[PATCH V2] KVM: sev: Fix potential overflow send|recieve_update_data

[Peter Gonda <pgonda@google.com>]

KVM_SEV_SEND_UPDATE_DATA and KVM_SEV_RECEIVE_UPDATE_DATA have an integer
overflow issue. Params.guest_len and offset are both 32bite wide, with a
large params.guest_len the check to confirm a page boundary is not
crossed can falsely pass:

    /* Check if we are crossing the page boundary *
    offset = params.guest_uaddr & (PAGE_SIZE - 1);
    if ((params.guest_len + offset > PAGE_SIZE))

Add an additional check to this conditional to confirm that
params.guest_len itself is not greater than PAGE_SIZE.

The current code is can only overflow with a params.guest_len of greater
than 0xfffff000. And the FW spec says these commands fail with lengths
greater than 16KB. So this issue should not be a security concern

Fixes: 15fb7de1a7f5 ("KVM: SVM: Add KVM_SEV_RECEIVE_UPDATE_DATA command")
Fixes: d3d1af85e2c7 ("KVM: SVM: Add KVM_SEND_UPDATE_DATA command")
Reported-by: Andy Nguyen <theflow@google.com>
Suggested-by: Thomas Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Peter Gonda <pgonda@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: kvm@vger.kernel.org
Cc: stable@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---

V2
 * Updated conditional based on feedback from Tom.

---
 arch/x86/kvm/svm/sev.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 273cba809328..3d74facaead8 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1294,7 +1294,7 @@ static int sev_send_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
 
 	/* Check if we are crossing the page boundary */
 	offset = params.guest_uaddr & (PAGE_SIZE - 1);
-	if ((params.guest_len + offset > PAGE_SIZE))
+	if (params.guest_len > PAGE_SIZE || (params.guest_len + offset) > PAGE_SIZE)
 		return -EINVAL;
 
 	/* Pin guest memory */
@@ -1474,7 +1474,7 @@ static int sev_receive_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
 
 	/* Check if we are crossing the page boundary */
 	offset = params.guest_uaddr & (PAGE_SIZE - 1);
-	if ((params.guest_len + offset > PAGE_SIZE))
+	if (params.guest_len > PAGE_SIZE || (params.guest_len + offset) > PAGE_SIZE)
 		return -EINVAL;
 
 	hdr = psp_copy_user_blob(params.hdr_uaddr, params.hdr_len);
-- 
2.39.1.519.gcb327c4b5f-goog