Re: [PATCH 6.6.y] KVM: arm64: Check the untrusted offset in FF-A memory share

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Nov 24, 2025 at 09:11:34AM -0500, Sasha Levin wrote:
> From: Sebastian Ene <sebastianene@google.com>
> 
> [ Upstream commit 103e17aac09cdd358133f9e00998b75d6c1f1518 ]
> 
> Verify the offset to prevent OOB access in the hypervisor
> FF-A buffer in case an untrusted large enough value
> [U32_MAX - sizeof(struct ffa_composite_mem_region) + 1, U32_MAX]
> is set from the host kernel.
> 
> Signed-off-by: Sebastian Ene <sebastianene@google.com>
> Acked-by: Will Deacon <will@kernel.org>
> Link: https://patch.msgid.link/20251017075710.2605118-1-sebastianene@google.com
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>  arch/arm64/kvm/hyp/nvhe/ffa.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> index 8d21ab904f1a9..eacf4ba1d88e9 100644
> --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> @@ -425,7 +425,7 @@ static void __do_ffa_mem_xfer(const u64 func_id,
>  	DECLARE_REG(u32, npages_mbz, ctxt, 4);
>  	struct ffa_composite_mem_region *reg;
>  	struct ffa_mem_region *buf;
> -	u32 offset, nr_ranges;
> +	u32 offset, nr_ranges, checked_offset;
>  	int ret = 0;
>  
>  	if (addr_mbz || npages_mbz || fraglen > len ||
> @@ -460,7 +460,12 @@ static void __do_ffa_mem_xfer(const u64 func_id,
>  		goto out_unlock;
>  	}
>  
> -	if (fraglen < offset + sizeof(struct ffa_composite_mem_region)) {
> +	if (check_add_overflow(offset, sizeof(struct ffa_composite_mem_region), &checked_offset)) {
> +		ret = FFA_RET_INVALID_PARAMETERS;
> +		goto out_unlock;
> +	}

I was told that a "straight" backport like this was not correct, so we
need a "better" one :(

Sebastian, can you provide the correct backport for 6.6.y please?

thanks,

greg k-h