If an AFS cell that has an unreachable (eg. ENETUNREACH) server listed (VL
server or fileserver), an asynchronous probe to one of its addresses may
fail immediately because sendmsg() returns an error. When this happens, a
refcount underflow can happen if certain events hit a very small window.
The way this occurs is:
(1) There are two levels of "call" object, the afs_call and the
rxrpc_call. Each of them can be transitioned to a "completed" state
in the event of success or failure.
(2) Asynchronous afs_calls are self-referential whilst they are active to
prevent them from evaporating when they're not being processed. This
reference is disposed of when the afs_call is completed.
Note that an afs_call may only be completed once; once completed
completing it again will do nothing.
(3) When a call transmission is made, the app-side rxrpc code queues a Tx
buffer for the rxrpc I/O thread to transmit. The I/O thread invokes
sendmsg() to transmit it - and in the case of failure, it transitions
the rxrpc_call to the completed state.
(4) When an rxrpc_call is completed, the app layer is notified. In this
case, the app is kafs and it schedules a work item to process events
pertaining to an afs_call.
(5) When the afs_call event processor is run, it goes down through the
RPC-specific handler to afs_extract_data() to retrieve data from rxrpc
- and, in this case, it picks up the error from the rxrpc_call and
returns it.
The error is then propagated to the afs_call and that is completed
too. At this point the self-reference is released.
(6) If the rxrpc I/O thread manages to complete the rxrpc_call within the
window between rxrpc_send_data() queuing the request packet and
checking for call completion on the way out, then
rxrpc_kernel_send_data() will return the error from sendmsg() to the
app.
(7) Then afs_make_call() will see an error and will jump to the error
handling path which will attempt to clean up the afs_call.
(8) The problem comes when the error handling path in afs_make_call()
tries to unconditionally drop an async afs_call's self-reference.
This self-reference, however, may already have been dropped by
afs_extract_data() completing the afs_call
(9) The refcount underflows when we return to afs_do_probe_vlserver() and
that tries to drop its reference on the afs_call.
Fix this by making afs_make_call() attempt to complete the afs_call rather
than unconditionally putting it. That way, if afs_extract_data() manages
to complete the call first, afs_make_call() won't do anything.
The bug can be forced by making do_udp_sendmsg() return -ENETUNREACH and
sticking an msleep() in rxrpc_send_data() after the 'success:' label to
widen the race window.
The error message looks something like:
refcount_t: underflow; use-after-free.
WARNING: CPU: 3 PID: 720 at lib/refcount.c:28 refcount_warn_saturate+0xba/0x110
...
RIP: 0010:refcount_warn_saturate+0xba/0x110
...
afs_put_call+0x1dc/0x1f0 [kafs]
afs_fs_get_capabilities+0x8b/0xe0 [kafs]
afs_fs_probe_fileserver+0x188/0x1e0 [kafs]
afs_lookup_server+0x3bf/0x3f0 [kafs]
afs_alloc_server_list+0x130/0x2e0 [kafs]
afs_create_volume+0x162/0x400 [kafs]
afs_get_tree+0x266/0x410 [kafs]
vfs_get_tree+0x25/0xc0
fc_mount+0xe/0x40
afs_d_automount+0x1b3/0x390 [kafs]
__traverse_mounts+0x8f/0x210
step_into+0x340/0x760
path_openat+0x13a/0x1260
do_filp_open+0xaf/0x160
do_sys_openat2+0xaf/0x170
or something like:
refcount_t: underflow; use-after-free.
...
RIP: 0010:refcount_warn_saturate+0x99/0xda
...
afs_put_call+0x4a/0x175
afs_send_vl_probes+0x108/0x172
afs_select_vlserver+0xd6/0x311
afs_do_cell_detect_alias+0x5e/0x1e9
afs_cell_detect_alias+0x44/0x92
afs_validate_fc+0x9d/0x134
afs_get_tree+0x20/0x2e6
vfs_get_tree+0x1d/0xc9
fc_mount+0xe/0x33
afs_d_automount+0x48/0x9d
__traverse_mounts+0xe0/0x166
step_into+0x140/0x274
open_last_lookups+0x1c1/0x1df
path_openat+0x138/0x1c3
do_filp_open+0x55/0xb4
do_sys_openat2+0x6c/0xb6
Fixes: 34fa47612b ("afs: Fix race in async call refcounting")
Reported-by: Bill MacAllister <bill@ca-zephyr.org>
Closes: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1052304
Suggested-by: Jeffrey E Altman <jaltman@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/2633992.1702073229@warthog.procyon.org.uk/ # v1
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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