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

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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]
This commit is contained in:
Junjie Cao authored and Helge Deller committed 2025-10-28 22:59:19 +01:00
1 parent 5f566c0ac5
commit 18c4ef4e76
1 file changed
+12 -4
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drivers/video/fbdev/core/bitblit.c
+12 -4
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@@ -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);