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 Krčmář 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 Krčmář <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>
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
