tracing: Fix WARN_ON in tracing_buffers_mmap_close for split VMAs

When a VMA is split (e.g., by partial munmap or MAP_FIXED), the kernel
calls vm_ops->close on each portion. For trace buffer mappings, this
results in ring_buffer_unmap() being called multiple times while
ring_buffer_map() was only called once.

This causes ring_buffer_unmap() to return -ENODEV on subsequent calls
because user_mapped is already 0, triggering a WARN_ON.

Trace buffer mappings cannot support partial mappings because the ring
buffer structure requires the complete buffer including the meta page.

Fix this by adding a may_split callback that returns -EINVAL to prevent
VMA splits entirely.

Cc: stable@vger.kernel.org
Fixes: cf9f0f7c4c5bb ("tracing: Allow user-space mapping of the ring-buffer")
Link: https://patch.msgid.link/20251119064019.25904-1-kartikey406@gmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a72c325b042aae6403c7
Tested-by: syzbot+a72c325b042aae6403c7@syzkaller.appspotmail.com
Reported-by: syzbot+a72c325b042aae6403c7@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index d1e527cf2aae07c3ab97cf89e11e71f4a9c0946c..304e93597126e5eb786d851e2166a4e1b49b808c 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8781,8 +8781,18 @@ static void tracing_buffers_mmap_close(struct vm_area_struct *vma)
        put_snapshot_map(iter->tr);
 }
 
+static int tracing_buffers_may_split(struct vm_area_struct *vma, unsigned long addr)
+{
+       /*
+        * Trace buffer mappings require the complete buffer including
+        * the meta page. Partial mappings are not supported.
+        */
+       return -EINVAL;
+}
+
 static const struct vm_operations_struct tracing_buffers_vmops = {
        .close          = tracing_buffers_mmap_close,
+       .may_split      = tracing_buffers_may_split,
 };
 
 static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma)