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(). Its 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: 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.)

Affected Software

References

• https://git.kernel.org/stable/c/00d6a284fcf3fad1b7e1b5bc3cd87cbfb60ce03f
• 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