[CIFS] fix bad handling of EAGAIN error on kernel_recvmsg in cifs_demultiplex_thread

When kernel_recvmsg returns -EAGAIN or -ERESTARTSYS, then
cifs_demultiplex_thread sleeps for a bit and then tries the read again.
When it does this, it's not zeroing out the length and that throws off
the value of total_read. Fix it to zero out the length.

Can cause memory corruption:
If kernel_recvmsg returns an error and total_read is a large enough
value, then we'll end up going through the loop again. total_read will
be a bogus value, as will (pdu_length-total_read). When this happens we
end up calling kernel_recvmsg with a bogus value (possibly larger than
the current iov_len).

At that point, memcpy_toiovec can overrun iov. It will start walking
up the stack, casting other things that are there to struct iovecs
(since it assumes that it's been passed an array of them). Any pointer
on the stack at an address above the kvec is a candidate for corruption
here.

Many thanks to Ulrich Obergfell for pointing this out.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/CHANGES

@@ -10,7 +10,8 @@ opened, read and written as if they were files).  When 1st tree
 connect fails (e.g. due to signing negotiation failure) fix
 leak that causes cifsd not to stop and rmmod to fail to cleanup
 cifs_request_buffers pool. Fix problem with POSIX Open/Mkdir on
-bigendian architectures.
+bigendian architectures. Fix possible memory corruption when
+EAGAIN returned on kern_recvmsg.
 
 Version 1.50
 ------------

fs/cifs/connect.c

@@ -415,7 +415,10 @@ incomplete_rcv:
 			msleep(1); /* minimum sleep to prevent looping
 				allowing socket to clear and app threads to set
 				tcpStatus CifsNeedReconnect if server hung */
-			continue;
+			if (pdu_length < 4)
+				goto incomplete_rcv;
+			else
+				continue;
 		} else if (length <= 0) {
 			if (server->tcpStatus == CifsNew) {
 				cFYI(1, ("tcp session abend after SMBnegprot"));
@@ -543,6 +546,7 @@ incomplete_rcv:
 					      allowing socket to clear and app
 					      threads to set tcpStatus
 					      CifsNeedReconnect if server hung*/
+				length = 0;
 				continue;
 			} else if (length <= 0) {
 				cERROR(1, ("Received no data, expecting %d",