CVE-2024-26798

In the Linux kernel, the following vulnerability has been resolved: fbcon:
always restore the old font data in fbcon_do_set_font() 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.)