Re: [PATCH] KVM: x86: restore IP after all far jump failures

[Nadav Amit]

I admit my wrongdoings, but I still think the fix should have been to
remove the entire recovery logic and just return X86EMUL_UNHANDLEABLE if
something goes wrong (exception). This will kill the misbehaving process
but keep the VM running.

Otherwise, a malicious VM process, which can somehow control descriptors
(LDT?) may modify the descriptor during the emulation and get the system
to inconsistent state and prevent the VM-entry.

No?

Nadav

> On Nov 22, 2016, at 11:34 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> 
> 
> 
> On 22/11/2016 20:21, Radim KrÄmÃÅ wrote:
>> em_jmp_far and em_ret_far assumed that setting IP can only fail in 64
>> bit mode, but syzkaller proved otherwise (and SDM agrees).
>> Code segment was restored upon failure, but it was left uninitialized
>> outside of long mode, which could lead to a leak of host kernel stack.
>> 
>> Found by syzkaller:
>> 
>>  WARNING: CPU: 2 PID: 3668 at arch/x86/kvm/emulate.c:2217 em_ret_far+0x428/0x480
>>  Kernel panic - not syncing: panic_on_warn set ...
>> 
>>  CPU: 2 PID: 3668 Comm: syz-executor Not tainted 4.9.0-rc4+ #49
>>  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>>   [...]
>>  Call Trace:
>>   [...] __dump_stack lib/dump_stack.c:15
>>   [...] dump_stack+0xb3/0x118 lib/dump_stack.c:51
>>   [...] panic+0x1b7/0x3a3 kernel/panic.c:179
>>   [...] __warn+0x1c4/0x1e0 kernel/panic.c:542
>>   [...] warn_slowpath_null+0x2c/0x40 kernel/panic.c:585
>>   [...] em_ret_far+0x428/0x480 arch/x86/kvm/emulate.c:2217
>>   [...] em_ret_far_imm+0x17/0x70 arch/x86/kvm/emulate.c:2227
>>   [...] x86_emulate_insn+0x87a/0x3730 arch/x86/kvm/emulate.c:5294
>>   [...] x86_emulate_instruction+0x520/0x1ba0 arch/x86/kvm/x86.c:5545
>>   [...] emulate_instruction arch/x86/include/asm/kvm_host.h:1116
>>   [...] complete_emulated_io arch/x86/kvm/x86.c:6870
>>   [...] complete_emulated_mmio+0x4e9/0x710 arch/x86/kvm/x86.c:6934
>>   [...] kvm_arch_vcpu_ioctl_run+0x3b7a/0x5a90 arch/x86/kvm/x86.c:6978
>>   [...] kvm_vcpu_ioctl+0x61e/0xdd0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2557
>>   [...] vfs_ioctl fs/ioctl.c:43
>>   [...] do_vfs_ioctl+0x18c/0x1040 fs/ioctl.c:679
>>   [...] SYSC_ioctl fs/ioctl.c:694
>>   [...] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685
>>   [...] entry_SYSCALL_64_fastpath+0x1f/0xc2
>> 
>> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
>> Fixes: d1442d85cc30 ("KVM: x86: Handle errors when RIP is set during far jumps")
>> Signed-off-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>> ---
>> Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
>> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>> Cc: Steve Rutherford <srutherford@xxxxxxxxxx>
>> Cc: Andrew Honig <ahonig@xxxxxxxxxx>
>> Cc: Prasad Pandit <ppandit@xxxxxxxxxx>
>> ---
>> arch/x86/kvm/emulate.c | 20 ++++++--------------
>> 1 file changed, 6 insertions(+), 14 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> index 7d4f9b7f06ee..d8a812b6204d 100644
>> --- a/arch/x86/kvm/emulate.c
>> +++ b/arch/x86/kvm/emulate.c
>> @@ -2137,10 +2137,7 @@ static int em_jmp_far(struct x86_emulate_ctxt *ctxt)
>> 	const struct x86_emulate_ops *ops = ctxt->ops;
>> 	u8 cpl = ctxt->ops->cpl(ctxt);
>> 
>> -	/* Assignment of RIP may only fail in 64-bit mode */
>> -	if (ctxt->mode == X86EMUL_MODE_PROT64)
>> -		ops->get_segment(ctxt, &old_sel, &old_desc, NULL,
>> -				 VCPU_SREG_CS);
>> +	ops->get_segment(ctxt, &old_sel, &old_desc, NULL, VCPU_SREG_CS);
>> 
>> 	memcpy(&sel, ctxt->src.valptr + ctxt->op_bytes, 2);
>> 
>> @@ -2151,12 +2148,10 @@ static int em_jmp_far(struct x86_emulate_ctxt *ctxt)
>> 		return rc;
>> 
>> 	rc = assign_eip_far(ctxt, ctxt->src.val, &new_desc);
>> -	if (rc != X86EMUL_CONTINUE) {
>> -		WARN_ON(ctxt->mode != X86EMUL_MODE_PROT64);
>> +	if (rc != X86EMUL_CONTINUE)
>> 		/* assigning eip failed; restore the old cs */
>> 		ops->set_segment(ctxt, old_sel, &old_desc, 0, VCPU_SREG_CS);
>> -		return rc;
>> -	}
>> +
>> 	return rc;
>> }
>> 
>> @@ -2221,9 +2216,7 @@ static int em_ret_far(struct x86_emulate_ctxt *ctxt)
>> 	struct desc_struct old_desc, new_desc;
>> 	const struct x86_emulate_ops *ops = ctxt->ops;
>> 
>> -	if (ctxt->mode == X86EMUL_MODE_PROT64)
>> -		ops->get_segment(ctxt, &old_cs, &old_desc, NULL,
>> -				 VCPU_SREG_CS);
>> +	ops->get_segment(ctxt, &old_cs, &old_desc, NULL, VCPU_SREG_CS);
>> 
>> 	rc = emulate_pop(ctxt, &eip, ctxt->op_bytes);
>> 	if (rc != X86EMUL_CONTINUE)
>> @@ -2240,10 +2233,9 @@ static int em_ret_far(struct x86_emulate_ctxt *ctxt)
>> 	if (rc != X86EMUL_CONTINUE)
>> 		return rc;
>> 	rc = assign_eip_far(ctxt, eip, &new_desc);
>> -	if (rc != X86EMUL_CONTINUE) {
>> -		WARN_ON(ctxt->mode != X86EMUL_MODE_PROT64);
>> +	if (rc != X86EMUL_CONTINUE)
>> 		ops->set_segment(ctxt, old_cs, &old_desc, 0, VCPU_SREG_CS);
>> -	}
>> +
>> 	return rc;
>> }
>> 
>> 
> 
> Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>