CVE-2023-52486

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

drm: Dont unref the same fb many times by mistake due to deadlock handling

If we get a deadlock after the fb lookup in drm_mode_page_flip_ioctl() we proceed to unref the fb and then retry the whole thing from the top. But we forget to reset the fb pointer back to NULL, and so if we then get another error during the retry, before the fb lookup, we proceed the unref the same fb again without having gotten another reference. The end result is that the fb will (eventually) end up being freed while its still in use.

Reset fb to NULL once weve unreffed it to avoid doing it again until weve done another fb lookup.

This turned out to be pretty easy to hit on a DG2 when doing async flips (and CONFIG_DEBUG_WW_MUTEX_SLOWPATH=y). The first symptom I saw that drm_closefb() simply got stuck in a busy loop while walking the framebuffer list. Fortunately I was able to convince it to oops instead, and from there it was easier to track down the culprit.

Weakness

The product does not properly acquire or release a lock on a resource, leading to unexpected resource state changes and behaviors.

Affected Software

Extended Description

Locking is a type of synchronization behavior that ensures that multiple independently-operating processes or threads do not interfere with each other when accessing the same resource. All processes/threads are expected to follow the same steps for locking. If these steps are not followed precisely - or if no locking is done at all - then another process/thread could modify the shared resource in a way that is not visible or predictable to the original process. This can lead to data or memory corruption, denial of service, etc.

Potential Mitigations

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/25.html
• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/27.html

References

• https://git.kernel.org/stable/c/376e21a9e4c2c63ee5d8d3aa74be5082c3882229
• https://git.kernel.org/stable/c/62f2e79cf9f4f47cc9dea9cebdf58d9f7b5695e0
• https://git.kernel.org/stable/c/9dd334a8245011ace45e53298175c7b659edb3e7
• https://git.kernel.org/stable/c/b4af63da9d94986c529d74499fdfe44289acd551
• https://git.kernel.org/stable/c/bfd0feb1b109cb63b87fdcd00122603787c75a1a
• https://git.kernel.org/stable/c/cb4daf271302d71a6b9a7c01bd0b6d76febd8f0c
• https://git.kernel.org/stable/c/d7afdf360f4ac142832b098b4de974e867cc063c
• https://git.kernel.org/stable/c/f55261469be87c55df13db76dc945f6bcd825105
• https://git.kernel.org/stable/c/376e21a9e4c2c63ee5d8d3aa74be5082c3882229
• https://git.kernel.org/stable/c/62f2e79cf9f4f47cc9dea9cebdf58d9f7b5695e0
• https://git.kernel.org/stable/c/9dd334a8245011ace45e53298175c7b659edb3e7
• https://git.kernel.org/stable/c/b4af63da9d94986c529d74499fdfe44289acd551
• https://git.kernel.org/stable/c/bfd0feb1b109cb63b87fdcd00122603787c75a1a
• https://git.kernel.org/stable/c/cb4daf271302d71a6b9a7c01bd0b6d76febd8f0c
• https://git.kernel.org/stable/c/d7afdf360f4ac142832b098b4de974e867cc063c
• https://git.kernel.org/stable/c/f55261469be87c55df13db76dc945f6bcd825105
• https://lists.debian.org/debian-lts-announce/2024/06/msg00017.html
• https://lists.debian.org/debian-lts-announce/2024/06/msg00020.html