CVE-2024-26798: fbcon: always restore the old font data in fbcon_do_set_font()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

Description
===========

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.)

The Linux kernel CVE team has assigned CVE-2024-26798 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.15.64 with commit ebd6f886aa24 and fixed in 5.15.151 with commit 20a4b5214f7b
	Issue introduced in 6.0 with commit a5a923038d70 and fixed in 6.1.81 with commit 2f91a96b892f
	Issue introduced in 6.0 with commit a5a923038d70 and fixed in 6.6.21 with commit 73a6bd68a134
	Issue introduced in 6.0 with commit a5a923038d70 and fixed in 6.7.9 with commit a2c881413dcc
	Issue introduced in 6.0 with commit a5a923038d70 and fixed in 6.8 with commit 00d6a284fcf3
	Issue introduced in 5.19.6 with commit f08ccb792d3e

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-26798
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/video/fbdev/core/fbcon.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/20a4b5214f7bee13c897477168c77bbf79683c3d
	https://git.kernel.org/stable/c/2f91a96b892fab2f2543b4a55740c5bee36b1a6b
	https://git.kernel.org/stable/c/73a6bd68a1342f3a44cac9dffad81ad6a003e520
	https://git.kernel.org/stable/c/a2c881413dcc5d801bdc9535e51270cc88cb9cd8
	https://git.kernel.org/stable/c/00d6a284fcf3fad1b7e1b5bc3cd87cbfb60ce03f