Subject: Re: [PATCH] KVM: arm64: Forward FFA_NOTIFICATION* calls to TrustZone

From: Sebastian Ene <[email-masked]>
Date: Fri, 8 May 2026 13:04:27 +0000

On Thu, May 07, 2026 at 03:21:46PM +0100, Marc Zyngier wrote:
> On Thu, 07 May 2026 15:13:06 +0100,
> Sebastian Ene <[email-masked]> wrote:
> > 
> > On Thu, May 07, 2026 at 02:36:46PM +0100, Marc Zyngier wrote:
> > > On Thu, 07 May 2026 11:48:46 +0100,
> > > Sebastian Ene <[email-masked]> wrote:
> > > > 
> > > > On Wed, May 06, 2026 at 05:29:22PM +0100, Marc Zyngier wrote:
> > > > 
> > > > Hello Marc,
> > > > 
> > > > > [+ Sudeep]
> > > > > 
> > > > > On Fri, 01 May 2026 12:44:48 +0100,
> > > > > Sebastian Ene <[email-masked]> wrote:
> > > > > > 
> > > > > > Remove the FFA_NOTIFICATION* calls from the blocklist used by the pKVM
> > > > > > FF-A proxy. This restriction was preventing the use of asynchronous
> > > > > > signaling mechanisms defined by the Arm FF-A specification to
> > > > > > communicate with the secure services.
> > > > > > While these calls are markes as optional, there is no reason why the
> > > > > > hypervisor proxy would block them because:
> > > > > > 
> > > > > > 1. Host is the Sole Non-Secure Endpoint: The Host operates as the
> > > > > >    only Non-Secure VM ID (VM ID 0) recognized by the Secure World.
> > > > > 
> > > > > Where is this enforced?
> > > > > 
> > > > 
> > > > There is no enforcement in place in the hypervisor since we don't proxy
> > > > FF-A from guest VMs, there is only one non-secure user of this which is the host.
> > > 
> > > And again: what makes that VM ID 0? Why can't the host pick VM ID 32
> > > and use that?
> > > 
> > 
> > The host discovers its id through the FFA_ID_GET and TZ returns 0 in
> 
> Does it? How do you verify this?
> 
 
It is written in the spec under 13.10 FFA_ID_GET ("ID value 0 must be
returned at the Non-secure physical FF-A instance"). If this contract is
broken and TZ in not spec compliant I am afraid there is not too much
that we can do.

> > this case. However if it wants to use VM ID 32 in any other call it
> > absolutely can but what would it be the attack here, what is your
> > concern ?
> 
> Let's be clear: I don't give a damn about a potential attack vector.
> The moment you add Secure to the mix, security is gone (funny, isn't
> it?). I care about being strict about the spec, and not letting
> through things that will eventually break.
>

Understood. 

> > 
> > > > > >    Because all forwarded notifications are inherently attributed to
> > > > > >    the Host by the SPMC, there is no risk of VM ID spoofing
> > > > > >    originating from the Normal World.
> > > > > 
> > > > > I don't understand: either the host is always using VM ID 0, and we
> > > > > have ways to check and enforce this (how?), or the simple fact that
> > > > > the request comes from NS is a guarantee that the SPMC will treat the
> > > > > VM ID as 0.
> > > > > 
> > > > > Which one is it?
> > > > 
> > > > My understanding is that when the hypervisor doesn't handle the allocation of
> > > > the non-secure IDs (through FFA_ID_GET), everything that comes from non-secure
> > > > is treated as having the VM ID 0 by the SPMC.
> > > 
> > > This looks terribly fragile. I'd rather you *enforce* these things
> > > rather than allowing any random stuff from the host and relying on
> > > the EL3 firmware to get it right (odds are that it won't).
> > > 
> > 
> > I can verify the vmid is 0 for the notification calls that I enable.
> 
> Yes, please.
>

Ack.

