Patch "KVM: s390: SCA must not cross page boundaries" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

KVM: s390: SCA must not cross page boundaries

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-s390-sca-must-not-cross-page-boundaries.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: David Hildenbrand <[email protected]>
Date: Mon, 26 Oct 2015 08:41:29 +0100
Subject: KVM: s390: SCA must not cross page boundaries

From: David Hildenbrand <[email protected]>

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.

---
 arch/s390/kvm/kvm-s390.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

@@ -1031,7 +1031,9 @@ int kvm_arch_init_vm(struct kvm *kvm, un
 	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);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/kvm-s390-sca-must-not-cross-page-boundaries.patch queue-4.1/kvm-s390-fix-wrong-lookup-of-vcpus-by-array-index.patch queue-4.1/kvm-s390-avoid-memory-overwrites-on-emergency-signal-injection.patch