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: syzbot+793cf822d213be1a74f2@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=793cf822d213be1a74f2 Tested-by: syzbot+793cf822d213be1a74f2@syzkaller.appspotmail.com Signed-off-by: Junjie Cao junjie.cao@intel.com
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 junjie.cao@intel.com AuthorDate: 2025-10-20 21:47:01 +0800 Commit: Greg Kroah-Hartman gregkh@linuxfoundation.org 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: syzbot+793cf822d213be1a74f2@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=793cf822d213be1a74f2 Tested-by: syzbot+793cf822d213be1a74f2@syzkaller.appspotmail.com Signed-off-by: Junjie Cao junjie.cao@intel.com Reviewed-by: Thomas Zimmermann tzimmermann@suse.de Signed-off-by: Helge Deller deller@gmx.de Cc: stable@vger.kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
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/5d237d1a-a528-4205-a4d8-71709134f1e1@sus... 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