> > 
> > > This also ties into this:
> > > 
> > > > > > diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > > > > > index 1af722771178..a82d0cd22a17 100644
> > > > > > --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> > > > > > +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > > > > > @@ -675,14 +675,6 @@ static bool ffa_call_supported(u64 func_id)
> > > > > >  	case FFA_RXTX_MAP:
> > > > > >  	case FFA_MEM_DONATE:
> > > > > >  	case FFA_MEM_RETRIEVE_REQ:
> > > > > > -       /* Optional notification interfaces added in FF-A 1.1 */
> > > > > > -	case FFA_NOTIFICATION_BITMAP_CREATE:
> > > > > > -	case FFA_NOTIFICATION_BITMAP_DESTROY:
> > > > > > -	case FFA_NOTIFICATION_BIND:
> > > > > > -	case FFA_NOTIFICATION_UNBIND:
> > > > > > -	case FFA_NOTIFICATION_SET:
> > > > > > -	case FFA_NOTIFICATION_GET:
> > > > > > -	case FFA_NOTIFICATION_INFO_GET:
> > > > > >  	/* Optional interfaces added in FF-A 1.2 */
> > > > > >  	case FFA_MSG_SEND_DIRECT_REQ2:		/* Optional per 7.5.1 */
> > > > > >  	case FFA_MSG_SEND_DIRECT_RESP2:		/* Optional per 7.5.1 */
> > > > > 
> > > > > Shouldn't these be sanitised in a way? A bunch of registers are SBZ in
> > > > > the spec, and I'd expect this to be enforced.
> > > 
> > > which still remains unanswered.
> > 
> > Missed this sorry. We can reject them in the hyp proxy if the caller
> > uses non zero values in those registers.
> 
> I think we need that indeed.

While at it I discovered that none of the FF-A calls in the proxy
currently check for these SBZ registers. Would you be ok with a diff that
fixes this before the patch with the notifications ?

Refactor the handling logic in pKVM FF-A proxy to support checking for
SBZ/MBZ values. While at it, drop the do_ffa_mem_xfer macro and replace
it with two functions that make it clear that we re-write the
function-id with a 64-bit variant, to keep the same behavior as before.
Keep each handler in an array of structures together with a mask that
corresponds to the SBZ registers the spec expects.

diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
index a82d0cd22a17..35443a894172 100644
--- a/arch/arm64/kvm/hyp/nvhe/ffa.c
+++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
@@ -561,13 +561,6 @@ static void __do_ffa_mem_xfer(const u64 func_id,
 	goto out_unlock;
 }
 
-#define do_ffa_mem_xfer(fid, res, ctxt)				\
-	do {							\
-		BUILD_BUG_ON((fid) != FFA_FN64_MEM_SHARE &&	\
-			     (fid) != FFA_FN64_MEM_LEND);	\
-		__do_ffa_mem_xfer((fid), (res), (ctxt));	\
-	} while (0);
-
 static void do_ffa_mem_reclaim(struct arm_smccc_1_2_regs *res,
 			       struct kvm_cpu_context *ctxt)
 {
@@ -854,9 +847,60 @@ static void do_ffa_part_get(struct arm_smccc_1_2_regs *res,
 	hyp_spin_unlock(&host_buffers.lock);
 }
 
