Re: [PATCH v6 5/6] KVM: arm64: Validate the offset to the mem access descriptor

[Sebastian Ene <sebastianene@google.com>]

On Wed, May 27, 2026 at 03:02:35PM +0000, Mostafa Saleh wrote:
> From: Sebastian Ene <sebastianene@google.com>
> 
> Prevent the pKVM hypervisor from making assumptions that the
> endpoint memory access descriptor (EMAD) comes right after the
> FF-A memory region header.
> Prior to FF-A version 1.1 the header of the memory region
> didn't contain an offset to the endpoint memory access descriptor.
> The layout of a memory transaction looks like this from 1.1 onward:
> Type | Field name | Offset
> [ Header | ffa_mem_region  | 0
>   EMAD 1 | ffa_mem_region_attributes) | ffa_mem_region.ep_mem_offset
> ]
> Verify that the offset to the first endpoint memory access descriptor
> is within the mailbox buffer bounds.
> 
> Also, fix one hardcoded sizeof(struct ffa_mem_region_attributes) that
> should be replaced ffa_emad_size_get() for compatibility with FFA v1.0.
> 
> [@Mostafa, Add missing call to ffa_rx_release() and use fraglen
>  as the max buffer size as it is the only intialised part]
> 
> Fixes: 42fb33dde42b ("KVM: arm64: Use FF-A 1.1 with pKVM")
> Signed-off-by: Sebastian Ene <sebastianene@google.com>
> Signed-off-by: Mostafa Saleh <smostafa@google.com>
> ---
>  arch/arm64/kvm/hyp/nvhe/ffa.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> index b6cf9ad82e12..a12e01883314 100644
> --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> @@ -476,10 +476,10 @@ static void __do_ffa_mem_xfer(const u64 func_id,
>  	DECLARE_REG(u32, fraglen, ctxt, 2);
>  	DECLARE_REG(u64, addr_mbz, ctxt, 3);
>  	DECLARE_REG(u32, npages_mbz, ctxt, 4);
> +	u32 offset, nr_ranges, checked_offset, em_mem_access_off;
>  	struct ffa_mem_region_attributes *ep_mem_access;
>  	struct ffa_composite_mem_region *reg;
>  	struct ffa_mem_region *buf;
> -	u32 offset, nr_ranges, checked_offset;
>  	int ret = 0;
>  
>  	if (addr_mbz || npages_mbz || fraglen > len ||
> @@ -489,7 +489,7 @@ static void __do_ffa_mem_xfer(const u64 func_id,
>  	}
>  

Hi Mostafa,

>  	if (fraglen < sizeof(struct ffa_mem_region) +
> -		      sizeof(struct ffa_mem_region_attributes)) {
> +		      ffa_emad_size_get(hyp_ffa_version)) {
>  		ret = FFA_RET_INVALID_PARAMETERS;
>  		goto out;
>  	}

Here sashiko complains about a compatibility break with v1.0 even though
it is not from this change. This is because the size of sizeof(struct
ffa_mem_region) was 36 bytes in v1.0 and now it is 48.

This check can cause "perfectly valid v1.0 memory transfers to be rejected"
(from sashiko).

Other than that this looks good. I will spin a new version of this
series.

> @@ -508,8 +508,13 @@ static void __do_ffa_mem_xfer(const u64 func_id,
>  	buf = hyp_buffers.tx;
>  	memcpy(buf, host_buffers.tx, fraglen);
>  
> -	ep_mem_access = (void *)buf +
> -			ffa_mem_desc_offset(buf, 0, hyp_ffa_version);
> +	em_mem_access_off = ffa_mem_desc_offset(buf, 0, hyp_ffa_version);
> +	if ((u64)em_mem_access_off + ffa_emad_size_get(hyp_ffa_version) > fraglen) {
> +		ret = FFA_RET_INVALID_PARAMETERS;
> +		goto out_unlock;
> +	}
> +
> +	ep_mem_access = (void *)buf + em_mem_access_off;
>  	offset = ep_mem_access->composite_off;
>  	if (!offset || buf->ep_count != 1 || buf->sender_id != HOST_FFA_ID) {
>  		ret = FFA_RET_INVALID_PARAMETERS;
> @@ -574,9 +579,9 @@ static void do_ffa_mem_reclaim(struct arm_smccc_1_2_regs *res,
>  	DECLARE_REG(u32, handle_lo, ctxt, 1);
>  	DECLARE_REG(u32, handle_hi, ctxt, 2);
>  	DECLARE_REG(u32, flags, ctxt, 3);
> +	u32 offset, len, fraglen, fragoff, em_mem_access_off;
>  	struct ffa_mem_region_attributes *ep_mem_access;
>  	struct ffa_composite_mem_region *reg;
> -	u32 offset, len, fraglen, fragoff;
>  	struct ffa_mem_region *buf;
>  	int ret = 0;
>  	u64 handle;
> @@ -599,8 +604,14 @@ static void do_ffa_mem_reclaim(struct arm_smccc_1_2_regs *res,
>  	len = res->a1;
>  	fraglen = res->a2;
>  
> -	ep_mem_access = (void *)buf +
> -			ffa_mem_desc_offset(buf, 0, hyp_ffa_version);
> +	em_mem_access_off = ffa_mem_desc_offset(buf, 0, hyp_ffa_version);
> +	if ((u64)em_mem_access_off + ffa_emad_size_get(hyp_ffa_version) > fraglen) {
> +		ret = FFA_RET_INVALID_PARAMETERS;
> +		ffa_rx_release(res);
> +		goto out_unlock;
> +	}
> +
> +	ep_mem_access = (void *)buf + em_mem_access_off;
>  	offset = ep_mem_access->composite_off;
>  	/*
>  	 * We can trust the SPMD to get this right, but let's at least
> -- 
> 2.54.0.746.g67dd491aae-goog
>

Thanks,
Sebastian