[PATCH] fbdev: bitblit: bound-check glyph index in bit_putcs*

[Junjie Cao]

bit_putcs_aligned()/unaligned() derived the glyph pointer from the
character value masked by 0xff/0x1ff, which may exceed the actual font's
glyph count and read past the end of the built-in font array.
Clamp the index to the actual glyph count before computing the address.

This fixes a global out-of-bounds read reported by syzbot.

Reported-by: syzbot+793cf822d213be1a74f2@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=793cf822d213be1a74f2
Tested-by: syzbot+793cf822d213be1a74f2@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Junjie Cao <junjie.cao@xxxxxxxxx>
---
 drivers/video/fbdev/core/bitblit.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/video/fbdev/core/bitblit.c b/drivers/video/fbdev/core/bitblit.c
index a9ec7f488522..710883831826 100644
--- a/drivers/video/fbdev/core/bitblit.c
+++ b/drivers/video/fbdev/core/bitblit.c
@@ -79,12 +79,15 @@ static inline void bit_putcs_aligned(struct vc_data *vc, struct fb_info *info,
 				     struct fb_image *image, u8 *buf, u8 *dst)
 {
 	u16 charmask = vc->vc_hi_font_mask ? 0x1ff : 0xff;
+	unsigned int charcnt = vc->vc_font.charcount;
 	u32 idx = vc->vc_font.width >> 3;
 	u8 *src;
 
 	while (cnt--) {
-		src = vc->vc_font.data + (scr_readw(s++)&
-					  charmask)*cellsize;
+		u16 ch = scr_readw(s++) & charmask;
+	if (ch >= charcnt)
+		ch = 0;
+	src = vc->vc_font.data + (unsigned int)ch * cellsize;
 
 		if (attr) {
 			update_attr(buf, src, attr, vc);
@@ -112,14 +115,17 @@ static inline void bit_putcs_unaligned(struct vc_data *vc,
 				       u8 *dst)
 {
 	u16 charmask = vc->vc_hi_font_mask ? 0x1ff : 0xff;
+	unsigned int charcnt = vc->vc_font.charcount;
 	u32 shift_low = 0, mod = vc->vc_font.width % 8;
 	u32 shift_high = 8;
 	u32 idx = vc->vc_font.width >> 3;
 	u8 *src;
 
 	while (cnt--) {
-		src = vc->vc_font.data + (scr_readw(s++)&
-					  charmask)*cellsize;
+		u16 ch = scr_readw(s++) & charmask;
+	if (ch >= charcnt)
+		ch = 0;
+	src = vc->vc_font.data + (unsigned int)ch * cellsize;
 
 		if (attr) {
 			update_attr(buf, src, attr, vc);
-- 
2.48.1