[PATCH] fbcon: fix NULL pointer dereference for a console without vc_data

[Ian Bridges <icb@fastmail.org>]

fbcon_new_modelist() runs when a framebuffer's modelist changes. For each
console mapped to it with fb_display[i].mode set, it reads vc_cons[i].d and
passes the vc_num to fbcon_set_disp(). This assumes a console with a mode
set has a vc_data, but it can be NULL. fbcon_set_disp() sets
fb_display[i].mode before it checks vc_data, and fbcon_deinit() leaves the
mode set after the vc_data is freed. fbcon_new_modelist() then dereferences
the NULL vc_data.

Keep fb_display[i].mode set only while the console has a vc_data. Check
vc_data before setting the mode in fbcon_set_disp(), and clear the mode in
fbcon_deinit(). The existing mode check in fbcon_new_modelist() then skips
such consoles.

Reported-by: syzbot+42525d636f430fd5d983@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=42525d636f430fd5d983
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Ian Bridges <icb@fastmail.org>
---
This patch fixes a NULL pointer dereference in the framebuffer console code.
fbcon_new_modelist() dereferences a NULL vc_data. It was found while writing
a reproducer for a separate use-after-free in store_modes(). Sashiko
independently flagged the same dereference in its review of the fix for that
use-after-free [1].

The dereference happens when a console has fb_display[i].mode set but no
vc_data, and the modelist is then replaced, as follows.

1. A console ends up with fb_display[i].mode set while vc_cons[i].d is NULL.
   Either fbcon_set_disp() sets the mode (fbcon.c:1446) before it checks
   vc_data, so mapping an unused console with FBIOPUT_CON2FBMAP leaves the
   mode set, or fbcon_deinit() (fbcon.c:1268) frees the vc_data but keeps the
   mode.
2. A write to the modes attribute calls store_modes() (fbsysfs.c:91), which
   replaces the modelist and calls fb_new_modelist() (fbsysfs.c:108).
3. fb_new_modelist() calls fbcon_new_modelist() (fbmem.c:770).
4. fbcon_new_modelist() walks the consoles mapped to the framebuffer, takes
   vc = vc_cons[i].d for the one with the mode set, and reads vc->vc_num with
   vc NULL (fbcon.c:3046). This is a NULL pointer dereference.

This patch does not change that line:

    fbcon_set_disp(info, &var, vc->vc_num);

vc is vc_cons[i].d, which is NULL.

syzbot reported the same crash, with the call stack store_modes() ->
fb_new_modelist() -> fbcon_new_modelist() [2]. That report had no reproducer
and was closed as obsolete.

It reproduces on a KASAN kernel with two framebuffers, two ways:

1. Map an unused console to one framebuffer and back to the other with
   FBIOPUT_CON2FBMAP, then write a modelist to that framebuffer's modes
   attribute.
2. Open and close /dev/ttyN, then write a modelist to its framebuffer's
   modes attribute.

The reproducer was written with the help of a coding agent (Claude Code). The
patch is against commit 3726ce7f6cef on the for-next branch of
git://git.kernel.org/pub/scm/linux/kernel/git/deller/linux-fbdev.git. The
file offsets above are from that commit. The dereference is present in the
initial 2.6.12-rc2 import, so there is no Fixes tag.

[1] https://lore.kernel.org/all/20260622080749.D7FC61F000E9@smtp.kernel.org/
[2] https://syzkaller.appspot.com/bug?extid=42525d636f430fd5d983

 drivers/video/fbdev/core/fbcon.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 9077d3b99357..97d4b836b26a 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -1273,6 +1273,7 @@ static void fbcon_deinit(struct vc_data *vc)
 	int idx;
 
 	fbcon_free_font(p);
+	p->mode = NULL;
 	idx = con2fb_map[vc->vc_num];
 
 	if (idx == -1)
@@ -1443,14 +1444,14 @@ static void fbcon_set_disp(struct fb_info *info, struct fb_var_screeninfo *var,
 
 	p = &fb_display[unit];
 
-	if (var_to_display(p, var, info))
-		return;
-
 	vc = vc_cons[unit].d;
 
 	if (!vc)
 		return;
 
+	if (var_to_display(p, var, info))
+		return;
+
 	default_mode = vc->vc_display_fg;
 	svc = *default_mode;
 	t = &fb_display[svc->vc_num];
-- 
2.47.3