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

Hi Andrew,

> On 18 Aug 2026, at 16:46, Andrew Cooper <[email protected]> wrote:
> 
> On 18/08/2026 2:28 pm, Bertrand Marquis wrote:
>> Hi Andrew,
>> 
>>> On 18 Aug 2026, at 14:40, Andrew Cooper <[email protected]> wrote:
>>> 
>>> 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.
>> the ACCESS_ONCE is protecting the access to guest_vers which is not on the stack
>> but a value on an internal context accessed by all VMs.
>> 
>> You probably mixed src_MSG with src_CTX ?
> Oh, maybe.  Those really ought to have more distinct names.

No worries. 
Would you consider renaming them a requirement for this patch? 
If not, I would prefer to keep this fix focused on adding the barrier and avoid unrelated churn.

Cheers
Bertrand

> 
> ~Andrew