| Message ID | 1571743294-14285-1-git-send-email-vinmenon@codeaurora.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm: do not free shared swap slots | expand |
On Tue, Oct 22, 2019 at 04:51:34PM +0530, Vinayak Menon wrote: > 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. > > Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org> > Suggested-by: Minchan Kim <minchan@google.com> > Cc: Michal Hocko <mhocko@suse.com> Acked-by: Minchan Kim <minchan@kernel.org> -stable material from v4.15. Thanks.
On Tue 29-10-19 17:24:47, Minchan Kim wrote: > On Tue, Oct 22, 2019 at 04:51:34PM +0530, Vinayak Menon wrote: > > 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. > > > > Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org> > > Suggested-by: Minchan Kim <minchan@google.com> > > Cc: Michal Hocko <mhocko@suse.com> > Acked-by: Minchan Kim <minchan@kernel.org> > > -stable material from v4.15. Which commit does this fix? Fixes: tag would be highly appreciated.
On Wed, Oct 30, 2019 at 11:24:09AM +0100, Michal Hocko wrote: > On Tue 29-10-19 17:24:47, Minchan Kim wrote: > > On Tue, Oct 22, 2019 at 04:51:34PM +0530, Vinayak Menon wrote: > > > 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. > > > > > > Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org> > > > Suggested-by: Minchan Kim <minchan@google.com> > > > Cc: Michal Hocko <mhocko@suse.com> > > Acked-by: Minchan Kim <minchan@kernel.org> > > > > -stable material from v4.15. > > Which commit does this fix? Fixes: tag would be highly appreciated. Oops, I missed to mention it. Fixes: aa8d22a11da9 (mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference) > > -- > Michal Hocko > SUSE Labs
On Thu 31-10-19 07:46:08, Minchan Kim wrote: > On Wed, Oct 30, 2019 at 11:24:09AM +0100, Michal Hocko wrote: > > On Tue 29-10-19 17:24:47, Minchan Kim wrote: > > > On Tue, Oct 22, 2019 at 04:51:34PM +0530, Vinayak Menon wrote: > > > > 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. > > > > > > > > Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org> > > > > Suggested-by: Minchan Kim <minchan@google.com> > > > > Cc: Michal Hocko <mhocko@suse.com> > > > Acked-by: Minchan Kim <minchan@kernel.org> > > > > > > -stable material from v4.15. > > > > Which commit does this fix? Fixes: tag would be highly appreciated. > > Oops, I missed to mention it. > Fixes: aa8d22a11da9 (mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference) Thanks!
diff --git a/mm/page_io.c b/mm/page_io.c index 83db25a..3a198de 100644 --- a/mm/page_io.c +++ b/mm/page_io.c @@ -74,6 +74,7 @@ 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 @@ -105,11 +106,10 @@ 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(entry) == 1) { unsigned long offset; - entry.val = page_private(page); offset = swp_offset(entry); SetPageDirty(page);
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. Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org> Suggested-by: Minchan Kim <minchan@google.com> Cc: Michal Hocko <mhocko@suse.com> --- mm/page_io.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)