KVM: x86: Fix KVM_CAP_SYNC_REGS's sync_regs() TOCTOU issues

In a spirit of using a sledgehammer to crack a nut, make sync_regs() feed
__set_sregs() and kvm_vcpu_ioctl_x86_set_vcpu_events() with kernel's own
copy of data.

Both __set_sregs() and kvm_vcpu_ioctl_x86_set_vcpu_events() assume they
have exclusive rights to structs they operate on. While this is true when
coming from an ioctl handler (caller makes a local copy of user's data),
sync_regs() breaks this contract; a pointer to a user-modifiable memory
(vcpu->run->s.regs) is provided. This can lead to a situation when incoming
data is checked and/or sanitized only to be re-set by a user thread running
in parallel.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Fixes: 01643c51bfcf ("KVM: x86: KVM_CAP_SYNC_REGS")
Link: https://lore.kernel.org/r/20230728001606.2275586-2-mhal@rbox.co
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a6b9bea62fb8ac4498cccc7686196f6b63a8c1e9..0145d844283befb32725f6349f81a013488d4c79 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11777,15 +11777,22 @@ static int sync_regs(struct kvm_vcpu *vcpu)
                __set_regs(vcpu, &vcpu->run->s.regs.regs);
                vcpu->run->kvm_dirty_regs &= ~KVM_SYNC_X86_REGS;
        }
+
        if (vcpu->run->kvm_dirty_regs & KVM_SYNC_X86_SREGS) {
-               if (__set_sregs(vcpu, &vcpu->run->s.regs.sregs))
+               struct kvm_sregs sregs = vcpu->run->s.regs.sregs;
+
+               if (__set_sregs(vcpu, &sregs))
                        return -EINVAL;
+
                vcpu->run->kvm_dirty_regs &= ~KVM_SYNC_X86_SREGS;
        }
+
        if (vcpu->run->kvm_dirty_regs & KVM_SYNC_X86_EVENTS) {
-               if (kvm_vcpu_ioctl_x86_set_vcpu_events(
-                               vcpu, &vcpu->run->s.regs.events))
+               struct kvm_vcpu_events events = vcpu->run->s.regs.events;
+
+               if (kvm_vcpu_ioctl_x86_set_vcpu_events(vcpu, &events))
                        return -EINVAL;
+
                vcpu->run->kvm_dirty_regs &= ~KVM_SYNC_X86_EVENTS;
        }