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

binder: fix UAF in binder_thread_release()

When a thread exits, binder_thread_release() walks its transaction stack to clear the t->from and t->to_proc that correspond with the exiting thread. However, a process dying in parallel might attempt to kfree some of these transactions. And if one of them has no associated t->to_proc, the t->to_proc->inner_lock will not be acquired.

This means that transaction accesses in binder_thread_release() after t->to_proc has been cleared might race with binder_free_transaction() and cause a use-after-free error as reported by KASAN:

================================================================== BUG: KASAN: slab-use-after-free in binder_thread_release+0x5d0/0x798 Write of size 8 at addr ffff000016627500 by task X/715

CPU: 17 UID: 0 PID: 715 Comm: X Not tainted 7.1.0-rc5-00149-g8fde5d1d47f6 #30 PREEMPT Hardware name: linux,dummy-virt (DT) Call trace: binder_thread_release+0x5d0/0x798 binder_ioctl+0x12c0/0x299c […]

Allocated by task 717 on cpu 18 at 67.267803s: __kasan_kmalloc+0xa0/0xbc __kmalloc_cache_noprof+0x174/0x444 binder_transaction+0x554/0x8150 binder_thread_write+0xa30/0x4354 binder_ioctl+0x20f0/0x299c […]

Freed by task 202 on cpu 18 at 90.416221s: __kasan_slab_free+0x58/0x80 kfree+0x1a0/0x4a4 binder_free_transaction+0x150/0x294 binder_send_failed_reply+0x398/0x6d8 binder_release_work+0x3e4/0x4ec binder_deferred_func+0xbd8/0x104c […]

In order to avoid this, make sure that binder_free_transaction() reads the t->to_proc under the transaction lock. This will serialize the transaction release with the accesses in binder_thread_release(). Plus, it matches the documented locking rules for @to_proc.

Affected Software

References

• https://git.kernel.org/stable/c/114a116aaa5f0295376cdf12da743c5bce3b20ce
• https://git.kernel.org/stable/c/1f96f8c0a6ed4f6d01d3dd29ad0cbf08dde96082
• https://git.kernel.org/stable/c/38e1a71728e5795b670cc159c18e286a40aeebb4
• https://git.kernel.org/stable/c/df1a17abba8d6fac5f965adcb8113ceace6e4949
• https://git.kernel.org/stable/c/e63032dc715026a96bcaa13d375a8e15c91caa84
• https://git.kernel.org/stable/c/ea02df466df60ecd758eb3b4df3f0cadc5c886ce
• https://git.kernel.org/stable/c/ef5439ba5b9ac93349f5df12ef88b42a0ce26340
• https://git.kernel.org/stable/c/faa070c7ad8ba25dcd0b12d7cdbb419e336f5391