CVE-2024-38613

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

m68k: Fix spinlock race in kernel thread creation

Context switching does take care to retain the correct lock owner across the switch from prev to next tasks. This does rely on interrupts remaining disabled for the entire duration of the switch.

This condition is guaranteed for normal process creation and context switching between already running processes, because both prev and next already have interrupts disabled in their saved copies of the status register.

The situation is different for newly created kernel threads. The status register is set to PS_S in copy_thread(), which does leave the IPL at 0. Upon restoring the next threads status register in switch_to() aka resume(), interrupts then become enabled prematurely. resume() then returns via ret_from_kernel_thread() and schedule_tail() where run queue lock is released (see finish_task_switch() and finish_lock_switch()).

A timer interrupt calling scheduler_tick() before the lock is released in finish_task_switch() will find the lock already taken, with the current task as lock owner. This causes a spinlock recursion warning as reported by Guenter Roeck.

As far as I can ascertain, this race has been opened in commit 533e6903bea0 (m68k: split ret_from_fork(), simplify kernel_thread()) but I havent done a detailed study of kernel history so it may well predate that commit.

Interrupts cannot be disabled in the saved status register copy for kernel threads (init will complain about interrupts disabled when finally starting user space). Disable interrupts temporarily when switching the tasks register sets in resume().

Note that a simple oriw 0x700,%sr after restoring sr is not enough here

• this leaves enough of a race for the spinlock recursion warning to still be observed.

Tested on ARAnyM and qemu (Quadra 800 emulation).

Weakness

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/0d9ae1253535f6e85a016e09c25ecbe6f7f59ef0
• https://git.kernel.org/stable/c/2a8d1d95302c7d52c6ac8fa5cb4a6948ae0d3a14
• https://git.kernel.org/stable/c/4eeffecc8e3cce25bb559502c2fd94a948bcde82
• https://git.kernel.org/stable/c/5213cc01d0464c011fdc09f318705603ed3a746b
• https://git.kernel.org/stable/c/77b2b67a0f8bce260c53907e5749d61466d90c87
• https://git.kernel.org/stable/c/95f00caf767b5968c2c51083957b38be4748a78a
• https://git.kernel.org/stable/c/da89ce46f02470ef08f0f580755d14d547da59ed
• https://git.kernel.org/stable/c/f1d4274a84c069be0f6098ab10c3443fc1f7134c
• https://git.kernel.org/stable/c/f3baf0f4f92af32943ebf27b960e0552c6c082fd
• https://git.kernel.org/stable/c/0d9ae1253535f6e85a016e09c25ecbe6f7f59ef0
• https://git.kernel.org/stable/c/2a8d1d95302c7d52c6ac8fa5cb4a6948ae0d3a14
• https://git.kernel.org/stable/c/4eeffecc8e3cce25bb559502c2fd94a948bcde82
• https://git.kernel.org/stable/c/5213cc01d0464c011fdc09f318705603ed3a746b
• https://git.kernel.org/stable/c/77b2b67a0f8bce260c53907e5749d61466d90c87
• https://git.kernel.org/stable/c/95f00caf767b5968c2c51083957b38be4748a78a
• https://git.kernel.org/stable/c/da89ce46f02470ef08f0f580755d14d547da59ed
• https://git.kernel.org/stable/c/f1d4274a84c069be0f6098ab10c3443fc1f7134c
• https://git.kernel.org/stable/c/f3baf0f4f92af32943ebf27b960e0552c6c082fd