Re: [PATCH] ring-buffer: Fix a race in rb_time_cmpxchg() for 32 bit archs

[Mathieu Desnoyers]

On 2023-12-12 11:53, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
>
> 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@xxxxxxxxxxxx/
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 10464b4aa605e ("ring-buffer: Add rb_time_t 64 bit operations for speeding up 32 bit")
> Reported-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

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

> ---
>   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