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From:   Khalid Masum <khalid.masum.92@gmail.com>
To:     linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        syzbot+14b0e8f3fd1612e35350@syzkaller.appspotmail.com
Cc:     khalid.masum.92@gmail.com, khalid.masum@aol.com
Subject: Re: [syzbot] KASAN: vmalloc-out-of-bounds Write in imageblit (2)
Date:   Sat, 30 Jul 2022 14:12:46 +0600
Message-Id: <20220730081246.4567-1-khalid.masum.92@gmail.com>
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Currently the if block's condition has an unhandled case, where the
result of ret might get greater than vc->vc_scr_end, and therefore
the corresponding handler in else block never gets executed. Which
eventually causes panic in fast_imageblit.

Add this extra check in the conditions to fix this breakage.

#syz-test: https://github.com/torvalds/linux.git e0dccc3b76fb 

---
 drivers/video/fbdev/core/fbcon.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 1a9aa12cf886..d026f3845b60 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2591,14 +2591,13 @@ static unsigned long fbcon_getxy(struct vc_data *vc, unsigned long pos,
 {
 	unsigned long ret;
 	int x, y;
+	unsigned long offset = (pos - vc->vc_origin) / 2;
+	x = offset % vc->vc_cols;
+	y = offset / vc->vc_cols;
+	ret = pos + (vc->vc_cols - x) * 2;
 
-	if (pos >= vc->vc_origin && pos < vc->vc_scr_end) {
-		unsigned long offset = (pos - vc->vc_origin) / 2;
-
-		x = offset % vc->vc_cols;
-		y = offset / vc->vc_cols;
-		ret = pos + (vc->vc_cols - x) * 2;
-	} else {
+	if (!pos >= vc->vc_origin || !pos < vc->vc_scr_end ||
+	                             !ret < vc->vc_scr_end) {
 		/* Should not happen */
 		x = y = 0;
 		ret = vc->vc_origin;
-- 
2.36.1