Re: [PATCH] xen/arm: ffa: Harden SEND2 against invented loads

On 18/08/2026 1:16 pm, Bertrand Marquis wrote:
> Research into compiler-invented loads has flagged FFA_MSG_SEND2 as a
> possible vulnerability.
>
> ffa_handle_msg_send2() copies the message header from the guest-writable
> TX buffer before validating and using its fields. A plain structure copy
> does not prevent the compiler from re-deriving later field accesses from
> the live TX mapping.
>
> For VM-to-VM messages, msg_offset and msg_size are validated against the
> source and destination buffers, then used to copy the payload. If a
> sibling vCPU changes the header and the compiler reloads either field,
> the checked and used values can differ. This can cause an out-of-bounds
> read from the sender's TX buffer or an out-of-bounds write into the
> receiver's RX buffer.
>
> The cross-VM path is gated by CONFIG_FFA_VM_TO_VM, which is disabled by
> default. The audit ranks the likelihood of such a reload as low, but the
> C semantics do not guarantee that later accesses use the stack copy.
>
> Add a compiler barrier immediately after copying the header so that
> validation and use consume the same snapshot.
>
> Link: https://github.com/xoreaxeaxeax/schrodingers-toctou/blob/main/observer-effect/audits/audit-xen-tee-mediator-RELEASE-4.21.1.md#tm-2--ff-a-txrx-buffers-ffa_shmc-ffa_msgc
> Fixes: 98af565b1e61 ("xen/arm: ffa: Add indirect message between VM")
> Signed-off-by: Bertrand Marquis <[email protected]>
> ---
>  xen/arch/arm/tee/ffa_msg.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/xen/arch/arm/tee/ffa_msg.c b/xen/arch/arm/tee/ffa_msg.c
> index 1eadc62870f2..39f561c8237f 100644
> --- a/xen/arch/arm/tee/ffa_msg.c
> +++ b/xen/arch/arm/tee/ffa_msg.c
> @@ -257,6 +257,11 @@ int32_t ffa_handle_msg_send2(struct cpu_user_regs *regs)
>  
>      /* create a copy of the message header */
>      memcpy(&src_msg, tx_buf, sizeof(src_msg));
> +    /*
> +     * Make sure that "tx_buf" which is shared with the guest isn't accessed
> +     * again after this point.
> +     */
> +    barrier();
>  
>      src_id = src_msg.send_recv_id >> 16;
>      dst_id = src_msg.send_recv_id & GENMASK(15,0);

This does look to be adequate to fix the potential issue, but you should
drop the ACCESS_ONCE(src_ctx->guest_vers) a little lower down.

With a safe copy on the stack, there's no need to further inhibit
optimisations around it.  In fact, it's unclear why e040b94d0fff added
the ACCESS_ONCE() in the first place, seeing as it was already an
on-stack object at the time.

~Andrew