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Message-ID: <e6aac320-846d-eecf-0016-23b56d7cd854@gmail.com>
Date: Sat, 10 Jan 2026 08:20:19 -0500
From: Woody Suwalski <terraluna977@...il.com>
To: Vitaly Chikunov <vt@...linux.org>, Junjie Cao <junjie.cao@...el.com>
Cc: Thomas Zimmermann <tzimmermann@...e.de>, Simona Vetter <simona@...ll.ch>,
Helge Deller <deller@....de>, Zsolt Kajtar <soci@....rulez.org>,
Albin Babu Varghese <albinbabuvarghese20@...il.com>,
linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
linux-kernel@...r.kernel.org, stable@...r.kernel.org,
regressions@...ts.linux.dev, Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2] fbdev: bitblit: bound-check glyph index in bit_putcs*
Vitaly Chikunov wrote:
> 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@...kaller.appspotmail.com
>> Closes: https://syzkaller.appspot.com/bug?extid=793cf822d213be1a74f2
>> Tested-by: syzbot+793cf822d213be1a74f2@...kaller.appspotmail.com
>> Signed-off-by: Junjie Cao <junjie.cao@...el.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@...el.com>
> AuthorDate: 2025-10-20 21:47:01 +0800
> Commit: Greg Kroah-Hartman <gregkh@...uxfoundation.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@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=793cf822d213be1a74f2
> Tested-by: syzbot+793cf822d213be1a74f2@...kaller.appspotmail.com
> Signed-off-by: Junjie Cao <junjie.cao@...el.com>
> Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>
> Signed-off-by: Helge Deller <deller@....de>
> Cc: stable@...r.kernel.org
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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@suse.de/
>> 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
>>
I have done the same bisecting work, too bad I did not notice Vitaly's
work earlier :-(
There is a "cheap" workaround for systems before 5.11, (not addressing
the root issue but) working:
diff --git a/drivers/video/fbdev/core/bitblit.c
b/drivers/video/fbdev/core/bitblit.c
index 7c2fc9f..c5a1a9d 100644
--- a/drivers/video/fbdev/core/bitblit.c
+++ b/drivers/video/fbdev/core/bitblit.c
@@ -86,7 +86,7 @@ static inline void bit_putcs_aligned(struct vc_data
*vc, struct fb_info *info,
while (cnt--) {
u16 ch = scr_readw(s++) & charmask;
- if (ch >= charcnt)
+ if (charcnt && ch >= charcnt)
ch = 0;
src = vc->vc_font.data + (unsigned int)ch * cellsize;
@@ -125,7 +125,7 @@ static inline void bit_putcs_unaligned(struct
vc_data *vc,
while (cnt--) {
u16 ch = scr_readw(s++) & charmask;
- if (ch >= charcnt)
+ if (charcnt && ch >= charcnt)
ch = 0;
src = vc->vc_font.data + (unsigned int)ch * cellsize;
I will try next to go full backport from 5.11 as Thorsten has suggested.
However the bigger problem is that the fbdev patch has landed in the
5.4.302 EOL, and essentially the 5.4 EOL kernel is now hanging broken :-(
Thanks, Woody
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