CVE-2022-48760

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

USB: core: Fix hang in usb_kill_urb by adding memory barriers

The syzbot fuzzer has identified a bug in which processes hang waiting for usb_kill_urb() to return. It turns out the issue is not unlinking the URB; that works just fine. Rather, the problem arises when the wakeup notification that the URB has completed is not received.

The reason is memory-access ordering on SMP systems. In outline form, usb_kill_urb() and __usb_hcd_giveback_urb() operating concurrently on different CPUs perform the following actions:

CPU 0 CPU 1

usb_kill_urb(): __usb_hcd_giveback_urb(): … … atomic_inc(&urb->reject); atomic_dec(&urb->use_count); … … wait_event(usb_kill_urb_queue, atomic_read(&urb->use_count) == 0); if (atomic_read(&urb->reject)) wake_up(&usb_kill_urb_queue);

Confining your attention to urb->reject and urb->use_count, you can see that the overall pattern of accesses on CPU 0 is:

write urb->reject, then read urb->use_count;

whereas the overall pattern of accesses on CPU 1 is:

write urb->use_count, then read urb->reject.

This pattern is referred to in memory-model circles as SB (for Store Buffering), and it is well known that without suitable enforcement of the desired order of accesses – in the form of memory barriers – it is entirely possible for one or both CPUs to execute their reads ahead of their writes. The end result will be that sometimes CPU 0 sees the old un-decremented value of urb->use_count while CPU 1 sees the old un-incremented value of urb->reject. Consequently CPU 0 ends up on the wait queue and never gets woken up, leading to the observed hang in usb_kill_urb().

The same pattern of accesses occurs in usb_poison_urb() and the failure pathway of usb_hcd_submit_urb().

The problem is fixed by adding suitable memory barriers. To provide proper memory-access ordering in the SB pattern, a full barrier is required on both CPUs. The atomic_inc() and atomic_dec() accesses themselves dont provide any memory ordering, but since they are present, we can use the optimized smp_mb__after_atomic() memory barrier in the various routines to obtain the desired effect.

This patch adds the necessary memory barriers.

Affected Software

References

• https://git.kernel.org/stable/c/26fbe9772b8c459687930511444ce443011f86bf
• https://git.kernel.org/stable/c/546ba238535d925254e0b3f12012a5c55801e2f3
• https://git.kernel.org/stable/c/5904dfd3ddaff3bf4a41c3baf0a8e8f31ed4599b
• https://git.kernel.org/stable/c/5f138ef224dffd15d5e5c5b095859719e0038427
• https://git.kernel.org/stable/c/9340226388c66a7e090ebb00e91ed64a753b6c26
• https://git.kernel.org/stable/c/9c61fce322ac2ef7fecf025285353570d60e41d6
• https://git.kernel.org/stable/c/b50f5ca60475710bbc9a3af32fbfc17b1e69c2f0
• https://git.kernel.org/stable/c/c9a18f7c5b071dce5e6939568829d40994866ab0
• https://git.kernel.org/stable/c/e3b131e30e612ff0e32de6c1cb4f69f89db29193