• Tony Battersby's avatar
    [SCSI] iscsi_tcp: fix potential lockup with write commands · 505f76b3
    Tony Battersby authored
    There is a race condition in iscsi_tcp.c that may cause it to forget
    that it received a R2T from the target.  This race may cause a data-out
    command (such as a write) to lock up.  The race occurs here:
    
    static int
    iscsi_send_unsol_pdu(struct iscsi_conn *conn, struct iscsi_cmd_task *ctask)
    {
    	struct iscsi_tcp_cmd_task *tcp_ctask = ctask->dd_data;
    	int rc;
    
    	if (tcp_ctask->xmstate & XMSTATE_UNS_HDR) {
    		BUG_ON(!ctask->unsol_count);
    		tcp_ctask->xmstate &= ~XMSTATE_UNS_HDR; <---- RACE
    		...
    
    static int
    iscsi_r2t_rsp(struct iscsi_conn *conn, struct iscsi_cmd_task *ctask)
    {
    	...
    	tcp_ctask->xmstate |= XMSTATE_SOL_HDR_INIT; <---- RACE
    	...
    
    While iscsi_xmitworker() (called from scsi_queue_work()) is preparing to
    send unsolicited data, iscsi_tcp_data_recv() (called from
    tcp_read_sock()) interrupts it upon receipt of a R2T from the target.
    Both contexts do read-modify-write of tcp_ctask->xmstate.  Usually, gcc
    on x86 will make &= and |= atomic on UP (not guaranteed of course), but
    in this case iscsi_send_unsol_pdu() reads the value of xmstate before
    clearing the bit, which causes gcc to read xmstate into a CPU register,
    test it, clear the bit, and then store it back to memory.  If the recv
    interrupt happens during this sequence, then the XMSTATE_SOL_HDR_INIT
    bit set by the recv interrupt will be lost, and the R2T will be
    forgotten.
    
    The patch below (against 2.6.24-rc1) converts accesses of xmstate to use
    set_bit, clear_bit, and test_bit instead of |= and &=.  I have tested
    this patch and verified that it fixes the problem.  Another possible
    approach would be to hold a lock during most of the rx/tx setup and
    post-processing, and drop the lock only for the actual rx/tx.
    Signed-off-by: default avatarTony Battersby <tonyb@cybernetics.com>
    Signed-off-by: default avatarMike Christie <michaelc@cs.wisc.edu>
    Signed-off-by: default avatarJames Bottomley <James.Bottomley@HansenPartnership.com>
    505f76b3
iscsi_tcp.h 5.06 KB