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. thanks, greg k-h > ------------------ 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); >
