rds: crash at rds_ib_inc_copy_to_user+104 due to NULL ptr reference

The customer hit this crash few times.

PID: 31556  TASK: ffff880f823caa00  CPU: 1   COMMAND: "cellsrv"
 #0 [ffff880f823db850] machine_kexec at ffffffff8105d93c
 #1 [ffff880f823db8b0] crash_kexec at ffffffff811103b3
 #2 [ffff880f823db980] oops_end at ffffffff8101a788
 #3 [ffff880f823db9b0] no_context at ffffffff8106b9cf
 #4 [ffff880f823dba20] __bad_area_nosemaphore at ffffffff8106bc9d
 #5 [ffff880f823dba70] bad_area at ffffffff8106be97
 #6 [ffff880f823dbaa0] __do_page_fault at ffffffff8106c71e
 #7 [ffff880f823dbb00] do_page_fault at ffffffff8106c81f
 #8 [ffff880f823dbb40] page_fault at ffffffff816b5a9f
    [exception RIP: rds_ib_inc_copy_to_user+104]
    RIP: ffffffffa04607b8  RSP: ffff880f823dbbf8  RFLAGS: 00010287
    RAX: 0000000000000340  RBX: 0000000000001000  RCX: 0000000000004000
    RDX: 0000000000001000  RSI: ffff88176cea2000  RDI: ffff8817d291f520
    RBP: ffff880f823dbc48   R8: 0000000000001340   R9: 0000000000001000
    R10: 0000000000001200  R11: ffff880f823dc000  R12: ffff880f823dbed0
    R13: 0000000000000000  R14: 0000000000000000  R15: 0000000000001000
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 #9 [ffff880f823dbc50] rds_recvmsg at ffffffffa041d837 [rds]

int rds_ib_inc_copy_to_user(struct rds_incoming *inc, struct iov_iter *to)
...
...
        ibinc = container_of(inc, struct rds_ib_incoming, ii_inc);
        frag = list_entry(ibinc->ii_frags.next, struct rds_page_frag, f_item);
        len = be32_to_cpu(inc->i_hdr.h_len);
        sg = frag->f_sg;

        while (iov_iter_count(to) && copied < len) {
                to_copy = min_t(unsigned long, iov_iter_count(to),
                                sg->length - frag_off);
                ...

sg is NULL and it crashes accessing sg->length above.

The cause looks like is due to ic->i_frag_sz returning incorrect value.
16KB when 4KB was expected.

                if (copied % ic->i_frag_sz == 0) {
                        frag = list_entry(frag->f_item.next,
                                          struct rds_page_frag, f_item);
                        frag_off = 0;
                        sg = frag->f_sg;
                }

The other end is using 4KB RDS fragsize (Solaris Super Cluster).
This end is UEK4 (4.1.12-94.8.4.el6uek.x86_64).

The message being copied arrived over 4KB RDS frag size connection.
But during the above check ic->i_frag_sz is 16KB.
This can happen during a reconnect at the connection setup phase.
We start off with ic->i_frag_sz as 16KB. Then settle down at 4KB.

Failing this check
  if (copied % ic->i_frag_sz == 0) {
can result in sg not getting set correctly.

Say, "copied" = 4KB but ic->i_frag_sz is 16KB when it should be 4KB.

During race condition with a reconnect, ic->i_frag_sz can be 16KB
even though once the connection is set up it settled down to 4KB.
It can change from 4KB to 16KB and back to 4KB during connection setup
due to reconnect.

We started seeing this crash after bug 26848749.
But prior to that the same scenario could result in data copied to user
from incorrect "sg" resulting in data corruption.

Orabug: 28748049

Reviewed-by: Rama Nichanamatlu <[email protected]>
Signed-off-by: Venkat Venkatsubra <[email protected]>

Author: venkatxvenkatsubra

net/rds/ib_recv.c

@@ -883,6 +883,12 @@ int rds_ib_inc_copy_to_user(struct rds_incoming *inc, struct iov_iter *to)
 			frag_off = 0;
 			sg = frag->f_sg;
 		}
+		if ((sg == NULL) && (copied < len)) {
+			frag = list_entry(frag->f_item.next,
+					  struct rds_page_frag, f_item);
+			frag_off = 0;
+			sg = frag->f_sg;
+		}
 
 	}