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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 18/08/2026 2:28 pm, Bertrand Marquis wrote:
> Hi Andrew,
>
>> On 18 Aug 2026, at 14:40, Andrew Cooper <andrew.cooper3@citrix.com> 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 <bertrand.marquis@arm.com>
>>> ---
>>> 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.

~Andrew