[PATCH] ring-buffer: Fix 32-bit rb_time_read() race with rb_time_cmpxchg()

[Mathieu Desnoyers]

The following race can cause rb_time_read() to observe a corrupted time
stamp:

rb_time_cmpxchg()
[...]
        if (!rb_time_read_cmpxchg(&t->msb, msb, msb2))
                return false;
        if (!rb_time_read_cmpxchg(&t->top, top, top2))
                return false;
<interrupted before updating bottom>
__rb_time_read()
[...]
        do {
                c = local_read(&t->cnt);
                top = local_read(&t->top);
                bottom = local_read(&t->bottom);
                msb = local_read(&t->msb);
        } while (c != local_read(&t->cnt));

        *cnt = rb_time_cnt(top);

        /* If top and msb counts don't match, this interrupted a write */
        if (*cnt != rb_time_cnt(msb))
                return false;
          ^ this check fails to catch that "bottom" is still not updated.

So the old "bottom" value is returned, which is wrong.

Fix this by checking that all three of msb, top, and bottom 2-bit cnt
values match.

The reason to favor checking all three fields over requiring a specific
update order for both rb_time_set() and rb_time_cmpxchg() is because
checking all three fields is more robust to handle partial failures of
rb_time_cmpxchg() when interrupted by nested rb_time_set().

Link: https://lore.kernel.org/lkml/20231211201324.652870-1-mathieu.desnoyers@xxxxxxxxxxxx/
Fixes: f458a1453424e ("ring-buffer: Test last update in 32bit version of __rb_time_read()")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: linux-trace-kernel@xxxxxxxxxxxxxxx
---
 kernel/trace/ring_buffer.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 8d2a4f00eca9..71c225ca2a2b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -644,8 +644,8 @@ static inline bool __rb_time_read(rb_time_t *t, u64 *ret, unsigned long *cnt)
 
 	*cnt = rb_time_cnt(top);
 
-	/* If top and msb counts don't match, this interrupted a write */
-	if (*cnt != rb_time_cnt(msb))
+	/* If top, msb or bottom counts don't match, this interrupted a write */
+	if (*cnt != rb_time_cnt(msb) || *cnt != rb_time_cnt(bottom))
 		return false;
 
 	/* The shift to msb will lose its cnt bits */
-- 
2.39.2