Re: [PATCH v3 4/5] xen/arm: ffa: Add indirect message between VM

[Jens Wiklander <jens.wiklander@xxxxxxxxxx>]

Hi Bertrand,

On Mon, Mar 24, 2025 at 10:15 AM Bertrand Marquis
<bertrand.marquis@xxxxxxx> wrote:
>
> Add support for indirect messages between VMs.
> This is only enabled if CONFIG_FFA_VM_TO_VM is selected.
>
> Signed-off-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
> ---
> Changes in v3:
> - Move vm to vm indirect message handling in a sub function to simplify
>   lock handling and make implementation easier to read
> Changes in v2:
> - Switch ifdef to IS_ENABLED
> ---
>  xen/arch/arm/tee/ffa_msg.c | 96 ++++++++++++++++++++++++++++++++++----
>  1 file changed, 88 insertions(+), 8 deletions(-)
>
> diff --git a/xen/arch/arm/tee/ffa_msg.c b/xen/arch/arm/tee/ffa_msg.c
> index ee594e737fc7..c21083b5e68f 100644
> --- a/xen/arch/arm/tee/ffa_msg.c
> +++ b/xen/arch/arm/tee/ffa_msg.c
> @@ -88,6 +88,72 @@ out:
>                   resp.a7 & mask);
>  }
>
> +static int32_t ffa_msg_send2_vm(uint16_t dst_id,
> +                                const struct ffa_part_msg_rxtx *src_msg)
> +{
> +    struct domain *dst_d;
> +    struct ffa_ctx *dst_ctx;
> +    struct ffa_part_msg_rxtx *dst_msg;
> +    int err;
> +    int32_t ret;
> +
> +    if ( dst_id == 0 )
> +        /* FF-A ID 0 is the hypervisor, this is not valid */
> +        return FFA_RET_INVALID_PARAMETERS;
> +
> +    /* This is also checking that dest is not src */
> +    err = rcu_lock_live_remote_domain_by_id(dst_id - 1, &dst_d);
> +    if ( err )
> +        return FFA_RET_INVALID_PARAMETERS;
> +
> +    if ( dst_d->arch.tee == NULL )
> +    {
> +        ret = FFA_RET_INVALID_PARAMETERS;
> +        goto out_unlock;
> +    }
> +
> +    dst_ctx = dst_d->arch.tee;
> +    if ( !dst_ctx->guest_vers )
> +    {
> +        ret = FFA_RET_INVALID_PARAMETERS;
> +        goto out_unlock;
> +    }
> +
> +    /* we need to have enough space in the destination buffer */
> +    if ( dst_ctx->page_count * FFA_PAGE_SIZE <
> +            (sizeof(struct ffa_part_msg_rxtx) + src_msg->msg_size) )
> +    {
> +        ret = FFA_RET_NO_MEMORY;
> +        goto out_unlock;
> +    }
> +
> +    /* This also checks that destination has set a Rx buffer */
> +    ret = ffa_rx_acquire(dst_d);
> +    if ( ret )
> +        goto out_unlock;
> +
> +    dst_msg = dst_ctx->rx;
> +
> +    /* prepare destination header */
> +    dst_msg->flags = 0;
> +    dst_msg->reserved = 0;
> +    dst_msg->msg_offset = sizeof(struct ffa_part_msg_rxtx);
> +    dst_msg->send_recv_id = src_msg->send_recv_id;
> +    dst_msg->msg_size = src_msg->msg_size;
> +
> +    memcpy(dst_ctx->rx + sizeof(struct ffa_part_msg_rxtx),
> +           ((void *)src_msg) + src_msg->msg_offset, src_msg->msg_size);

Is it possible for another core in the VM to modify
src_msg->msg_offset or src_msg->msg_size after we check them but
before we use them here (TOC/TOU)?

Cheers,
Jens

> +
> +    /* receiver rx buffer will be released by the receiver*/
> +
> +out_unlock:
> +    rcu_unlock_domain(dst_d);
> +    if ( !ret )
> +        ffa_raise_rx_buffer_full(dst_d);
> +
> +    return ret;
> +}
> +
>  int32_t ffa_handle_msg_send2(struct cpu_user_regs *regs)
>  {
>      struct domain *src_d = current->domain;
> @@ -96,9 +162,6 @@ int32_t ffa_handle_msg_send2(struct cpu_user_regs *regs)
>      uint16_t dst_id, src_id;
>      int32_t ret;
>
> -    if ( !ffa_fw_supports_fid(FFA_MSG_SEND2) )
> -        return FFA_RET_NOT_SUPPORTED;
> -
>      if ( !spin_trylock(&src_ctx->tx_lock) )
>          return FFA_RET_BUSY;
>
> @@ -106,10 +169,10 @@ int32_t ffa_handle_msg_send2(struct cpu_user_regs *regs)
>      src_id = src_msg->send_recv_id >> 16;
>      dst_id = src_msg->send_recv_id & GENMASK(15,0);
>
> -    if ( src_id != ffa_get_vm_id(src_d) || !FFA_ID_IS_SECURE(dst_id) )
> +    if ( src_id != ffa_get_vm_id(src_d) )
>      {
>          ret = FFA_RET_INVALID_PARAMETERS;
> -        goto out_unlock_tx;
> +        goto out;
>      }
>
>      /* check source message fits in buffer */
> @@ -118,13 +181,30 @@ int32_t ffa_handle_msg_send2(struct cpu_user_regs *regs)
>           src_msg->msg_offset < sizeof(struct ffa_part_msg_rxtx) )
>      {
>          ret = FFA_RET_INVALID_PARAMETERS;
> -        goto out_unlock_tx;
> +        goto out;
>      }
>
> -    ret = ffa_simple_call(FFA_MSG_SEND2,
> +    if ( FFA_ID_IS_SECURE(dst_id) )
> +    {
> +        /* Message for a secure partition */
> +        if ( !ffa_fw_supports_fid(FFA_MSG_SEND2) )
> +        {
> +            ret = FFA_RET_NOT_SUPPORTED;
> +            goto out;
> +        }
> +
> +        ret = ffa_simple_call(FFA_MSG_SEND2,
>                            ((uint32_t)ffa_get_vm_id(src_d)) << 16, 0, 0, 0);
> +    }
> +    else if ( IS_ENABLED(CONFIG_FFA_VM_TO_VM) )
> +    {
> +        /* Message for a VM */
> +        ret = ffa_msg_send2_vm(dst_id, src_msg);
> +    }
> +    else
> +        ret = FFA_RET_INVALID_PARAMETERS;
>
> -out_unlock_tx:
> +out:
>      spin_unlock(&src_ctx->tx_lock);
>      return ret;
>  }
> --
> 2.47.1
>