+static void do_ffa_mem_share(struct arm_smccc_1_2_regs *res, struct kvm_cpu_context *ctxt)
+{
+	__do_ffa_mem_xfer(FFA_FN64_MEM_SHARE, res, ctxt);
+}
+
+static void do_ffa_mem_lend(struct arm_smccc_1_2_regs *res, struct kvm_cpu_context *ctxt)
+{
+	__do_ffa_mem_xfer(FFA_FN64_MEM_LEND, res, ctxt);
+}
+
+struct ffa_handler {
+	u32 func_id;
+	void (* do_ffa_handle)(struct arm_smccc_1_2_regs *res, struct kvm_cpu_context *ctxt);
+	u32 sbz_mask;
+};
+
+#define REG_RANGE_SBZ	GENMASK
+#define FFA_HANDLER(fid, cb, sbz) {	\
+	.func_id = (fid),	\
+	.do_ffa_handle = (cb),	\
+	.sbz_mask = (sbz),	\
+}
+
+static const struct ffa_handler host_handlers[] = {
+	FFA_HANDLER(FFA_FN64_RXTX_MAP,	do_ffa_rxtx_map,	REG_RANGE_SBZ(17, 4)),
+	FFA_HANDLER(FFA_RXTX_UNMAP,	do_ffa_rxtx_unmap,	REG_RANGE_SBZ(17, 2)),
+	FFA_HANDLER(FFA_MEM_SHARE,	do_ffa_mem_share,	REG_RANGE_SBZ(17, 5)),
+	FFA_HANDLER(FFA_FN64_MEM_SHARE, do_ffa_mem_share,	REG_RANGE_SBZ(17, 5)),
+	FFA_HANDLER(FFA_MEM_RECLAIM,	do_ffa_mem_reclaim,	REG_RANGE_SBZ(17, 4)),
+	FFA_HANDLER(FFA_MEM_LEND,	do_ffa_mem_lend,	REG_RANGE_SBZ(17, 5)),
+	FFA_HANDLER(FFA_FN64_MEM_LEND,	do_ffa_mem_lend,	REG_RANGE_SBZ(17, 5)),
+	FFA_HANDLER(FFA_MEM_FRAG_TX,	do_ffa_mem_frag_tx,	REG_RANGE_SBZ(17, 5)),
+	FFA_HANDLER(FFA_VERSION,	do_ffa_version,		REG_RANGE_SBZ(17, 2)),
+	FFA_HANDLER(FFA_PARTITION_INFO_GET,	do_ffa_part_get,	REG_RANGE_SBZ(17, 6)),
+};
+
+static bool is_sbz_error(const struct ffa_handler *cb, struct kvm_cpu_context *ctxt)
+{
+	int reg_idx, reg_end = fls(cb->sbz_mask);
+
+	if (!ARM_SMCCC_IS_64(cb->func_id) && reg_end > 7)
+		reg_end = 7;
+
+	for (reg_idx = 0; reg_idx <= reg_end; reg_idx++)
+		if (((BIT(reg_idx) & cb->sbz_mask)) && cpu_reg(ctxt, reg_idx))
+			return true;
+
+	return false;
+}
+
 bool kvm_host_ffa_handler(struct kvm_cpu_context *host_ctxt, u32 func_id)
 {
 	struct arm_smccc_1_2_regs res;
+	const struct ffa_handler *cb;
 
 	/*
 	 * There's no way we can tell what a non-standard SMC call might
@@ -880,37 +924,22 @@ bool kvm_host_ffa_handler(struct kvm_cpu_context *host_ctxt, u32 func_id)
 		goto out_handled;
 	}
 
-	switch (func_id) {
-	case FFA_FEATURES:
+	if (func_id == FFA_FEATURES) {
 		if (!do_ffa_features(&res, host_ctxt))
 			return false;
 		goto out_handled;
-	/* Memory management */
-	case FFA_FN64_RXTX_MAP:
-		do_ffa_rxtx_map(&res, host_ctxt);
-		goto out_handled;
-	case FFA_RXTX_UNMAP:
-		do_ffa_rxtx_unmap(&res, host_ctxt);
-		goto out_handled;
-	case FFA_MEM_SHARE:
-	case FFA_FN64_MEM_SHARE:
-		do_ffa_mem_xfer(FFA_FN64_MEM_SHARE, &res, host_ctxt);
-		goto out_handled;
-	case FFA_MEM_RECLAIM:
-		do_ffa_mem_reclaim(&res, host_ctxt);
-		goto out_handled;
-	case FFA_MEM_LEND:
-	case FFA_FN64_MEM_LEND:
-		do_ffa_mem_xfer(FFA_FN64_MEM_LEND, &res, host_ctxt);
-		goto out_handled;
-	case FFA_MEM_FRAG_TX:
-		do_ffa_mem_frag_tx(&res, host_ctxt);
-		goto out_handled;
-	case FFA_VERSION:
-		do_ffa_version(&res, host_ctxt);
-		goto out_handled;
-	case FFA_PARTITION_INFO_GET:
-		do_ffa_part_get(&res, host_ctxt);
+	}
+
+	for (cb = host_handlers; cb < host_handlers + ARRAY_SIZE(host_handlers); cb++) {
+		if (cb->func_id != func_id)
+			continue;
+
+		if (is_sbz_error(cb, host_ctxt)) {
+			ffa_to_smccc_error(&res, FFA_RET_INVALID_PARAMETERS);
+			goto out_handled;
+		}
+
+		cb->do_ffa_handle(&res, host_ctxt);
 		goto out_handled;
 	}
 
-- 
2.54.0.563.g4f69b47b94-goog

> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.

Thanks,
Sebastian