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

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Andrew,

> On 18 Aug 2026, at 14:40, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 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@xxxxxxx>
>> ---
>> 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 ?

Cheers
Bertrand

> 
> ~Andrew