CVE-2024-26798 in Linux
Summary
by MITRE • 04/04/2024
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.)
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Analysis
by VulDB Data Team • 02/06/2026
The vulnerability described in CVE-2024-26798 resides within the Linux kernel's framebuffer console subsystem, specifically in the fbcon_do_set_font() function. This issue represents a critical flaw in font data management during console resizing operations that can lead to system crashes and potential privilege escalation. The vulnerability stems from an incomplete implementation of font restoration logic that was partially addressed in commit a5a923038d70, which aimed to properly revert changes when vc_resize() fails. However, this fix only applies to user fonts while neglecting system or internal fonts, creating a dangerous inconsistency in the kernel's memory management approach. The flaw manifests when the first invocation of fbcon_do_set_font() encounters a vc_resize() failure, as the system fails to restore the old font data entirely for internal fonts, leaving the console in an inconsistent state.
The technical nature of this vulnerability aligns with CWE-248, which addresses "Uncaught Exception" in software systems, and specifically relates to improper handling of error conditions in kernel space operations. When vc_resize() fails during font setting operations, the kernel should consistently restore all font data regardless of whether it's user-defined or system-managed. The current implementation fails to restore system fonts, which creates a memory access violation when subsequent font operations attempt to reference previously allocated but now corrupted font structures. This issue is particularly dangerous because it operates at the kernel level where memory corruption can lead to complete system compromise and arbitrary code execution. The crash pattern observed in the Syzkaller testing environment demonstrates a page fault error with address 0xffffffffffffffff, indicating a null pointer dereference that occurs during font_get() operations, which is a direct consequence of the incomplete font restoration mechanism.
The operational impact of this vulnerability extends beyond simple system crashes to encompass potential security implications for kernel integrity and system stability. Attackers could exploit this flaw through fault injection techniques to force allocation failures during vc_resize() operations, ultimately leading to denial of service conditions or privilege escalation opportunities. The vulnerability affects the core framebuffer console functionality that many Linux systems depend upon for display operations, particularly in headless server environments or embedded systems where console access is critical. The issue demonstrates a fundamental flaw in the kernel's defensive programming practices, where error handling paths are not consistently applied across all code branches. The fix proposed in the patch ensures that font data restoration occurs regardless of font type, addressing the root cause by removing the conditional logic that previously restricted restoration to user fonts only. This aligns with ATT&CK technique T1068, which involves exploitation of legitimate credentials and privileges, as the vulnerability could potentially be leveraged to escalate privileges through kernel memory corruption.
Mitigation strategies for this vulnerability should focus on applying the kernel patch that implements universal font restoration across all font types. System administrators should prioritize updating their kernel versions to include the fix from commit a5a923038d70 and subsequent related commits that address this issue. Monitoring for unusual console behavior or font-related system crashes should be implemented as part of security operations procedures. Additionally, the vulnerability highlights the importance of comprehensive testing for error handling paths in kernel subsystems, particularly those involving memory allocation and deallocation operations. Organizations should consider implementing kernel hardening measures such as KASAN (Kernel Address Sanitizer) and KMSAN (Kernel Memory Sanitizer) to detect similar issues during development and testing phases. The fix ensures that the old_data pointer is properly checked and restored in all cases, preventing the scenario where the system attempts to access freed or corrupted memory structures during subsequent font operations, thereby maintaining system stability and preventing potential exploitation by malicious actors who might attempt to leverage this flaw for unauthorized access or system compromise.