On 11/18/2019 10:10 PM, Greg KH wrote: > On Mon, Nov 18, 2019 at 05:37:16PM +0100, gregkh@xxxxxxxxxxxxxxxxxxx wrote: >> The patch below does not apply to the 4.19-stable tree. >> If someone wants it applied there, or to any other stable or longterm >> tree, then please email the backport, including the original git commit >> id to <stable@xxxxxxxxxxxxxxx>. > Note, this applies, but just breaks the build, so it needs a backport > for 4.19 if people want to see it there. The version below fixes the build on 4.19. > > thanks, > > greg k-h >From 3745cc4bb5cddbc1058889e4f779492060d5e550 Mon Sep 17 00:00:00 2001 From: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx> Date: Fri, 15 Nov 2019 17:35:00 -0800 Subject: [PATCH] mm/page_io.c: do not free shared swap slots commit 5df373e95689b9519b8557da7c5bd0db0856d776 upstream. 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@xxxxxxxxxxxxxx Fixes: aa8d22a11da9 ("mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference") Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx> Suggested-by: Minchan Kim <minchan@xxxxxxxxxx> Acked-by: Minchan Kim <minchan@xxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/page_io.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/mm/page_io.c b/mm/page_io.c index abc1466..e763047 100644 --- a/mm/page_io.c +++ b/mm/page_io.c @@ -77,6 +77,7 @@ 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 @@ -108,11 +109,11 @@ 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); -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation > >> ------------------ original commit in Linus's tree ------------------ >> >> >From 5df373e95689b9519b8557da7c5bd0db0856d776 Mon Sep 17 00:00:00 2001 >> From: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx> >> Date: Fri, 15 Nov 2019 17:35:00 -0800 >> Subject: [PATCH] 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@xxxxxxxxxxxxxx >> Fixes: aa8d22a11da9 ("mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference") >> Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx> >> Suggested-by: Minchan Kim <minchan@xxxxxxxxxx> >> Acked-by: Minchan Kim <minchan@xxxxxxxxxx> >> Cc: Michal Hocko <mhocko@xxxxxxxx> >> Cc: Hugh Dickins <hughd@xxxxxxxxxx> >> Cc: <stable@xxxxxxxxxxxxxxx> >> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> >> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> >> >> diff --git a/mm/page_io.c b/mm/page_io.c >> index 24ee600f9131..60a66a58b9bf 100644 >> --- a/mm/page_io.c >> +++ b/mm/page_io.c >> @@ -73,6 +73,7 @@ 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 >> @@ -104,11 +105,10 @@ 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); >>
