Re: [PATCH v5 13/15] coresight: trbe: Work around write to out of range

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 10/15/21 4:01 AM, Suzuki K Poulose wrote:
> TRBE implementations affected by Arm erratum (2253138 or 2224489), could
> write to the next address after the TRBLIMITR.LIMIT, instead of wrapping
> to the TRBBASER. This implies that the TRBE could potentially corrupt :
> 
>   - A page used by the rest of the kernel/user (if the LIMIT = end of
>     perf ring buffer)
>   - A page within the ring buffer, but outside the driver's range.
>     [head, head + size]. This may contain some trace data, may be
>     consumed by the userspace.
> 
> We workaround this erratum by :
>   - Making sure that there is at least an extra PAGE space left in the
>     TRBE's range than we normally assign. This will be additional to other
>     restrictions (e.g, the TRBE alignment for working around
>     TRBE_WORKAROUND_OVERWRITE_IN_FILL_MODE, where there is a minimum of
>     PAGE_SIZE. Thus we would have 2 * PAGE_SIZE)
> 
>   - Adjust the LIMIT to leave the last PAGE_SIZE out of the TRBE's allowed
>     range (i.e, TRBEBASER...TRBLIMITR.LIMIT), by :
> 
>         TRBLIMITR.LIMIT -= PAGE_SIZE
> 
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
> Cc: Mike Leach <mike.leach@linaro.org>
> Cc: Leo Yan <leo.yan@linaro.org>
> Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

> ---
>  drivers/hwtracing/coresight/coresight-trbe.c | 63 +++++++++++++++++++-
>  1 file changed, 62 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> index 5fb9f49eab33..eca2e7e94079 100644
> --- a/drivers/hwtracing/coresight/coresight-trbe.c
> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> @@ -90,9 +90,11 @@ struct trbe_buf {
>   *   - Streamlined detection of erratum across the system
>   */
>  #define TRBE_WORKAROUND_OVERWRITE_FILL_MODE	0
> +#define TRBE_WORKAROUND_WRITE_OUT_OF_RANGE	1
>  
>  static int trbe_errata_cpucaps[] = {
>  	[TRBE_WORKAROUND_OVERWRITE_FILL_MODE] = ARM64_WORKAROUND_TRBE_OVERWRITE_FILL_MODE,
> +	[TRBE_WORKAROUND_WRITE_OUT_OF_RANGE] = ARM64_WORKAROUND_TRBE_WRITE_OUT_OF_RANGE,
>  	-1,		/* Sentinel, must be the last entry */
>  };
>  
> @@ -160,6 +162,11 @@ static inline bool trbe_may_overwrite_in_fill_mode(struct trbe_cpudata *cpudata)
>  	return trbe_has_erratum(cpudata, TRBE_WORKAROUND_OVERWRITE_FILL_MODE);
>  }
>  
> +static inline bool trbe_may_write_out_of_range(struct trbe_cpudata *cpudata)
> +{
> +	return trbe_has_erratum(cpudata, TRBE_WORKAROUND_WRITE_OUT_OF_RANGE);
> +}
> +
>  static int trbe_alloc_node(struct perf_event *event)
>  {
>  	if (event->cpu == -1)
> @@ -305,7 +312,21 @@ static unsigned long trbe_snapshot_offset(struct perf_output_handle *handle)
>  
>  static u64 trbe_min_trace_buf_size(struct perf_output_handle *handle)
>  {
> -	return TRBE_TRACE_MIN_BUF_SIZE;
> +	u64 size = TRBE_TRACE_MIN_BUF_SIZE;
> +	struct trbe_buf *buf = etm_perf_sink_config(handle);
> +	struct trbe_cpudata *cpudata = buf->cpudata;
> +
> +	/*
> +	 * When the TRBE is affected by an erratum that could make it
> +	 * write to the next "virtually addressed" page beyond the LIMIT.
> +	 * We need to make sure there is always a PAGE after the LIMIT,
> +	 * within the buffer. Thus we ensure there is at least an extra
> +	 * page than normal. With this we could then adjust the LIMIT
> +	 * pointer down by a PAGE later.
> +	 */
> +	if (trbe_may_write_out_of_range(cpudata))
> +		size += PAGE_SIZE;
> +	return size;
>  }
>  
>  /*
> @@ -611,6 +632,17 @@ static unsigned long trbe_get_trace_size(struct perf_output_handle *handle,
>  	/*
>  	 * If the TRBE has wrapped around the write pointer has
>  	 * wrapped and should be treated as limit.
> +	 *
> +	 * When the TRBE is affected by TRBE_WORKAROUND_WRITE_OUT_OF_RANGE,
> +	 * it may write upto 64bytes beyond the "LIMIT". The driver already
> +	 * keeps a valid page next to the LIMIT and we could potentially
> +	 * consume the trace data that may have been collected there. But we
> +	 * cannot be really sure it is available, and the TRBPTR may not
> +	 * indicate the same. Also, affected cores are also affected by another
> +	 * erratum which forces the PAGE_SIZE alignment on the TRBPTR, and thus
> +	 * could potentially pad an entire PAGE_SIZE - 64bytes, to get those
> +	 * 64bytes. Thus we ignore the potential triggering of the erratum
> +	 * on WRAP and limit the data to LIMIT.
>  	 */
>  	if (wrap)
>  		write = get_trbe_limit_pointer();
> @@ -864,6 +896,35 @@ static int trbe_apply_work_around_before_enable(struct trbe_buf *buf)
>  		buf->trbe_write += TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP_BYTES;
>  	}
>  
> +	/*
> +	 * TRBE_WORKAROUND_WRITE_OUT_OF_RANGE could cause the TRBE to write to
> +	 * the next page after the TRBLIMITR.LIMIT. For perf, the "next page"
> +	 * may be:
> +	 *     - The page beyond the ring buffer. This could mean, TRBE could
> +	 *       corrupt another entity (kernel / user)
> +	 *     - A portion of the "ring buffer" consumed by the userspace.
> +	 *       i.e, a page outisde [head, head + size].
> +	 *
> +	 * We work around this by:
> +	 *     - Making sure that we have at least an extra space of PAGE left
> +	 *       in the ring buffer [head, head + size], than we normally do
> +	 *       without the erratum. See trbe_min_trace_buf_size().
> +	 *
> +	 *     - Adjust the TRBLIMITR.LIMIT to leave the extra PAGE outside
> +	 *       the TRBE's range (i.e [TRBBASER, TRBLIMITR.LIMI] ).
> +	 */
> +	if (trbe_has_erratum(buf->cpudata, TRBE_WORKAROUND_WRITE_OUT_OF_RANGE)) {
> +		s64 space = buf->trbe_limit - buf->trbe_write;
> +		/*
> +		 * We must have more than a PAGE_SIZE worth space in the proposed
> +		 * range for the TRBE.
> +		 */
> +		if (WARN_ON(space <= PAGE_SIZE ||
> +			    !IS_ALIGNED(buf->trbe_limit, PAGE_SIZE)))
> +			return -EINVAL;
> +		buf->trbe_limit -= PAGE_SIZE;
> +	}
> +
>  	return 0;
>  }
>  
>