Message-ID: <ccbbf777-cf4e-4c66-856e-282dd9d61970@pobox.com>
Date: Fri, 26 Dec 2025 18:04:04 -0800
From: "Barry K. Nathan" <barryn@...ox.com>
To: Vitaly Chikunov <vt@...linux.org>, Junjie Cao <junjie.cao@...el.com>,
 Thomas Zimmermann <tzimmermann@...e.de>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Peilin Ye <yepeilin.cs@...il.com>, Daniel Vetter
 <daniel.vetter@...ll.ch>, Shigeru Yoshida <syoshida@...hat.com>,
 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
Subject: Re: [PATCH v2] fbdev: bitblit: bound-check glyph index in bit_putcs*

On 12/26/25 4:21 AM, Vitaly Chikunov wrote:
> Dear linux-fbdev, stable,
> 
> On Fri, Dec 26, 2025 at 01:29:13AM +0300, Vitaly Chikunov wrote:
>>
>> 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;
> 
> Perhaps, vc->vc_font.charcount (which is relied upon in the following
> comparison) is not always set correctly in v5.10.247. At least two
> commits that set vc_font.charcount are missing from v5.10.247:
> 
>    a1ac250a82a5 ("fbcon: Avoid using FNTCHARCNT() and hard-coded built-in font charcount")
>    a5a923038d70 ("fbdev: fbcon: Properly revert changes when vc_resize() failed")
> 
> Thanks,

I was just about to report this.

I found two ways to fix this bug. One is to revert this patch; the other 
is to apply the following 3 patches, which are already present in 5.11 
and later:

7a089ec7d77fe7d50f6bb7b178fa25eec9fd822b
     console: Delete unused con_font_copy() callback implementations

4ee573086bd88ff3060dda07873bf755d332e9ba
     Fonts: Add charcount field to font_desc

a1ac250a82a5e97db71f14101ff7468291a6aaef
     fbcon: Avoid using FNTCHARCNT() and hard-coded built-in font
     charcount

(Oh, by the way, this same regression also affects 5.4.302, and the same 
3 patches fix the regression on 5.4 as well, once you manually fix merge 
conflicts. Maybe it would be better to backport other additional commits 
instead of fixing the merge conflicts manually, but since 5.4 is now EOL 
I didn't dig that deep.)

Once these 3 patches are applied, I wonder if a5a923038d70 now becomes 
necessary for 5.10.y. For what it's worth, it applies fine and the 
resulting kernel seems to run OK in brief testing.

-- 
-Barry K. Nathan  <barryn@...ox.com>

Powered by blists - more mailing lists