[PATCH 6.18 176/314] tracing: Fix WARN_ON in tracing_buffers_mmap_close

Sasha Levin
2026-03-10T11:27:29Z

From: Qing Wang <wangqing7171@gmail.com>

commit e39bb9e02b68942f8e9359d2a3efe7d37ae6be0e upstream.

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 <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Vincent Donnefort <vdonnefort@google.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
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 <wangqing7171@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/ring_buffer.h | 1 +
kernel/trace/ring_buffer.c | 21 +++++++++++++++++++++
kernel/trace/trace.c | 13 +++++++++++++
3 files changed, 35 insertions(+)

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index 876358cfe1b12..d862fa610270b 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 */
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 54d70bd0a3cb9..98ca4beabf023 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7283,6 +7283,27 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
return err;
}

+/*
+ * This is called when a VMA is duplicated (e.g., on fork()) to increment
+ * the user_mapped counter without remapping pages.
+ */
+void ring_buffer_map_dup(struct trace_buffer *buffer, int cpu)
+{
+ struct ring_buffer_per_cpu *cpu_buffer;
+
+ if (WARN_ON(!cpumask_test_cpu(cpu, buffer->cpumask)))
+ return;
+
+ cpu_buffer = buffer->buffers[cpu];
+
+ guard(mutex)(&cpu_buffer->mapping_lock);
+
+ if (cpu_buffer->user_mapped)
+ __rb_inc_dec_mapped(cpu_buffer, true);
+ else
+ WARN(1, "Unexpected buffer stat, it should be mapped");
+}
+
int ring_buffer_unmap(struct trace_buffer *buffer, int cpu)
{
struct ring_buffer_per_cpu *cpu_buffer;
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d2eabc162205c..38fab063c3687 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8784,6 +8784,18 @@ static inline int get_snapshot_map(struct trace_array *tr) { return 0; }
static inline void put_snapshot_map(struct trace_array *tr) { }
#endif

+/*
+ * This is called when a VMA is duplicated (e.g., on fork()) to increment
+ * the user_mapped counter without remapping pages.
+ */
+static void tracing_buffers_mmap_open(struct vm_area_struct *vma)
+{
+ struct ftrace_buffer_info *info = vma->vm_file->private_data;
+ struct trace_iterator *iter = &info->iter;
+
+ ring_buffer_map_dup(iter->array_buffer->buffer, iter->cpu_file);
+}
+
static void tracing_buffers_mmap_close(struct vm_area_struct *vma)
{
struct ftrace_buffer_info *info = vma->vm_file->private_data;
@@ -8803,6 +8815,7 @@ static int tracing_buffers_may_split(struct vm_area_struct *vma, unsigned long a
}

static const struct vm_operations_struct tracing_buffers_vmops = {
+ .open = tracing_buffers_mmap_open,
.close = tracing_buffers_mmap_close,
.may_split = tracing_buffers_may_split,
};
--
2.51.0