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Message-ID: <aU58SeZZPxScVPad@altlinux.org>
Date: Fri, 26 Dec 2025 15:21:12 +0300
From: Vitaly Chikunov <vt@...linux.org>
To: 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*
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,
> > 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
> >
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