[PATCH] vc_screen: don't clobber return value in vcs_read

[linux@weissschuh.net]

From: Thomas Weißschuh <linux@weissschuh.net>

Commit 226fae124b2d
("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.

Reported-by: Storm Dragon <stormdragon2976@gmail.com>
Link: https://lore.kernel.org/lkml/Y%2FKS6vdql2pIsCiI@hotmail.com/
Fixes: 226fae124b2d ("vc_screen: move load of struct vc_data pointer in vcs_read() to avoid UAF")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

---

@Storm Could you validate this patch?
---
 drivers/tty/vt/vc_screen.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
index f566eb1839dc..2ef519a40a87 100644
--- a/drivers/tty/vt/vc_screen.c
+++ b/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)
+		if (!vc) {
+			ret = -ENXIO;
 			goto unlock_out;
+		}
 
 		/* Check whether we are above size each round,
 		 * as copy_to_user at the end of this loop

base-commit: c9c3395d5e3dcc6daee66c6908354d47bf98cb0c
-- 
2.39.2