| Message ID | 20240315095556.GC581298@cmpxchg.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: cachestat: fix two shmem bugs | expand |
On 2024/3/15 17:55, Johannes Weiner wrote: > When cachestat on shmem races with swapping and invalidation, there > are two possible bugs: > > 1) A swapin error can have resulted in a poisoned swap entry in the > shmem inode's xarray. Calling get_shadow_from_swap_cache() on it > will result in an out-of-bounds access to swapper_spaces[]. > > Validate the entry with non_swap_entry() before going further. > > 2) When we find a valid swap entry in the shmem's inode, the shadow > entry in the swapcache might not exist yet: swap IO is still in > progress and we're before __remove_mapping; swapin, invalidation, > or swapoff have removed the shadow from swapcache after we saw the > shmem swap entry. > > This will send a NULL to workingset_test_recent(). The latter > purely operates on pointer bits, so it won't crash - node 0, memcg > ID 0, eviction timestamp 0, etc. are all valid inputs - but it's a > bogus test. In theory that could result in a false "recently > evicted" count. > > Such a false positive wouldn't be the end of the world. But for > code clarity and (future) robustness, be explicit about this case. > > Bail on get_shadow_from_swap_cache() returning NULL. > > Fixes: cf264e1329fb ("cachestat: implement cachestat syscall") > Cc: stable@vger.kernel.org [v6.5+] > Reported-by: Chengming Zhou <chengming.zhou@linux.dev> [Bug #1] > Reported-by: Jann Horn <jannh@google.com> [Bug #2] > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Looks good to me. Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev> Thanks. > --- > mm/filemap.c | 16 ++++++++++++++++ > 1 file changed, 16 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index 222adac7c9c5..0aa91bf6c1f7 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -4198,7 +4198,23 @@ static void filemap_cachestat(struct address_space *mapping, > /* shmem file - in swap cache */ > swp_entry_t swp = radix_to_swp_entry(folio); > > + /* swapin error results in poisoned entry */ > + if (non_swap_entry(swp)) > + goto resched; > + > + /* > + * Getting a swap entry from the shmem > + * inode means we beat > + * shmem_unuse(). rcu_read_lock() > + * ensures swapoff waits for us before > + * freeing the swapper space. However, > + * we can race with swapping and > + * invalidation, so there might not be > + * a shadow in the swapcache (yet). > + */ > shadow = get_shadow_from_swap_cache(swp); > + if (!shadow) > + goto resched; > } > #endif > if (workingset_test_recent(shadow, true, &workingset))
On Fri, Mar 15, 2024 at 4:55 PM Johannes Weiner <hannes@cmpxchg.org> wrote: > > When cachestat on shmem races with swapping and invalidation, there > are two possible bugs: > > 1) A swapin error can have resulted in a poisoned swap entry in the > shmem inode's xarray. Calling get_shadow_from_swap_cache() on it > will result in an out-of-bounds access to swapper_spaces[]. > > Validate the entry with non_swap_entry() before going further. > > 2) When we find a valid swap entry in the shmem's inode, the shadow > entry in the swapcache might not exist yet: swap IO is still in > progress and we're before __remove_mapping; swapin, invalidation, > or swapoff have removed the shadow from swapcache after we saw the > shmem swap entry. > > This will send a NULL to workingset_test_recent(). The latter > purely operates on pointer bits, so it won't crash - node 0, memcg > ID 0, eviction timestamp 0, etc. are all valid inputs - but it's a > bogus test. In theory that could result in a false "recently > evicted" count. > > Such a false positive wouldn't be the end of the world. But for > code clarity and (future) robustness, be explicit about this case. > > Bail on get_shadow_from_swap_cache() returning NULL. > > Fixes: cf264e1329fb ("cachestat: implement cachestat syscall") > Cc: stable@vger.kernel.org [v6.5+] > Reported-by: Chengming Zhou <chengming.zhou@linux.dev> [Bug #1] > Reported-by: Jann Horn <jannh@google.com> [Bug #2] > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Nice catch! Thanks for the report, Chengming and Jann, and thanks for the fix, Johannes! Reviewed-by: Nhat Pham <nphamcs@gmail.com> > --- > mm/filemap.c | 16 ++++++++++++++++ > 1 file changed, 16 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index 222adac7c9c5..0aa91bf6c1f7 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -4198,7 +4198,23 @@ static void filemap_cachestat(struct address_space *mapping, > /* shmem file - in swap cache */ > swp_entry_t swp = radix_to_swp_entry(folio); > > + /* swapin error results in poisoned entry */ > + if (non_swap_entry(swp)) > + goto resched; > + > + /* > + * Getting a swap entry from the shmem > + * inode means we beat > + * shmem_unuse(). rcu_read_lock() > + * ensures swapoff waits for us before > + * freeing the swapper space. However, > + * we can race with swapping and > + * invalidation, so there might not be > + * a shadow in the swapcache (yet). > + */ > shadow = get_shadow_from_swap_cache(swp); > + if (!shadow) > + goto resched; > } > #endif > if (workingset_test_recent(shadow, true, &workingset)) > -- > 2.44.0 >
diff --git a/mm/filemap.c b/mm/filemap.c index 222adac7c9c5..0aa91bf6c1f7 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -4198,7 +4198,23 @@ static void filemap_cachestat(struct address_space *mapping, /* shmem file - in swap cache */ swp_entry_t swp = radix_to_swp_entry(folio); + /* swapin error results in poisoned entry */ + if (non_swap_entry(swp)) + goto resched; + + /* + * Getting a swap entry from the shmem + * inode means we beat + * shmem_unuse(). rcu_read_lock() + * ensures swapoff waits for us before + * freeing the swapper space. However, + * we can race with swapping and + * invalidation, so there might not be + * a shadow in the swapcache (yet). + */ shadow = get_shadow_from_swap_cache(swp); + if (!shadow) + goto resched; } #endif if (workingset_test_recent(shadow, true, &workingset))
When cachestat on shmem races with swapping and invalidation, there are two possible bugs: 1) A swapin error can have resulted in a poisoned swap entry in the shmem inode's xarray. Calling get_shadow_from_swap_cache() on it will result in an out-of-bounds access to swapper_spaces[]. Validate the entry with non_swap_entry() before going further. 2) When we find a valid swap entry in the shmem's inode, the shadow entry in the swapcache might not exist yet: swap IO is still in progress and we're before __remove_mapping; swapin, invalidation, or swapoff have removed the shadow from swapcache after we saw the shmem swap entry. This will send a NULL to workingset_test_recent(). The latter purely operates on pointer bits, so it won't crash - node 0, memcg ID 0, eviction timestamp 0, etc. are all valid inputs - but it's a bogus test. In theory that could result in a false "recently evicted" count. Such a false positive wouldn't be the end of the world. But for code clarity and (future) robustness, be explicit about this case. Bail on get_shadow_from_swap_cache() returning NULL. Fixes: cf264e1329fb ("cachestat: implement cachestat syscall") Cc: stable@vger.kernel.org [v6.5+] Reported-by: Chengming Zhou <chengming.zhou@linux.dev> [Bug #1] Reported-by: Jann Horn <jannh@google.com> [Bug #2] Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> --- mm/filemap.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+)