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

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Michal,

> On 19 Aug 2026, at 09:47, Orzel, Michal <Michal.Orzel@xxxxxxx> wrote:
> 
> 
> 
> On 18-Aug-26 14:16, 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.
> This is a bit misleading because it *is* accessed in ffa_msg_send2_vm. The
> comment should say what you wrote as the last paragraph in the commit msg.

Yes, this should be something around:
Ensure validation and use of the message header use the same snapshot.

Do you agree ?

> 
> With that:
> Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
> 

Thanks.

Could that be fixed on commit or do you want me to send a v2 ?

Cheers
Bertrand

> ~Michal
>