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authorDave Wysochanski <dwysocha@redhat.com>2022-12-07 14:49:15 +0100
committerLinus Torvalds <torvalds@linux-foundation.org>2022-12-07 20:49:18 +0100
commitb5b52de3214a29911f949459a79f6640969b5487 (patch)
tree5588d71fd73c31833b67d712bea8db119151b72d /fs/fscache
parentmedia: videobuf2-core: take mmap_lock in vb2_get_unmapped_area() (diff)
downloadlinux-b5b52de3214a29911f949459a79f6640969b5487.tar.xz
linux-b5b52de3214a29911f949459a79f6640969b5487.zip
fscache: Fix oops due to race with cookie_lru and use_cookie
If a cookie expires from the LRU and the LRU_DISCARD flag is set, but the state machine has not run yet, it's possible another thread can call fscache_use_cookie and begin to use it. When the cookie_worker finally runs, it will see the LRU_DISCARD flag set, transition the cookie->state to LRU_DISCARDING, which will then withdraw the cookie. Once the cookie is withdrawn the object is removed the below oops will occur because the object associated with the cookie is now NULL. Fix the oops by clearing the LRU_DISCARD bit if another thread uses the cookie before the cookie_worker runs. BUG: kernel NULL pointer dereference, address: 0000000000000008 ... CPU: 31 PID: 44773 Comm: kworker/u130:1 Tainted: G E 6.0.0-5.dneg.x86_64 #1 Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 08/26/2022 Workqueue: events_unbound netfs_rreq_write_to_cache_work [netfs] RIP: 0010:cachefiles_prepare_write+0x28/0x90 [cachefiles] ... Call Trace: netfs_rreq_write_to_cache_work+0x11c/0x320 [netfs] process_one_work+0x217/0x3e0 worker_thread+0x4a/0x3b0 kthread+0xd6/0x100 Fixes: 12bb21a29c19 ("fscache: Implement cookie user counting and resource pinning") Reported-by: Daire Byrne <daire.byrne@gmail.com> Signed-off-by: Dave Wysochanski <dwysocha@redhat.com> Signed-off-by: David Howells <dhowells@redhat.com> Tested-by: Daire Byrne <daire@dneg.com> Link: https://lore.kernel.org/r/20221117115023.1350181-1-dwysocha@redhat.com/ # v1 Link: https://lore.kernel.org/r/20221117142915.1366990-1-dwysocha@redhat.com/ # v2 Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'fs/fscache')
-rw-r--r--fs/fscache/cookie.c8
1 files changed, 8 insertions, 0 deletions
diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index 451d8a077e12..bce2492186d0 100644
--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -605,6 +605,14 @@ again:
set_bit(FSCACHE_COOKIE_DO_PREP_TO_WRITE, &cookie->flags);
queue = true;
}
+ /*
+ * We could race with cookie_lru which may set LRU_DISCARD bit
+ * but has yet to run the cookie state machine. If this happens
+ * and another thread tries to use the cookie, clear LRU_DISCARD
+ * so we don't end up withdrawing the cookie while in use.
+ */
+ if (test_and_clear_bit(FSCACHE_COOKIE_DO_LRU_DISCARD, &cookie->flags))
+ fscache_see_cookie(cookie, fscache_cookie_see_lru_discard_clear);
break;
case FSCACHE_COOKIE_STATE_FAILED: