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 <[email protected]> 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 <[email protected]> >> Cc: [email protected] >> Fixes: d1442d85cc30 ("KVM: x86: Handle errors when RIP is set during far >> jumps") >> Signed-off-by: Radim Krčmář <[email protected]> >> --- >> Cc: Nadav Amit <[email protected]> >> Cc: Dmitry Vyukov <[email protected]> >> Cc: Steve Rutherford <[email protected]> >> Cc: Andrew Honig <[email protected]> >> Cc: Prasad Pandit <[email protected]> >> --- >> 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 <[email protected]>
