Linus,

ring-buffer fixes for v6.18:

- Do not allow mmapped ring buffer to be split

  When the ring buffer VMA is split by a partial munmap or a MAP_FIXED, the
  kernel calls vm_ops->close() on each portion. This causes the
  ring_buffer_unmap() to be called multiple times. This causes subsequent
  calls to return -ENODEV and triggers a warning.

  There's no reason to allow user space to split up memory mapping of the
  ring buffer. Have it return -EINVAL when that happens.

Please pull the latest trace-ringbuffer-v6.18-rc7 tree, which can be found at:


  git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git
trace-ringbuffer-v6.18-rc7

Tag SHA1: 0f396c7eb7f1d3ec728312e75cd42213d69916ff
Head SHA1: b042fdf18e89a347177a49e795d8e5184778b5b6


Deepanshu Kartikey (1):
      tracing: Fix WARN_ON in tracing_buffers_mmap_close for split VMAs

----
 kernel/trace/trace.c | 10 ++++++++++
 1 file changed, 10 insertions(+)
---------------------------
commit b042fdf18e89a347177a49e795d8e5184778b5b6
Author: Deepanshu Kartikey <kartikey406@gmail.com>
Date:   Wed Nov 19 12:10:19 2025 +0530

    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 d1e527cf2aae..304e93597126 100644
--- 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)

The pull request you sent on Wed, 26 Nov 2025 15:31:41 -0500:

> git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git trace-ringbuffer-v6.18-rc7

has been merged into torvalds/linux.git:
https://git.kernel.org/torvalds/c/4941a17751c99e17422be743c02c923ad706f888

Thank you!

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