6.8-stable review patch. If anyone has any objections, please let me know. ------------------ From: Johannes Weiner <hannes@xxxxxxxxxxx> commit d5d39c707a4cf0bcc84680178677b97aa2cb2627 upstream. 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. Link: https://lkml.kernel.org/r/20240315095556.GC581298@xxxxxxxxxxx Fixes: cf264e1329fb ("cachestat: implement cachestat syscall") Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> Reported-by: Chengming Zhou <chengming.zhou@xxxxxxxxx> [Bug #1] Reported-by: Jann Horn <jannh@xxxxxxxxxx> [Bug #2] Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx> Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> [v6.5+] Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/filemap.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) --- a/mm/filemap.c +++ b/mm/filemap.c @@ -4153,7 +4153,23 @@ static void filemap_cachestat(struct add /* 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))
