Dear linux-fbdev, stable,
On Mon, Oct 20, 2025 at 09:47:01PM +0800, Junjie Cao wrote:
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: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=793cf822d213be1a74f2
Tested-by: [email protected]
Signed-off-by: Junjie Cao <[email protected]>
This commit is applied to v5.10.247 and causes a regression: when
switching VT with ctrl-alt-f2 the screen is blank or completely filled
with angle characters, then new text is not appearing (or not visible).
This commit is found with git bisect from v5.10.246 to v5.10.247:
0998a6cb232674408a03e8561dc15aa266b2f53b is the first bad commit
commit 0998a6cb232674408a03e8561dc15aa266b2f53b
Author: Junjie Cao <[email protected]>
AuthorDate: 2025-10-20 21:47:01 +0800
Commit: Greg Kroah-Hartman <[email protected]>
CommitDate: 2025-12-07 06:08:07 +0900
fbdev: bitblit: bound-check glyph index in bit_putcs*
commit 18c4ef4e765a798b47980555ed665d78b71aeadf upstream.
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:
[email protected]
Closes:
https://syzkaller.appspot.com/bug?extid=793cf822d213be1a74f2
Tested-by: [email protected]
Signed-off-by: Junjie Cao <[email protected]>
Reviewed-by: Thomas Zimmermann <[email protected]>
Signed-off-by: Helge Deller <[email protected]>
Cc: [email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
drivers/video/fbdev/core/bitblit.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
The minimal reproducer in cli, after kernel is booted:
date >/dev/tty2; chvt 2
and the date does not appear.
Thanks,
#regzbot introduced: 0998a6cb232674408a03e8561dc15aa266b2f53b
---
v1:
https://lore.kernel.org/linux-fbdev/[email protected]/
v1 -> v2:
- Fix indentation and add blank line after declarations with the
.pl helper
- No functional changes
drivers/video/fbdev/core/bitblit.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/drivers/video/fbdev/core/bitblit.c
b/drivers/video/fbdev/core/bitblit.c
index 9d2e59796c3e..085ffb44c51a 100644
--- a/drivers/video/fbdev/core/bitblit.c
+++ b/drivers/video/fbdev/core/bitblit.c
@@ -79,12 +79,16 @@ 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 +116,18 @@ 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
