CVE-2024-49924

In the Linux kernel, the following vulnerability has been resolved:

fbdev: pxafb: Fix possible use after free in pxafb_task()

In the pxafb_probe function, it calls the pxafb_init_fbinfo function, after which &fbi->task is associated with pxafb_task. Moreover, within this pxafb_init_fbinfo function, the pxafb_blank function within the &pxafb_ops struct is capable of scheduling work.

If we remove the module which will call pxafb_remove to make cleanup, it will call unregister_framebuffer function which can call do_unregister_framebuffer to free fbi->fb through put_fb_info(fb_info), while the work mentioned above will be used. The sequence of operations that may lead to a UAF bug is as follows:

CPU0 CPU1

                               | pxafb_task

pxafb_remove | unregister_framebuffer(info) | do_unregister_framebuffer(fb_info) | put_fb_info(fb_info) | // free fbi->fb | set_ctrlr_state(fbi, state) | __pxafb_lcd_power(fbi, 0) | fbi->lcd_power(on, &fbi->fb.var) | //use fbi->fb

Fix it by ensuring that the work is canceled before proceeding with the cleanup in pxafb_remove.

Note that only root user can remove the driver at runtime.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/3c0d416eb4bef705f699213cee94bf54b6acdacd
• https://git.kernel.org/stable/c/4a6921095eb04a900e0000da83d9475eb958e61e
• https://git.kernel.org/stable/c/4cda484e584be34d55ee17436ebf7ad11922b97a
• https://git.kernel.org/stable/c/6d0a07f68b66269e167def6c0b90a219cd3e7473
• https://git.kernel.org/stable/c/a3a855764dbacbdb1cc51e15dc588f2d21c93e0e
• https://git.kernel.org/stable/c/aaadc0cb05c999ccd8898a03298b7e5c31509b08
• https://git.kernel.org/stable/c/e657fa2df4429f3805a9b3e47fb1a4a1b02a72bd
• https://git.kernel.org/stable/c/e6897e299f57b103e999e62010b88e363b3eebae
• https://git.kernel.org/stable/c/fdda354f60a576d52dcf90351254714681df4370