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fbdev: bitblit: bound-check glyph index in bit_putcs*

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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>
This commit is contained in:
Junjie Cao 2025-10-20 21:47:01 +08:00 • committed by Greg Kroah-Hartman
commit a10cede006
1 changed files with 12 additions and 4 deletions
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16
drivers/video/fbdev/core/bitblit.c
View file

@ -80,12 +80,16 @@ 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);
@ -113,14 +117,18 @@ 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);