From e39bb9e02b68942f8e9359d2a3efe7d37ae6be0e Mon Sep 17 00:00:00 2001 From: Qing Wang Date: Fri, 27 Feb 2026 10:58:42 +0800 Subject: tracing: Fix WARN_ON in tracing_buffers_mmap_close When a process forks, the child process copies the parent's VMAs but the user_mapped reference count is not incremented. As a result, when both the parent and child processes exit, tracing_buffers_mmap_close() is called twice. On the second call, user_mapped is already 0, causing the function to return -ENODEV and triggering a WARN_ON. Normally, this isn't an issue as the memory is mapped with VM_DONTCOPY set. But this is only a hint, and the application can call madvise(MADVISE_DOFORK) which resets the VM_DONTCOPY flag. When the application does that, it can trigger this issue on fork. Fix it by incrementing the user_mapped reference count without re-mapping the pages in the VMA's open callback. Cc: stable@vger.kernel.org Cc: Masami Hiramatsu Cc: Mathieu Desnoyers Cc: Vincent Donnefort Cc: Lorenzo Stoakes Link: https://patch.msgid.link/20260227025842.1085206-1-wangqing7171@gmail.com Fixes: cf9f0f7c4c5bb ("tracing: Allow user-space mapping of the ring-buffer") Reported-by: syzbot+3b5dd2030fe08afdf65d@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3b5dd2030fe08afdf65d Tested-by: syzbot+3b5dd2030fe08afdf65d@syzkaller.appspotmail.com Signed-off-by: Qing Wang Signed-off-by: Steven Rostedt (Google) --- include/linux/ring_buffer.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include/linux') diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h index 876358cfe1b1..d862fa610270 100644 --- a/include/linux/ring_buffer.h +++ b/include/linux/ring_buffer.h @@ -248,6 +248,7 @@ int trace_rb_cpu_prepare(unsigned int cpu, struct hlist_node *node); int ring_buffer_map(struct trace_buffer *buffer, int cpu, struct vm_area_struct *vma); +void ring_buffer_map_dup(struct trace_buffer *buffer, int cpu); int ring_buffer_unmap(struct trace_buffer *buffer, int cpu); int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu); #endif /* _LINUX_RING_BUFFER_H */ -- cgit v1.2.3