mm/page_io.c
View file

@ -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);