[PATCH v2 2/2] KVM: arm64: Validate the offset to the mem access descriptor

[Sebastian Ene <sebastianene@google.com>]

Prevent the pKVM hypervisor from making assumptions that the
endpoint memory access descriptor (EMAD) comes right after the
FF-A memory region header.
Prior to FF-A version 1.1 the header of the memory region
didn't contain an offset to the endpoint memory access descriptor.
The layout of a memory transaction looks like this from 1.1 onward:
Type | Field name | Offset
[ Header | ffa_mem_region  | 0
  EMAD 1 | ffa_mem_region_attributes) | ffa_mem_region.ep_mem_offset
]
Verify that the offset to the first endpoint memory access descriptor
is within the mailbox buffer bounds.

Signed-off-by: Sebastian Ene <sebastianene@google.com>

---
v1 -> v2:
 * don't enforce a strict placement for the ep_mem_offset since this is
   not compliant with the spec and we should not make assumptions about
   the driver.

Link to v1: https://lore.kernel.org/all/ae9KN9nkOgDYJcGP@google.com/T/#t

---
---
 arch/arm64/kvm/hyp/nvhe/ffa.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
index 1af722771178..8e9017fc8e39 100644
--- a/arch/arm64/kvm/hyp/nvhe/ffa.c
+++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
@@ -479,7 +479,7 @@ static void __do_ffa_mem_xfer(const u64 func_id,
 	struct ffa_mem_region_attributes *ep_mem_access;
 	struct ffa_composite_mem_region *reg;
 	struct ffa_mem_region *buf;
-	u32 offset, nr_ranges, checked_offset;
+	u32 offset, nr_ranges, checked_offset, em_mem_access_off;
 	int ret = 0;
 
 	if (addr_mbz || npages_mbz || fraglen > len ||
@@ -508,8 +508,14 @@ static void __do_ffa_mem_xfer(const u64 func_id,
 	buf = hyp_buffers.tx;
 	memcpy(buf, host_buffers.tx, fraglen);
 
-	ep_mem_access = (void *)buf +
-			ffa_mem_desc_offset(buf, 0, hyp_ffa_version);
+	em_mem_access_off = ffa_mem_desc_offset(buf, 0, hyp_ffa_version);
+	if (em_mem_access_off >
+	    KVM_FFA_MBOX_NR_PAGES * PAGE_SIZE - (struct ffa_mem_region_attributes)) {
+		ret = FFA_RET_INVALID_PARAMETERS;
+		goto out_unlock;
+	}
+
+	ep_mem_access = (void *)buf + em_mem_access_off;
 	offset = ep_mem_access->composite_off;
 	if (!offset || buf->ep_count != 1 || buf->sender_id != HOST_FFA_ID) {
 		ret = FFA_RET_INVALID_PARAMETERS;
@@ -576,7 +582,7 @@ static void do_ffa_mem_reclaim(struct arm_smccc_1_2_regs *res,
 	DECLARE_REG(u32, flags, ctxt, 3);
 	struct ffa_mem_region_attributes *ep_mem_access;
 	struct ffa_composite_mem_region *reg;
-	u32 offset, len, fraglen, fragoff;
+	u32 offset, len, fraglen, fragoff, em_mem_access_off;
 	struct ffa_mem_region *buf;
 	int ret = 0;
 	u64 handle;
@@ -599,8 +605,14 @@ static void do_ffa_mem_reclaim(struct arm_smccc_1_2_regs *res,
 	len = res->a1;
 	fraglen = res->a2;
 
-	ep_mem_access = (void *)buf +
-			ffa_mem_desc_offset(buf, 0, hyp_ffa_version);
+	em_mem_access_off = ffa_mem_desc_offset(buf, 0, hyp_ffa_version);
+	if (em_mem_access_off >
+	    KVM_FFA_MBOX_NR_PAGES * PAGE_SIZE - (struct ffa_mem_region_attributes)) {
+		ret = FFA_RET_INVALID_PARAMETERS;
+		goto out_unlock;
+	}
+
+	ep_mem_access = (void *)buf + em_mem_access_off;
 	offset = ep_mem_access->composite_off;
 	/*
 	 * We can trust the SPMD to get this right, but let's at least
-- 
2.54.0.545.g6539524ca2-goog