Re: [PATCH] bus: mhi: host: Add alignment check for event ring read pointer

[Manivannan Sadhasivam <mani@kernel.org>]

On Fri, Oct 27, 2023 at 08:19:44AM -0600, Jeffrey Hugo wrote:
> On 10/27/2023 7:09 AM, Manivannan Sadhasivam wrote:
> > On Mon, Oct 23, 2023 at 03:13:06PM +0530, Krishna chaitanya chundru wrote:
> > > Though we do check the event ring read pointer by "is_valid_ring_ptr"
> > > to make sure it is in the buffer range, but there is another risk the
> > > pointer may be not aligned.  Since we are expecting event ring elements
> > > are 128 bits(struct mhi_tre) aligned, an unaligned read pointer could lead
> > 
> > "mhi_tre" got renamed to "mhi_ring_element"
> > 
> > > to multiple issues like DoS or ring buffer memory corruption.
> > > 
> > > So add a alignment check for event ring read pointer.
> > > 
> > 
> > Since this is a potential fix, you should add the fixes tag and CC stable.
> > 
> > > Signed-off-by: Krishna chaitanya chundru <quic_krichai@quicinc.com>
> > > ---
> > >   drivers/bus/mhi/host/main.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
> > > index 499590437e9b..c907bbb67fb2 100644
> > > --- a/drivers/bus/mhi/host/main.c
> > > +++ b/drivers/bus/mhi/host/main.c
> > > @@ -268,7 +268,7 @@ static void mhi_del_ring_element(struct mhi_controller *mhi_cntrl,
> > >   static bool is_valid_ring_ptr(struct mhi_ring *ring, dma_addr_t addr)
> > >   {
> > > -	return addr >= ring->iommu_base && addr < ring->iommu_base + ring->len;
> > > +	return addr >= ring->iommu_base && addr < ring->iommu_base + ring->len && addr % 16 == 0;
> > 
> > How about,
> > 
> > !(addr % 16)
> 
> We are guaranteed that the ring allocation is 16 byte aligned, right?
> 
> I think using "struct mhi_ring_element" instead of "16" would be better.
> 
> I'm also thinking that perhaps doing a bit-wise & with a mask would be
> better than the % operator.  Not only is that how these alignment checks
> seem to normally be done elsewhere, but this check is in a critical patch
> for the MHI stack.
> 

Yes, both of your suggestions sounds good to me.

Chaitanya, please use below check:

	!(addr & (sizeof(struct mhi_ring_element) - 1))

- Mani

> -Jeff
> 

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