* [PATCH] mm: do not free shared swap slots
@ 2019-10-22 11:21 Vinayak Menon
2019-10-30 0:24 ` Minchan Kim
0 siblings, 1 reply; 5+ messages in thread
From: Vinayak Menon @ 2019-10-22 11:21 UTC (permalink / raw)
To: minchan, linux-mm; +Cc: kernel-team, Vinayak Menon, Michal Hocko
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(-)
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);
--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation
^ permalink raw reply related [flat|nested] 5+ messages in thread* Re: [PATCH] mm: do not free shared swap slots
2019-10-22 11:21 [PATCH] mm: do not free shared swap slots Vinayak Menon
@ 2019-10-30 0:24 ` Minchan Kim
2019-10-30 10:24 ` Michal Hocko
0 siblings, 1 reply; 5+ messages in thread
From: Minchan Kim @ 2019-10-30 0:24 UTC (permalink / raw)
To: Vinayak Menon; +Cc: linux-mm, kernel-team, Michal Hocko
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.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] mm: do not free shared swap slots
2019-10-30 0:24 ` Minchan Kim
@ 2019-10-30 10:24 ` Michal Hocko
2019-10-31 14:46 ` Minchan Kim
0 siblings, 1 reply; 5+ messages in thread
From: Michal Hocko @ 2019-10-30 10:24 UTC (permalink / raw)
To: Minchan Kim; +Cc: Vinayak Menon, linux-mm, kernel-team
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.
--
Michal Hocko
SUSE Labs
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] mm: do not free shared swap slots
2019-10-30 10:24 ` Michal Hocko
@ 2019-10-31 14:46 ` Minchan Kim
2019-10-31 17:15 ` Michal Hocko
0 siblings, 1 reply; 5+ messages in thread
From: Minchan Kim @ 2019-10-31 14:46 UTC (permalink / raw)
To: Michal Hocko; +Cc: Vinayak Menon, linux-mm, kernel-team
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
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] mm: do not free shared swap slots
2019-10-31 14:46 ` Minchan Kim
@ 2019-10-31 17:15 ` Michal Hocko
0 siblings, 0 replies; 5+ messages in thread
From: Michal Hocko @ 2019-10-31 17:15 UTC (permalink / raw)
To: Minchan Kim; +Cc: Vinayak Menon, linux-mm, kernel-team
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!
--
Michal Hocko
SUSE Labs
^ permalink raw reply [flat|nested] 5+ messages in thread
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2019-10-22 11:21 [PATCH] mm: do not free shared swap slots Vinayak Menon
2019-10-30 0:24 ` Minchan Kim
2019-10-30 10:24 ` Michal Hocko
2019-10-31 14:46 ` Minchan Kim
2019-10-31 17:15 ` Michal Hocko
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