Patch "ring-buffer: Do not attempt to read past "commit"" has been added to the 5.15-stable tree

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This is a note to let you know that I've just added the patch titled

    ring-buffer: Do not attempt to read past "commit"

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     ring-buffer-do-not-attempt-to-read-past-commit.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit db01d7ff7d3bd510d51304053c0eb0f1bdbffd90
Author: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Date:   Thu Sep 7 12:28:20 2023 -0400

    ring-buffer: Do not attempt to read past "commit"
    
    [ Upstream commit 95a404bd60af6c4d9d8db01ad14fe8957ece31ca ]
    
    When iterating over the ring buffer while the ring buffer is active, the
    writer can corrupt the reader. There's barriers to help detect this and
    handle it, but that code missed the case where the last event was at the
    very end of the page and has only 4 bytes left.
    
    The checks to detect the corruption by the writer to reads needs to see the
    length of the event. If the length in the first 4 bytes is zero then the
    length is stored in the second 4 bytes. But if the writer is in the process
    of updating that code, there's a small window where the length in the first
    4 bytes could be zero even though the length is only 4 bytes. That will
    cause rb_event_length() to read the next 4 bytes which could happen to be off the
    allocated page.
    
    To protect against this, fail immediately if the next event pointer is
    less than 8 bytes from the end of the commit (last byte of data), as all
    events must be a minimum of 8 bytes anyway.
    
    Link: https://lore.kernel.org/all/20230905141245.26470-1-Tze-nan.Wu@xxxxxxxxxxxx/
    Link: https://lore.kernel.org/linux-trace-kernel/20230907122820.0899019c@xxxxxxxxxxxxxxxxxx
    
    Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
    Cc: Mark Rutland <mark.rutland@xxxxxxx>
    Reported-by: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx>
    Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index b15d72284c7f7..69db849ae7dad 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2352,6 +2352,11 @@ rb_iter_head_event(struct ring_buffer_iter *iter)
 	 */
 	commit = rb_page_commit(iter_head_page);
 	smp_rmb();
+
+	/* An event needs to be at least 8 bytes in size */
+	if (iter->head > commit - 8)
+		goto reset;
+
 	event = __rb_page_index(iter_head_page, iter->head);
 	length = rb_event_length(event);
 



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