[PATCH 5.10 11/19] vc_screen: dont clobber return value in vcs_read

Greg Kroah-Hartman
2023-03-01T18:10:04Z

From: Thomas Weißschuh <linux@weissschuh.net>

commit ae3419fbac845b4d3f3a9fae4cc80c68d82cdf6e upstream.

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.

[ 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: 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>
Link: https://lore.kernel.org/lkml/64981d94-d00c-4b31-9063-43ad0a384bde@t-8ch.de/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/tty/vt/vc_screen.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

--- 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
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