[PATCH AUTOSEL 5.4 227/330] mm/swapfile: fix data races in try_to_unuse()

Sasha Levin
2020-09-18T02:59:57Z

From: Qian Cai <cai@lca.pw>

[ Upstream commit 218209487c3da2f6d861b236c11226b6eca7b7b7 ]

si->inuse_pages could be accessed concurrently as noticed by KCSAN,

write to 0xffff98b00ebd04dc of 4 bytes by task 82262 on cpu 92:
swap_range_free+0xbe/0x230
swap_range_free at mm/swapfile.c:719
swapcache_free_entries+0x1be/0x250
free_swap_slot+0x1c8/0x220
__swap_entry_free.constprop.19+0xa3/0xb0
free_swap_and_cache+0x53/0xa0
unmap_page_range+0x7e0/0x1ce0
unmap_single_vma+0xcd/0x170
unmap_vmas+0x18b/0x220
exit_mmap+0xee/0x220
mmput+0xe7/0x240
do_exit+0x598/0xfd0
do_group_exit+0x8b/0x180
get_signal+0x293/0x13d0
do_signal+0x37/0x5d0
prepare_exit_to_usermode+0x1b7/0x2c0
ret_from_intr+0x32/0x42

read to 0xffff98b00ebd04dc of 4 bytes by task 82499 on cpu 46:
try_to_unuse+0x86b/0xc80
try_to_unuse at mm/swapfile.c:2185
__x64_sys_swapoff+0x372/0xd40
do_syscall_64+0x91/0xb05
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The plain reads in try_to_unuse() are outside si->lock critical section
which result in data races that could be dangerous to be used in a loop.
Fix them by adding READ_ONCE().

Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Marco Elver <elver@google.com>
Cc: Hugh Dickins <hughd@google.com>
Link: http://lkml.kernel.org/r/1582578903-29294-1-git-send-email-cai@lca.pw
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/swapfile.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 646fd0a8e3202..2f59495782dd4 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2132,7 +2132,7 @@ int try_to_unuse(unsigned int type, bool frontswap,
swp_entry_t entry;
unsigned int i;

- if (!si->inuse_pages)
+ if (!READ_ONCE(si->inuse_pages))
return 0;

if (!frontswap)
@@ -2148,7 +2148,7 @@ retry:

spin_lock(&mmlist_lock);
p = &init_mm.mmlist;
- while (si->inuse_pages &&
+ while (READ_ONCE(si->inuse_pages) &&
!signal_pending(current) &&
(p = p->next) != &init_mm.mmlist) {

@@ -2177,7 +2177,7 @@ retry:
mmput(prev_mm);

i = 0;
- while (si->inuse_pages &&
+ while (READ_ONCE(si->inuse_pages) &&
!signal_pending(current) &&
(i = find_next_to_unuse(si, i, frontswap)) != 0) {

@@ -2219,7 +2219,7 @@ retry:
* been preempted after get_swap_page(), temporarily hiding that swap.
* It's easy and robust (though cpu-intensive) just to keep retrying.
*/
- if (si->inuse_pages) {
+ if (READ_ONCE(si->inuse_pages)) {
if (!signal_pending(current))
goto retry;
retval = -EINTR;
--
2.25.1