scsi: sg: sg_new_write(): don't bother with access_ok

... just use copy_from_user().  We copy only SZ_SG_IO_HDR bytes, so that
would, strictly speaking, loosen the check.  However, for call chains via
->write() the caller has actually checked the entire range and SG_IO passes
exactly SZ_SG_IO_HDR for count.  So no visible behaviour changes happen if
we check only what we really need for copyin.

Link: https://lore.kernel.org/r/20191017193925.25539-5-viro@ZenIV.linux.org.ukSigned-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/sg.c

@@ -717,8 +717,6 @@ sg_new_write(Sg_fd *sfp, struct file *file, const char __user *buf,
 
 	if (count < SZ_SG_IO_HDR)
 		return -EINVAL;
-	if (!access_ok(buf, count))
-		return -EFAULT; /* protects following copy_from_user()s + get_user()s */
 
 	sfp->cmd_q = 1;	/* when sg_io_hdr seen, set command queuing on */
 	if (!(srp = sg_add_request(sfp))) {
@@ -728,7 +726,7 @@ sg_new_write(Sg_fd *sfp, struct file *file, const char __user *buf,
 	}
 	srp->sg_io_owned = sg_io_owned;
 	hp = &srp->header;
-	if (__copy_from_user(hp, buf, SZ_SG_IO_HDR)) {
+	if (copy_from_user(hp, buf, SZ_SG_IO_HDR)) {
 		sg_remove_request(sfp, srp);
 		return -EFAULT;
 	}