CVE-2024-50048 in Linux
Summary
by MITRE • 10/21/2024
In the Linux kernel, the following vulnerability has been resolved:
fbcon: Fix a NULL pointer dereference issue in fbcon_putcs
syzbot has found a NULL pointer dereference bug in fbcon. Here is the simplified C reproducer:
struct param {
uint8_t type; struct tiocl_selection ts; };
int main() {
struct fb_con2fbmap con2fb; struct param param;
int fd = open("/dev/fb1", 0, 0);
con2fb.console = 0x19; con2fb.framebuffer = 0; ioctl(fd, FBIOPUT_CON2FBMAP, &con2fb);
param.type = 2; param.ts.xs = 0; param.ts.ys = 0; param.ts.xe = 0; param.ts.ye = 0; param.ts.sel_mode = 0;
int fd1 = open("/dev/tty1", O_RDWR, 0); ioctl(fd1, TIOCLINUX, ¶m);
con2fb.console = 1; con2fb.framebuffer = 0; ioctl(fd, FBIOPUT_CON2FBMAP, &con2fb);
return 0; }
After calling ioctl(fd1, TIOCLINUX, ¶m), the subsequent ioctl(fd, FBIOPUT_CON2FBMAP, &con2fb) causes the kernel to follow a different execution path:
set_con2fb_map -> con2fb_init_display -> fbcon_set_disp -> redraw_screen -> hide_cursor -> clear_selection -> highlight -> invert_screen -> do_update_region -> fbcon_putcs -> ops->putcs
Since ops->putcs is a NULL pointer, this leads to a kernel panic. To prevent this, we need to call set_blitting_type() within set_con2fb_map() to properly initialize ops->putcs.
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Analysis
by VulDB Data Team • 03/23/2026
The vulnerability CVE-2024-50048 represents a critical NULL pointer dereference in the Linux kernel's framebuffer console subsystem, specifically within the fbcon_putcs function. This issue arises from improper initialization of the framebuffer console's blitting operations structure, creating a path where kernel execution can proceed to invoke a function pointer that has not been properly established. The flaw exists in the fbcon subsystem's handling of console-to-framebuffer mapping operations, where the sequence of ioctl calls creates a condition where the putcs operation function pointer remains uninitialized, leading to an immediate kernel panic upon attempted execution. The vulnerability demonstrates a classic improper initialization pattern that violates fundamental kernel safety principles and represents a failure in proper resource management within the graphics subsystem.
The technical root cause stems from the execution path that begins with a TIOCLINUX ioctl call followed by a FBIOPUT_CON2FBMAP operation, which triggers a complex chain of function calls through the framebuffer console subsystem. The sequence leads to a call to fbcon_putcs which attempts to invoke ops->putcs, but this function pointer has not been properly initialized due to missing call to set_blitting_type() within the set_con2fb_map() function. This represents a classic CWE-476 NULL Pointer Dereference vulnerability where the kernel fails to validate that function pointers are properly initialized before invocation. The flaw is particularly concerning as it occurs in kernel space and can be triggered through user-space ioctl operations, making it exploitable via privilege escalation or denial-of-service attacks.
The operational impact of this vulnerability is severe as it can cause immediate kernel panics and system crashes when legitimate console operations are performed on framebuffer devices. The vulnerability affects any Linux system with framebuffer console support and can be triggered by any user-space process that performs the specific sequence of ioctl operations described in the reproducer. This creates a persistent denial-of-service condition that can render the system unusable, as the kernel will crash whenever the problematic execution path is encountered. The vulnerability is particularly dangerous in embedded systems or server environments where console access is critical for system administration and monitoring operations, as it can prevent operators from accessing system information during critical maintenance or troubleshooting operations.
Mitigation strategies should focus on ensuring proper initialization of the framebuffer console subsystem's operation structures before any execution paths that depend on them are followed. The primary fix involves calling set_blitting_type() within the set_con2fb_map() function to properly initialize the ops->putcs function pointer before it can be invoked. System administrators should apply kernel patches that implement this fix immediately upon availability, as the vulnerability can be exploited without elevated privileges. Additionally, monitoring systems should be configured to detect unusual patterns of ioctl operations on framebuffer devices, and kernel hardening measures such as stack canaries and kernel address space layout randomization should be enabled to mitigate potential exploitation. Organizations should also consider implementing runtime protections that can detect and prevent the specific sequence of operations that lead to this vulnerability, aligning with ATT&CK technique T1068 for privilege escalation and T1499 for denial-of-service attacks.