Re: [PATCH v3] bus: mhi: host: Use cached values for calculating the shared write pointer

[Manivannan Sadhasivam <mani@kernel.org>]

On Wed, Apr 13, 2022 at 10:41:53AM -0600, Jeffrey Hugo wrote:
> mhi_recycle_ev_ring() computes the shared write pointer for the ring
> (ctxt_wp) using a read/modify/write pattern where the ctxt_wp value in the
> shared memory is read, incremented, and written back.  There are no checks
> on the read value, it is assumed that it is kept in sync with the locally
> cached value.  Per the MHI spec, this is correct.  The device should only
> read ctxt_wp, never write it.
> 
> However, there are devices in the wild that violate the spec, and can
> update the ctxt_wp in a specific scenario.  This can cause corruption, and
> violate the above assumption that the ctxt_wp is in sync with the cached
> value.
> 
> This can occur when the device has loaded firmware from the host, and is
> transitioning from the SBL EE to the AMSS EE.  As part of shutting down
> SBL, the SBL flushes it's local MHI context to the shared memory since
> the local context will not persist across an EE change.  In the case of
> the event ring, SBL will flush its entire context, not just the parts that
> it is allowed to update.  This means SBL will write to ctxt_wp, and
> possibly corrupt it.
> 
> An example:
> 
> Host				Device
> ----				---
> Update ctxt_wp to 0x1f0
> 				SBL observes 0x1f0
> Update ctxt_wp to 0x0
> 				Starts transition to AMSS EE
> 				Context flush, writes 0x1f0 to ctxt_wp
> Update ctxt_wp to 0x200
> Update ctxt_wp to 0x210
> 				AMSS observes 0x210
> 				0x210 exceeds ring size
> 				AMSS signals syserr
> 
> The reason the ctxt_wp goes off the end of the ring is that the rollover
> check is only performed on the cached wp, which is out of sync with
> ctxt_wp.
> 
> Since the host is the authority of the value of ctxt_wp per the MHI spec,
> we can fix this issue by not reading ctxt_wp from the shared memory, and
> instead compute it based on the cached value.  If SBL corrupts ctxt_wp,
> the host won't observe it, and will correct the value at some point later.
> 
> Signed-off-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
> Reviewed-by: Hemant Kumar <hemantk@codeaurora.org>
> Reviewed-by: Bhaumik Bhatt <bbhatt@codeaurora.org>

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

Thanks,
Mani

> ---
> 
> v3:
> Rebase to -next
> 
> v2:
> Fix typo on the ring base
> 
>  drivers/bus/mhi/host/main.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
> index 142eea1..f3aef77a 100644
> --- a/drivers/bus/mhi/host/main.c
> +++ b/drivers/bus/mhi/host/main.c
> @@ -534,18 +534,13 @@ irqreturn_t mhi_intvec_handler(int irq_number, void *dev)
>  static void mhi_recycle_ev_ring_element(struct mhi_controller *mhi_cntrl,
>  					struct mhi_ring *ring)
>  {
> -	dma_addr_t ctxt_wp;
> -
>  	/* Update the WP */
>  	ring->wp += ring->el_size;
> -	ctxt_wp = le64_to_cpu(*ring->ctxt_wp) + ring->el_size;
>  
> -	if (ring->wp >= (ring->base + ring->len)) {
> +	if (ring->wp >= (ring->base + ring->len))
>  		ring->wp = ring->base;
> -		ctxt_wp = ring->iommu_base;
> -	}
>  
> -	*ring->ctxt_wp = cpu_to_le64(ctxt_wp);
> +	*ring->ctxt_wp = cpu_to_le64(ring->iommu_base + (ring->wp - ring->base));
>  
>  	/* Update the RP */
>  	ring->rp += ring->el_size;
> -- 
> 2.7.4
>