linux-kernel - [PATCH] vc_screen: break from vcs_read() while loop if vcs

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Message-Id: <1675704844-17228-1-git-send-email-george.kennedy@oracle.com>
Date:   Mon,  6 Feb 2023 12:34:04 -0500
From:   George Kennedy <george.kennedy@...cle.com>
To:     gregkh@...uxfoundation.org, jslaby@...e.cz,
        torvalds@...ux-foundation.org
Cc:     george.kennedy@...cle.com, sfr@...b.auug.org.au,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-serial@...r.kernel.org
Subject: [PATCH] vc_screen: break from vcs_read() while loop if vcs_vc() returns NULL

If vcs_vc() returns NULL in vcs_read(), break if partial read,
else if no reads have been done, go to unlock_out and return ENXIO.

Fixes: 226fae124b2d ("vc_screen: move load of struct vc_data pointer in vcs_read() to avoid UAF")
Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: George Kennedy <george.kennedy@...cle.com>
---
 drivers/tty/vt/vc_screen.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
index f566eb1839dc..29288401cf9e 100644
--- a/drivers/tty/vt/vc_screen.c
+++ b/drivers/tty/vt/vc_screen.c
@@ -403,10 +403,13 @@ 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) {
+			if (read)
+				break;
+			ret = -ENXIO;
 			goto unlock_out;
+		}
 
 		/* Check whether we are above size each round,
 		 * as copy_to_user at the end of this loop
-- 
2.31.1