Description
In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: RFCOMM: take rfcomm_mutex for the deferred setup accept

rfcomm_sock_recvmsg() completes a deferred setup by calling
rfcomm_dlc_accept() without holding any RFCOMM lock:

if (test_and_clear_bit(RFCOMM_DEFER_SETUP, &d->flags)) {
rfcomm_dlc_accept(d);
return 0;
}

and rfcomm_dlc_accept() dereferences the session on its first line:

struct sock *sk = d->session->sock->sk;

Every other path that touches d->session runs under rfcomm_mutex:
rfcomm_dlc_open(), rfcomm_dlc_close(), rfcomm_dlc_exists(),
rfcomm_dlc_send_rpn(), and the RFCOMM thread through
rfcomm_process_sessions(). rfcomm_connect_ind() is even documented as
"called under rfcomm_lock()". This call site is the only one that skips
it.

The RFCOMM_DEFER_SETUP bit looks like it serialises the accept against
teardown, since __rfcomm_dlc_close() returns early when it wins the
test_and_clear. But rfcomm_recv_disc() forces the state first:

d->state = BT_CLOSED;
__rfcomm_dlc_close(d, err);

and the early return only covers BT_CONNECT, BT_CONFIG, BT_OPEN and
BT_CONNECT2. With the state already BT_CLOSED that switch does not
match, the bit is never consulted, and __rfcomm_dlc_close() falls
through to rfcomm_dlc_unlink(), which sets d->session = NULL.

So a remote DISC on a deferred dlc clears the session while leaving
RFCOMM_DEFER_SETUP set. The next recvmsg() then passes the
test_and_clear and dereferences a NULL session. No timing window is
needed: once the DISC has been processed, the dereference is
unconditional.

Give rfcomm_dlc_accept() the same shape as rfcomm_dlc_open() and
rfcomm_dlc_close(): an exported wrapper that takes rfcomm_mutex and
re-checks the session, around a __rfcomm_dlc_accept() that the two
in-core callers, which already hold the mutex, keep using.

Reproduced on a KASAN + PROVE_LOCKING kernel with a BR/EDR peer emulated
over /dev/vhci: the peer brings up an ACL link, opens L2CAP on the
RFCOMM PSM, starts a session, opens a dlc on a channel bound with
BT_DEFER_SETUP, and sends DISC after the socket is accepted. recv() on
the accepted socket then hits:

Oops: general protection fault
KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
RIP: 0010:rfcomm_dlc_accept+0x54/0x350
Call Trace:
rfcomm_sock_recvmsg+0x1cd/0x230
sock_recvmsg+0x166/0x1c0
__sys_recvfrom+0x20d/0x300

0x10 is the offset of sock in struct rfcomm_session. With this patch the
same run completes with recv() returning 0 and no report, and lockdep
stays quiet, confirming rfcomm_mutex is still taken before lock_sock on
this path as it is on the thread side.
Published: 2026-09-04
Score: n/a
EPSS: n/a
KEV: No
Impact: n/a
Action: n/a
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History

Fri, 04 Sep 2026 17:45:00 +0000

Type Values Removed Values Added
Weaknesses CWE-476

Fri, 04 Sep 2026 15:30:00 +0000

Type Values Removed Values Added
Description In the Linux kernel, the following vulnerability has been resolved: Bluetooth: RFCOMM: take rfcomm_mutex for the deferred setup accept rfcomm_sock_recvmsg() completes a deferred setup by calling rfcomm_dlc_accept() without holding any RFCOMM lock: if (test_and_clear_bit(RFCOMM_DEFER_SETUP, &d->flags)) { rfcomm_dlc_accept(d); return 0; } and rfcomm_dlc_accept() dereferences the session on its first line: struct sock *sk = d->session->sock->sk; Every other path that touches d->session runs under rfcomm_mutex: rfcomm_dlc_open(), rfcomm_dlc_close(), rfcomm_dlc_exists(), rfcomm_dlc_send_rpn(), and the RFCOMM thread through rfcomm_process_sessions(). rfcomm_connect_ind() is even documented as "called under rfcomm_lock()". This call site is the only one that skips it. The RFCOMM_DEFER_SETUP bit looks like it serialises the accept against teardown, since __rfcomm_dlc_close() returns early when it wins the test_and_clear. But rfcomm_recv_disc() forces the state first: d->state = BT_CLOSED; __rfcomm_dlc_close(d, err); and the early return only covers BT_CONNECT, BT_CONFIG, BT_OPEN and BT_CONNECT2. With the state already BT_CLOSED that switch does not match, the bit is never consulted, and __rfcomm_dlc_close() falls through to rfcomm_dlc_unlink(), which sets d->session = NULL. So a remote DISC on a deferred dlc clears the session while leaving RFCOMM_DEFER_SETUP set. The next recvmsg() then passes the test_and_clear and dereferences a NULL session. No timing window is needed: once the DISC has been processed, the dereference is unconditional. Give rfcomm_dlc_accept() the same shape as rfcomm_dlc_open() and rfcomm_dlc_close(): an exported wrapper that takes rfcomm_mutex and re-checks the session, around a __rfcomm_dlc_accept() that the two in-core callers, which already hold the mutex, keep using. Reproduced on a KASAN + PROVE_LOCKING kernel with a BR/EDR peer emulated over /dev/vhci: the peer brings up an ACL link, opens L2CAP on the RFCOMM PSM, starts a session, opens a dlc on a channel bound with BT_DEFER_SETUP, and sends DISC after the socket is accepted. recv() on the accepted socket then hits: Oops: general protection fault KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017] RIP: 0010:rfcomm_dlc_accept+0x54/0x350 Call Trace: rfcomm_sock_recvmsg+0x1cd/0x230 sock_recvmsg+0x166/0x1c0 __sys_recvfrom+0x20d/0x300 0x10 is the offset of sock in struct rfcomm_session. With this patch the same run completes with recv() returning 0 and no report, and lockdep stays quiet, confirming rfcomm_mutex is still taken before lock_sock on this path as it is on the thread side.
Title Bluetooth: RFCOMM: take rfcomm_mutex for the deferred setup accept
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References

Subscriptions

Linux Linux Kernel
cve-icon MITRE

Status: PUBLISHED

Assigner: Linux

Published:

Updated: 2026-09-04T15:13:40.309Z

Reserved: 2026-08-26T14:34:25.795Z

Link: CVE-2026-80819

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Received

Published: 2026-09-04T16:18:09.583

Modified: 2026-09-04T16:18:09.583

Link: CVE-2026-80819

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-09-04T17:30:17Z

Weaknesses