net/9p: fix race condition on rdma->state in trans_rdma.c

Overview

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

net/9p: fix race condition on rdma->state in trans_rdma.c

The rdma->state field is modified without holding req_lock in both recv_done() and p9_cm_event_handler(), while rdma_request() accesses the same field under the req_lock spinlock. This inconsistent locking creates a race condition:

• recv_done() running in softirq completion context sets rdma->state = P9_RDMA_FLUSHING without acquiring req_lock

• p9_cm_event_handler() modifies rdma->state at multiple points (ADDR_RESOLVED, ROUTE_RESOLVED, ESTABLISHED, CLOSED) without req_lock

• rdma_request() uses spin_lock_irqsave(&rdma->req_lock, flags) to protect the read-modify-write of rdma->state

The race can cause lost state transitions: recv_done() or the CM event handler could set state to FLUSHING/CLOSED while rdma_request() is concurrently checking or modifying state under the lock, leading to the FLUSHING transition being silently overwritten by CLOSING. This corrupts the connection state machine and can cause use-after-free on RDMA request objects during teardown.

Fix by adding req_lock protection to all rdma->state modifications in recv_done() and p9_cm_event_handler(), matching the pattern already used in rdma_request(). Use spin_lock_irqsave/spin_unlock_irqrestore in the CM event handler since it can race with recv_done() which runs in softirq context.

Tested with a kernel module that races two threads (simulating rdma_request and recv_done/CM handler) on rdma->state with proper locking: 5.5M+ FLUSHING writes over 27M iterations with 0 lost transitions.

Affected

• Linux >= 473c7dd1d7b59ff8f88a5154737e3eac78a96e5b < 4cee2b8766045059d5e0b8114837b4a8efe827ac
• Linux >= 473c7dd1d7b59ff8f88a5154737e3eac78a96e5b < 5424138848eb7d7d8a7196676e90a2cbf2142454
• Linux >= 473c7dd1d7b59ff8f88a5154737e3eac78a96e5b < 3970a19a80de530b801b6256354d4a529a9a2d6c
• Linux >= 473c7dd1d7b59ff8f88a5154737e3eac78a96e5b < 8aadc136d8e8d8fc95d7982d213cfe2234dfcf2b
• Linux >= 473c7dd1d7b59ff8f88a5154737e3eac78a96e5b < 151f8cf5b23d8a534d884432a82a6d54d5a61989
• Linux >= 473c7dd1d7b59ff8f88a5154737e3eac78a96e5b < 13bf9879b778b2f4b260b45bed18f31806120d1e
• Linux >= 473c7dd1d7b59ff8f88a5154737e3eac78a96e5b < ebbcbe5c0db215feecc17def06178da443f4eea6
• Linux >= 473c7dd1d7b59ff8f88a5154737e3eac78a96e5b < 7d54894a1ee265a72d70f7cae1da6cc774cccc71
• Linux 3479b3c35e82ed10aa0ca2ee9e78c4eded06ba62
• Linux c01ddaa54d7411e964ffd250c018d8469c5852f2
• Linux 9e69c673fe077b8dc491cd8406c9bdcb1f76dee2
• Linux e48e7e27e4dfd00c81e0381e7cee610cce021452
• Linux >= 4.4.185 < 4.5
• Linux >= 4.9.185 < 4.10
• Linux >= 4.14.132 < 4.15
• Linux >= 4.19.57 < 4.20
• Linux 4.20

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.