net/smc: avoid NULL deref of conn->lnk in smc_msg_event tracepoint

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

net/smc: avoid NULL deref of conn->lnk in smc_msg_event tracepoint

The smc_msg_event tracepoint class, shared by smc_tx_sendmsg and

smc_rx_recvmsg, unconditionally dereferences smc->conn.lnk:

__string(name, smc->conn.lnk->ibname)

conn->lnk is only set for SMC-R; for SMC-D it is NULL. Other code on

these paths already handles this (e.g. !conn->lnk in

SMC_STAT_RMB_TX_SIZE_SMALL()). With the tracepoint enabled, the first

sendmsg()/recvmsg() on an SMC-D socket crashes:

Oops: general protection fault, probably for non-canonical address

KASAN: null-ptr-deref in range [...]

RIP: 0010:strlen+0x1e/0xa0

Call Trace:

trace_event_raw_event_smc_msg_event (net/smc/smc_tracepoint.h:44)

smc_rx_recvmsg (net/smc/smc_rx.c:515)

smc_recvmsg (net/smc/af_smc.c:2859)

__sys_recvfrom (net/socket.c:2315)

__x64_sys_recvfrom (net/socket.c:2326)

do_syscall_64

The faulting address 0x3e0 is offsetof(struct smc_link, ibname),

confirming the NULL ->lnk deref. Enabling the tracepoint requires

root, but the trigger itself is unprivileged: socket(AF_SMC, ...) has

no capability check, and SMC-D negotiation needs no admin step on

s390 or on x86 with the loopback ISM device loaded.

Log an empty device name for SMC-D instead of dereferencing NULL.