Re: [PATCH v3 2/2] KVM: arm64: Validate the offset to the mem access descriptor

[Mostafa Saleh <smostafa@google.com>]

On Tue, May 12, 2026 at 12:44:42PM +0000, Sebastian Ene wrote:
> 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.
> 
> Signed-off-by: Sebastian Ene <sebastianene@google.com>

Reviewed-by: Mostafa Saleh <smostafa@google.com>

Thanks,
Mostafa

> ---
>  arch/arm64/kvm/hyp/nvhe/ffa.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> index 1af722771178..34927bc1239b 100644
> --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> @@ -479,7 +479,7 @@ static void __do_ffa_mem_xfer(const u64 func_id,
>  	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;
> +	u32 offset, nr_ranges, checked_offset, em_mem_access_off;
>  	int ret = 0;
>  
>  	if (addr_mbz || npages_mbz || fraglen > len ||
> @@ -508,8 +508,14 @@ 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 (em_mem_access_off >
> +	    KVM_FFA_MBOX_NR_PAGES * PAGE_SIZE - sizeof(struct ffa_mem_region_attributes)) {
> +		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;
> @@ -576,7 +582,7 @@ static void do_ffa_mem_reclaim(struct arm_smccc_1_2_regs *res,
>  	DECLARE_REG(u32, flags, ctxt, 3);
>  	struct ffa_mem_region_attributes *ep_mem_access;
>  	struct ffa_composite_mem_region *reg;
> -	u32 offset, len, fraglen, fragoff;
> +	u32 offset, len, fraglen, fragoff, em_mem_access_off;
>  	struct ffa_mem_region *buf;
>  	int ret = 0;
>  	u64 handle;
> @@ -599,8 +605,14 @@ 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 (em_mem_access_off >
> +	    KVM_FFA_MBOX_NR_PAGES * PAGE_SIZE - sizeof(struct ffa_mem_region_attributes)) {
> +		ret = FFA_RET_INVALID_PARAMETERS;
> +		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
> -- 
> 2.54.0.563.g4f69b47b94-goog
>