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>
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