In the Linux kernel, the following vulnerability has been resolved:btrfs: fix use-after-free when COWing tree bock and tracing is enabledWhen a COWing a tree block, at btrfs_cow_block(), and we have thetracepoint trace_btrfs_cow_block() enabled and preemption is also enabled(CONFIG_PREEMPT=y), we can trigger a use-after-free in the COWed extentbuffer while inside the tracepoint code. This is because in some pathsthat call btrfs_cow_block(), such as btrfs_search_slot(), we are holdingthe last reference on the extent buffer @buf so btrfs_force_cow_block()drops the last reference on the @buf extent buffer when it callsfree_extent_buffer_stale(buf), which schedules the release of the extentbuffer 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() tobtrfs_force_cow_block() before the COWed extent buffer is freed.This also has a side effect of invoking the tracepoint in the tree defragcode, at defrag.c:btrfs_realloc_node(), since btrfs_force_cow_block() iscalled there, but this is fine and it was actually missing there.
