[PATCH 3.16.y-ckt 045/126] KVM: s390: SCA must not cross page boundaries

[Luis Henriques]

3.16.7-ckt21 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Hildenbrand <dahi@xxxxxxxxxxxxxxxxxx>

commit c5c2c393468576bad6d10b2b5fefff8cd25df3f4 upstream.

We seemed to have missed a few corner cases in commit f6c137ff00a4
("KVM: s390: randomize sca address").

The SCA has a maximum size of 2112 bytes. By setting the sca_offset to
some unlucky numbers, we exceed the page.

0x7c0 (1984) -> Fits exactly
0x7d0 (2000) -> 16 bytes out
0x7e0 (2016) -> 32 bytes out
0x7f0 (2032) -> 48 bytes out

One VCPU entry is 32 bytes long.

For the last two cases, we actually write data to the other page.
1. The address of the VCPU.
2. Injection/delivery/clearing of SIGP externall calls via SIGP IF.

Especially the 2. happens regularly. So this could produce two problems:
1. The guest losing/getting external calls.
2. Random memory overwrites in the host.

So this problem happens on every 127 + 128 created VM with 64 VCPUs.

Acked-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Signed-off-by: David Hildenbrand <dahi@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 arch/s390/kvm/kvm-s390.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 5e9217d96cfe..fd8a8373c275 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -429,7 +429,9 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	if (!kvm->arch.sca)
 		goto out_err;
 	spin_lock(&kvm_lock);
-	sca_offset = (sca_offset + 16) & 0x7f0;
+	sca_offset += 16;
+	if (sca_offset + sizeof(struct sca_block) > PAGE_SIZE)
+		sca_offset = 0;
 	kvm->arch.sca = (struct sca_block *) ((char *) kvm->arch.sca + sca_offset);
 	spin_unlock(&kvm_lock);
 
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