Description

In the Linux kernel, the following vulnerability has been resolved: tracing: Fix warning in trace_buffered_event_disable() Warning happened in trace_buffered_event_disable() at WARN_ON_ONCE(!trace_buffered_event_ref) Call Trace: ? __warn+0xa5/0x1b0 ? trace_buffered_event_disable+0x189/0x1b0 __ftrace_event_enable_disable+0x19e/0x3e0 free_probe_data+0x3b/0xa0 unregister_ftrace_function_probe_func+0x6b8/0x800 event_enable_func+0x2f0/0x3d0 ftrace_process_regex.isra.0+0x12d/0x1b0 ftrace_filter_write+0xe6/0x140 vfs_write+0x1c9/0x6f0 [...] The cause of the warning is in __ftrace_event_enable_disable(), trace_buffered_event_enable() was called once while trace_buffered_event_disable() was called twice. Reproduction script show as below, for analysis, see the comments: ``` #!/bin/bash cd /sys/kernel/tracing/ # 1. Register a 'disable_event' command, then: # 1) SOFT_DISABLED_BIT was set; # 2) trace_buffered_event_enable() was called first time; echo 'cmdline_proc_show:disable_event:initcall:initcall_finish' > \ set_ftrace_filter # 2. Enable the event registered, then: # 1) SOFT_DISABLED_BIT was cleared; # 2) trace_buffered_event_disable() was called first time; echo 1 > events/initcall/initcall_finish/enable # 3. Try to call into cmdline_proc_show(), then SOFT_DISABLED_BIT was # set again!!! cat /proc/cmdline # 4. Unregister the 'disable_event' command, then: # 1) SOFT_DISABLED_BIT was cleared again; # 2) trace_buffered_event_disable() was called second time!!! echo '!cmdline_proc_show:disable_event:initcall:initcall_finish' > \ set_ftrace_filter ``` To fix it, IIUC, we can change to call trace_buffered_event_enable() at fist time soft-mode enabled, and call trace_buffered_event_disable() at last time soft-mode disabled.

References 8

• git.kernel.org https://git.kernel.org/stable/c/1488d782c9e43087a3f341b8186cd25f3cf75583
• git.kernel.org https://git.kernel.org/stable/c/528c9d73153754defb748f0b96ad33308668d817
• git.kernel.org https://git.kernel.org/stable/c/813cede7b2f5a4b1b75d2d4bb4e705cc8e063b20
• git.kernel.org https://git.kernel.org/stable/c/a3a3c7bddab9b6c5690b20796ef5e332b8c48afb
• git.kernel.org https://git.kernel.org/stable/c/a6d2fd1703cdc8ecfc3e73987e0fb7474ae2b074
• git.kernel.org https://git.kernel.org/stable/c/b4f4ab423107dc1ba8e9cc6488c645be6403d3f5
• git.kernel.org https://git.kernel.org/stable/c/cdcc35e6454133feb61561b4e0d0c80e52cbc2ba
• git.kernel.org https://git.kernel.org/stable/c/dea499781a1150d285c62b26659f62fb00824fce

Remediation