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

["Radim Krčmář" <rkrcmar@redhat.com>]

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@google.com>
Cc: stable@vger.kernel.org
Fixes: d1442d85cc30 ("KVM: x86: Handle errors when RIP is set during far jumps")
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
---
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Steve Rutherford <srutherford@google.com>
Cc: Andrew Honig <ahonig@google.com>
Cc: Prasad Pandit <ppandit@redhat.com>
---
 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;
 }
 
-- 
2.10.2