btrfs: fix use-after-free when COWing tree bock and tracing is enabled

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: btrfs: fix use-after-free when COWing tree bock and tracing is enabled When a COWing a tree block, at btrfs_cow_block(), and we have the tracepoint trace_btrfs_cow_block() enabled and preemption is also enabled (CONFIG_PREEMPT=y), we can trigger a use-after-free in the COWed extent buffer while inside the tracepoint code. This is because in some paths that call btrfs_cow_block(), such as btrfs_search_slot(), we are holding the last reference on the extent buffer @buf so btrfs_force_cow_block() drops the last reference on the @buf extent buffer when it calls free_extent_buffer_stale(buf), which schedules the release of the extent buffer with RCU. This means that if we are on a kernel with preemption, the current task may be preempted before calling trace_btrfs_cow_block() and the extent buffer already released by the time trace_btrfs_cow_block() is called, resulting in a use-after-free. Fix this by moving the trace_btrfs_cow_block() from btrfs_cow_block() to btrfs_force_cow_block() before the COWed extent buffer is freed. This also has a side effect of invoking the tracepoint in the tree defrag code, at defrag.c:btrfs_realloc_node(), since btrfs_force_cow_block() is called there, but this is fine and it was actually missing there.

References

• https://git.kernel.org/stable/c/ba5120a2fb5f23b4d39d302e181aa5d4e28a90d1
• https://git.kernel.org/stable/c/526ff5b27f090fb15040471f892cd2c9899ce314
• https://git.kernel.org/stable/c/66376f1a73cba57fd0af2631d7888605b738e499
• https://git.kernel.org/stable/c/9a466b8693b9add05de99af00c7bdff8259ecf19
• https://git.kernel.org/stable/c/c3a403d8ce36f5a809a492581de5ad17843e4701
• https://git.kernel.org/stable/c/44f52bbe96dfdbe4aca3818a2534520082a07040