KVM: x86: invalid opcode oops on SET_SREGS with OSXSAVE bit set (CVE-2012-4461)

commit 6d1068b3a98519247d8ba4ec85cd40ac136dbdf9 upstream.

On hosts without the XSAVE support unprivileged local user can trigger
oops similar to the one below by setting X86_CR4_OSXSAVE bit in guest
cr4 register using KVM_SET_SREGS ioctl and later issuing KVM_RUN
ioctl.

invalid opcode: 0000 [#2] SMP
Modules linked in: tun ip6table_filter ip6_tables ebtable_nat ebtables
...
Pid: 24935, comm: zoog_kvm_monito Tainted: G      D      3.2.0-3-686-pae
EIP: 0060:[<f8b9550c>] EFLAGS: 00210246 CPU: 0
EIP is at kvm_arch_vcpu_ioctl_run+0x92a/0xd13 [kvm]
EAX: 00000001 EBX: 000f387e ECX: 00000000 EDX: 00000000
ESI: 00000000 EDI: 00000000 EBP: ef5a0060 ESP: d7c63e70
 DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
Process zoog_kvm_monito (pid: 24935, ti=d7c62000 task=ed84a0c0
task.ti=d7c62000)
Stack:
 00000001 f70a1200 f8b940a9 ef5a0060 00000000 00200202 f8769009 00000000
 ef5a0060 000f387e eda5c020 8722f9c8 00015bae 00000000 ed84a0c0 ed84a0c0
 c12bf02d 0000ae80 ef7f8740 fffffffb f359b740 ef5a0060 f8b85dc1 0000ae80
Call Trace:
 [<f8b940a9>] ? kvm_arch_vcpu_ioctl_set_sregs+0x2fe/0x308 [kvm]
...
 [<c12bfb44>] ? syscall_call+0x7/0xb
Code: 89 e8 e8 14 ee ff ff ba 00 00 04 00 89 e8 e8 98 48 ff ff 85 c0 74
1e 83 7d 48 00 75 18 8b 85 08 07 00 00 31 c9 8b 95 0c 07 00 00 <0f> 01
d1 c7 45 48 01 00 00 00 c7 45 1c 01 00 00 00 0f ae f0 89
EIP: [<f8b9550c>] kvm_arch_vcpu_ioctl_run+0x92a/0xd13 [kvm] SS:ESP
0068:d7c63e70

QEMU first retrieves the supported features via KVM_GET_SUPPORTED_CPUID
and then sets them later. So guest's X86_FEATURE_XSAVE should be masked
out on hosts without X86_FEATURE_XSAVE, making kvm_set_cr4 with
X86_CR4_OSXSAVE fail. Userspaces that allow specifying guest cpuid with
X86_FEATURE_XSAVE even on hosts that do not support it, might be
susceptible to this attack from inside the guest as well.

Allow setting X86_CR4_OSXSAVE bit only if host has XSAVE support.

Signed-off-by: Petr Matousek <pmatouse@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
[bwh: Backported to 3.2: both functions are in arch/x86/kvm/x86.c]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4fc53233ae7e47e1402edf3108cb0638fbfa6c9d..f4063fdf22d0e567d82c29eecabb889f92764a79 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -578,6 +578,9 @@ static bool guest_cpuid_has_xsave(struct kvm_vcpu *vcpu)
 {
        struct kvm_cpuid_entry2 *best;
 
+       if (!static_cpu_has(X86_FEATURE_XSAVE))
+               return 0;
+
        best = kvm_find_cpuid_entry(vcpu, 1, 0);
        return best && (best->ecx & bit(X86_FEATURE_XSAVE));
 }
@@ -6149,6 +6152,9 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
        int pending_vec, max_bits, idx;
        struct desc_ptr dt;
 
+       if (!guest_cpuid_has_xsave(vcpu) && (sregs->cr4 & X86_CR4_OSXSAVE))
+               return -EINVAL;
+
        dt.size = sregs->idt.limit;
        dt.address = sregs->idt.base;
        kvm_x86_ops->set_idt(vcpu, &dt);