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SubjectRe: [PATCH] ring-buffer: Fix a race in rb_time_cmpxchg() for 32 bit archs
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On 2023-12-12 11:53, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
>
> Mathieu Desnoyers pointed out an issue in the rb_time_cmpxchg() for 32 bit
> architectures. That is:
>
> static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
> {
> unsigned long cnt, top, bottom, msb;
> unsigned long cnt2, top2, bottom2, msb2;
> u64 val;
>
> /* The cmpxchg always fails if it interrupted an update */
> if (!__rb_time_read(t, &val, &cnt2))
> return false;
>
> if (val != expect)
> return false;
>
> <<<< interrupted here!
>
> cnt = local_read(&t->cnt);
>
> The problem is that the synchronization counter in the rb_time_t is read
> *after* the value of the timestamp is read. That means if an interrupt
> were to come in between the value being read and the counter being read,
> it can change the value and the counter and the interrupted process would
> be clueless about it!
>
> The counter needs to be read first and then the value. That way it is easy
> to tell if the value is stale or not. If the counter hasn't been updated,
> then the value is still good.
>
> Link: https://lore.kernel.org/linux-trace-kernel/20231211201324.652870-1-mathieu.desnoyers@efficios.com/
>
> Cc: stable@vger.kernel.org
> Fixes: 10464b4aa605e ("ring-buffer: Add rb_time_t 64 bit operations for speeding up 32 bit")
> Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

> ---
> kernel/trace/ring_buffer.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 1d9caee7f542..e110cde685ea 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -706,6 +706,9 @@ static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
> unsigned long cnt2, top2, bottom2, msb2;
> u64 val;
>
> + /* Any interruptions in this function should cause a failure */
> + cnt = local_read(&t->cnt);
> +
> /* The cmpxchg always fails if it interrupted an update */
> if (!__rb_time_read(t, &val, &cnt2))
> return false;
> @@ -713,7 +716,6 @@ static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
> if (val != expect)
> return false;
>
> - cnt = local_read(&t->cnt);
> if ((cnt & 3) != cnt2)
> return false;
>

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

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