[PATCH] KVM/Emulate: Mask linear address with actual address width in order to avoid conflict with UNMAPPED_GVA

[Lan Tianyu]

Reported by syzkaller:
    WARNING: CPU: 0 PID: 27962 at arch/x86/kvm/emulate.c:5631 x86_emulate_insn+0x557/0x15f0 [kvm]
    Modules linked in: kvm_intel kvm [last unloaded: kvm]
    CPU: 0 PID: 27962 Comm: syz-executor Tainted: G    B   W        4.15.0-rc2-next-20171208+ #32
    Hardware name: Intel Corporation S1200SP/S1200SP, BIOS S1200SP.86B.01.03.0006.040720161253 04/07/2016
    RIP: 0010:x86_emulate_insn+0x557/0x15f0 [kvm]
    RSP: 0018:ffff8807234476d0 EFLAGS: 00010282
    RAX: 0000000000000000 RBX: ffff88072d0237a0 RCX: ffffffffa0065c4d
    RDX: 1ffff100e5a046f9 RSI: 0000000000000003 RDI: ffff88072d0237c8
    RBP: ffff880723447728 R08: ffff88072d020000 R09: ffffffffa008d240
    R10: 0000000000000002 R11: ffffed00e7d87db3 R12: ffff88072d0237c8
    R13: ffff88072d023870 R14: ffff88072d0238c2 R15: ffffffffa008d080
    FS:  00007f8a68666700(0000) GS:ffff880802200000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 000000002009506c CR3: 000000071fec4005 CR4: 00000000003626f0
    Call Trace:
     x86_emulate_instruction+0x3bc/0xb70 [kvm]
     ? reexecute_instruction.part.162+0x130/0x130 [kvm]
     vmx_handle_exit+0x46d/0x14f0 [kvm_intel]
     ? trace_event_raw_event_kvm_entry+0xe7/0x150 [kvm]
     ? handle_vmfunc+0x2f0/0x2f0 [kvm_intel]
     ? wait_lapic_expire+0x25/0x270 [kvm]
     vcpu_enter_guest+0x720/0x1ef0 [kvm]
     ...

Syzkaller tests KVM emulation code path with setting RSP 0xffffffffffffffff
and running in 64bit non paging mode. It triggers warning that exception
vector is > 1f(it's initialized to be 0xff in x86_emulate_instruction())
during emulation of pop instruction. This is due to pop emulation callback
em_pop() returns X86EMUL_PROPAGATE_FAULT while not populate exception
vector. POP emulation code accesses RSP with linear address
0xffffffffffffffff and KVM mmu returns GVA as GPA during nonpaging mode.
In this case, GPA 0xffffffffffffffff(~0) conflicts with error code
UNMAPPED_GVA which is defined to be (~(u64)0). Caller vcpu_mmio_gva_to
_gpa() treats the return address as error and this also causes emulator
_read_write_onepage() returns X86EMUL_PROPAGATE_FAULT without a validated
exception vector.

This patch is to mask linear address with address width to fix such issue
since linear address won't occupy all 64-bit even if in 5 level paging
mode.

Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Jim Mattson <jmattson@xxxxxxxxxx>
Cc: Wanpeng Li <kernellwp@xxxxxxxxx>
Signed-off-by: Lan Tianyu <tianyu.lan@xxxxxxxxx>
---
 arch/x86/kvm/emulate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index abe74f7..9b21412 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -697,8 +697,8 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
 	*max_size = 0;
 	switch (mode) {
 	case X86EMUL_MODE_PROT64:
-		*linear = la;
 		va_bits = ctxt_virt_addr_bits(ctxt);
+		*linear = la & ((1ul << va_bits) - 1);
 		if (get_canonical(la, va_bits) != la)
 			goto bad;
 
-- 
1.8.3.1