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KVM: x86: avoid calling x86 emulator without a decoded instruction

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commit fee060cd52d69c114b62d1a2948ea9648b5131f9 upstream.

Whenever x86_decode_emulated_instruction() detects a breakpoint, it
returns the value that kvm_vcpu_check_breakpoint() writes into its
pass-by-reference second argument.  Unfortunately this is completely
bogus because the expected outcome of x86_decode_emulated_instruction
is an EMULATION_* value.

Then, if kvm_vcpu_check_breakpoint() does "*r = 0" (corresponding to
a KVM_EXIT_DEBUG userspace exit), it is misunderstood as EMULATION_OK
and x86_emulate_instruction() is called without having decoded the
instruction.  This causes various havoc from running with a stale
emulation context.

The fix is to move the call to kvm_vcpu_check_breakpoint() where it was
before commit 4aa2691dcbd3 ("KVM: x86: Factor out x86 instruction
emulation with decoding") introduced x86_decode_emulated_instruction().
The other caller of the function does not need breakpoint checks,
because it is invoked as part of a vmexit and the processor has already
checked those before executing the instruction that #GP'd.

This fixes CVE-2022-1852.

Reported-by: Qiuhao Li <qiuhao@sysec.org>
Reported-by: Gaoning Pan <pgn@zju.edu.cn>
Reported-by: Yongkang Jia <kangel@zju.edu.cn>
Fixes: 4aa2691dcbd3 ("KVM: x86: Factor out x86 instruction emulation with decoding")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220311032801.3467418-2-seanjc@google.com>
[Rewrote commit message according to Qiuhao's report, since a patch
 already existed to fix the bug. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Sean Christopherson 2022-03-11 03:27:41 +00:00 committed by Greg Kroah-Hartman
parent a2a3fa5b61
commit 3d8fc6e28f
1 changed files with 19 additions and 12 deletions
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31
arch/x86/kvm/x86.c
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@ -7295,7 +7295,7 @@ int kvm_skip_emulated_instruction(struct kvm_vcpu *vcpu)
}
EXPORT_SYMBOL_GPL(kvm_skip_emulated_instruction);
static bool kvm_vcpu_check_breakpoint(struct kvm_vcpu *vcpu, int *r)
static bool kvm_vcpu_check_code_breakpoint(struct kvm_vcpu *vcpu, int *r)
{
if (unlikely(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP) &&
(vcpu->arch.guest_debug_dr7 & DR7_BP_EN_MASK)) {
@ -7364,25 +7364,23 @@ static bool is_vmware_backdoor_opcode(struct x86_emulate_ctxt *ctxt)
}
/*
* Decode to be emulated instruction. Return EMULATION_OK if success.
* Decode an instruction for emulation. The caller is responsible for handling
* code breakpoints. Note, manually detecting code breakpoints is unnecessary
* (and wrong) when emulating on an intercepted fault-like exception[*], as
* code breakpoints have higher priority and thus have already been done by
* hardware.
*
* [*] Except #MC, which is higher priority, but KVM should never emulate in
* response to a machine check.
*/
int x86_decode_emulated_instruction(struct kvm_vcpu *vcpu, int emulation_type,
void *insn, int insn_len)
{
int r = EMULATION_OK;
struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
int r;
init_emulate_ctxt(vcpu);
/*
* We will reenter on the same instruction since we do not set
* complete_userspace_io. This does not handle watchpoints yet,
* those would be handled in the emulate_ops.
*/
if (!(emulation_type & EMULTYPE_SKIP) &&
kvm_vcpu_check_breakpoint(vcpu, &r))
return r;
ctxt->ud = emulation_type & EMULTYPE_TRAP_UD;
r = x86_decode_insn(ctxt, insn, insn_len);
@ -7417,6 +7415,15 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
if (!(emulation_type & EMULTYPE_NO_DECODE)) {
kvm_clear_exception_queue(vcpu);
/*
* Return immediately if RIP hits a code breakpoint, such #DBs
* are fault-like and are higher priority than any faults on
* the code fetch itself.
*/
if (!(emulation_type & EMULTYPE_SKIP) &&
kvm_vcpu_check_code_breakpoint(vcpu, &r))
return r;
r = x86_decode_emulated_instruction(vcpu, emulation_type,
insn, insn_len);
if (r != EMULATION_OK) {