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

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

> On 19 Aug 2026, at 10:09, Orzel, Michal <michal.orzel@xxxxxxx> wrote:
> 
> 
> 
> On 19-Aug-26 10:01, Bertrand Marquis wrote:
>> 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 ?
> Yes. I'll fix on commit.

Thanks :-)

Bertrand

> 
> ~Michal