vc_screen: don't clobber return value in vcs_read

stable inclusion
from stable-v5.10.171
commit 80653a6e6e287eb982be9aa9f60f94382b6767b5
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I7JRBO?from=project-issue

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=80653a6e6e287eb982be9aa9f60f94382b6767b5

--------------------------------

commit ae3419fb upstream.

Commit 226fae12 ("vc_screen: move load of struct vc_data pointer in
vcs_read() to avoid UAF") moved the call to vcs_vc() into the loop.

While doing this it also moved the unconditional assignment of

	ret = -ENXIO;

This unconditional assignment was valid outside the loop but within it
it clobbers the actual value of ret.

To avoid this only assign "ret = -ENXIO" when actually needed.

[ Also, the 'goto unlock_out" needs to be just a "break", so that it
  does the right thing when it exits on later iterations when partial
  success has happened - Linus ]
Reported-by: NStorm Dragon <stormdragon2976@gmail.com>
Link: https://lore.kernel.org/lkml/Y%2FKS6vdql2pIsCiI@hotmail.com/
Fixes: 226fae12 ("vc_screen: move load of struct vc_data pointer in vcs_read() to avoid UAF")
Signed-off-by: NThomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/lkml/64981d94-d00c-4b31-9063-43ad0a384bde@t-8ch.de/Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NLu Jialin <lujialin4@huawei.com>

drivers/tty/vt/vc_screen.c

@@ -403,10 +403,11 @@ vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 		unsigned int this_round, skip = 0;
 		int size;
 
-		ret = -ENXIO;
 		vc = vcs_vc(inode, &viewed);
-		if (!vc)
-			goto unlock_out;
+		if (!vc) {
+			ret = -ENXIO;
+			break;
+		}
 
 		/* Check whether we are above size each round,
 		 * as copy_to_user at the end of this loop