[PATCH 6.6 059/143] fbcon: always restore the old font data in fbcon_do_set_font()

Greg Kroah-Hartman
2024-03-04T21:38:30Z

From: Jiri Slaby (SUSE) <jirislaby@kernel.org>

[ Upstream commit 00d6a284fcf3fad1b7e1b5bc3cd87cbfb60ce03f ]

Commit a5a923038d70 (fbdev: fbcon: Properly revert changes when
vc_resize() failed) started restoring old font data upon failure (of
vc_resize()). But it performs so only for user fonts. It means that the
"system"/internal fonts are not restored at all. So in result, the very
first call to fbcon_do_set_font() performs no restore at all upon
failing vc_resize().

This can be reproduced by Syzkaller to crash the system on the next
invocation of font_get(). It's rather hard to hit the allocation failure
in vc_resize() on the first font_set(), but not impossible. Esp. if
fault injection is used to aid the execution/failure. It was
demonstrated by Sirius:
BUG: unable to handle page fault for address: fffffffffffffff8
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD cb7b067 P4D cb7b067 PUD cb7d067 PMD 0
Oops: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 8007 Comm: poc Not tainted 6.7.0-g9d1694dc91ce #20
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
RIP: 0010:fbcon_get_font+0x229/0x800 drivers/video/fbdev/core/fbcon.c:2286
Call Trace:
<TASK>
con_font_get drivers/tty/vt/vt.c:4558 [inline]
con_font_op+0x1fc/0xf20 drivers/tty/vt/vt.c:4673
vt_k_ioctl drivers/tty/vt/vt_ioctl.c:474 [inline]
vt_ioctl+0x632/0x2ec0 drivers/tty/vt/vt_ioctl.c:752
tty_ioctl+0x6f8/0x1570 drivers/tty/tty_io.c:2803
vfs_ioctl fs/ioctl.c:51 [inline]
...

So restore the font data in any case, not only for user fonts. Note the
later 'if' is now protected by 'old_userfont' and not 'old_data' as the
latter is always set now. (And it is supposed to be non-NULL. Otherwise
we would see the bug above again.)

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Fixes: a5a923038d70 ("fbdev: fbcon: Properly revert changes when vc_resize() failed")
Reported-and-tested-by: Ubisectech Sirius <bugreport@ubisectech.com>
Cc: Ubisectech Sirius <bugreport@ubisectech.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240208114411.14604-1-jirislaby@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/video/fbdev/core/fbcon.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index f157a5a1dffcf..24035b4f2cd70 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2398,11 +2398,9 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
struct fbcon_ops *ops = info->fbcon_par;
struct fbcon_display *p = &fb_display[vc->vc_num];
int resize, ret, old_userfont, old_width, old_height, old_charcount;
- char *old_data = NULL;
+ u8 *old_data = vc->vc_font.data;

resize = (w != vc->vc_font.width) || (h != vc->vc_font.height);
- if (p->userfont)
- old_data = vc->vc_font.data;
vc->vc_font.data = (void *)(p->fontdata = data);
old_userfont = p->userfont;
if ((p->userfont = userfont))
@@ -2436,13 +2434,13 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
update_screen(vc);
}

- if (old_data && (--REFCOUNT(old_data) == 0))
+ if (old_userfont && (--REFCOUNT(old_data) == 0))
kfree(old_data - FONT_EXTRA_WORDS * sizeof(int));
return 0;

err_out:
p->fontdata = old_data;
- vc->vc_font.data = (void *)old_data;
+ vc->vc_font.data = old_data;

if (userfont) {
p->userfont = old_userfont;
--
2.43.0