Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid
On Wed, Jan 13, 2021, Paolo Bonzini wrote:
> On 12/01/21 17:53, Sean Christopherson wrote:
> > And, masking bits 7:6 is architecturally wrong. Both the SDM and APM state
> > that
> > bits 7:0 contain the number of PA bits.
>
> They cannot be higher than 52,
Drat, I was going to argue that it could be >52 with a new paging mode, but both
the SDM and APM explicitly call out 52 as the max. Spending cycles on the stuff
that really matters here... :-)
> therefore bits 7:6 are (architecturally)
> always zero. In other words, I interpret "bit 7:0 contain the number of PA
> bits" as "you need not do an '& 63' yourself", which is basically the
> opposite of "bit 7:6 might be nonzero". If masking made any difference, it
> would be outside the spec already.
>
> In fact another possibility to avoid UB is to do "& 63" of both s and e in
> rsvd_bits. This would also be masking bits 7:6 of the CPUID leaf, just done
> differently.
Hmm, 'e' is hardcoded in all call sites except kvm_mmu_reset_all_pte_masks(),
and so long as 'e <= 63' holds true, 's &= 63' is unnecessary. What if we add
compile-time asserts on hardcoded values, and mask 'e' for the rare case where
the upper bound isn't hardcoded? That way bogus things like rsvd_bits(63, 65)
will fail the build.
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 581925e476d6..261be1d2032b 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -44,8 +44,15 @@
#define PT32_ROOT_LEVEL 2
#define PT32E_ROOT_LEVEL 3
-static inline u64 rsvd_bits(int s, int e)
+static __always_inline u64 rsvd_bits(int s, int e)
{
+ BUILD_BUG_ON(__builtin_constant_p(e) && __builtin_constant_p(s) && e <
s);
+
+ if (__builtin_constant_p(e))
+ BUILD_BUG_ON(e > 63);
+ else
+ e &= 63;
+
if (e < s)
return 0;
Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid
On 12/01/21 17:53, Sean Christopherson wrote: On Tue, Jan 12, 2021, Paolo Bonzini wrote: On 12/01/21 00:01, Sean Christopherson wrote: Perhaps cpuid_query_maxphyaddr() should just look at the low 5 bits of CPUID.8008H:EAX? The low 6 bits I guess---yes, that would make sense and it would have also fixed the bug. No, that wouldn't have fixed this specific bug. In this case, the issue was CPUID.8008H:AL == 0; masking off bits 7:6 wouldn't have changed anything. Right. And, masking bits 7:6 is architecturally wrong. Both the SDM and APM state that bits 7:0 contain the number of PA bits. They cannot be higher than 52, therefore bits 7:6 are (architecturally) always zero. In other words, I interpret "bit 7:0 contain the number of PA bits" as "you need not do an '& 63' yourself", which is basically the opposite of "bit 7:6 might be nonzero". If masking made any difference, it would be outside the spec already. In fact another possibility to avoid UB is to do "& 63" of both s and e in rsvd_bits. This would also be masking bits 7:6 of the CPUID leaf, just done differently. Paolo KVM could reject guest.MAXPA > host.MAXPA, but arbitrarily dropping bits would be wrong.
Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid
On Tue, Jan 12, 2021, Paolo Bonzini wrote: > On 12/01/21 00:01, Sean Christopherson wrote: > > > Perhaps cpuid_query_maxphyaddr() should just look at the low 5 bits of > > > CPUID.8008H:EAX? > > The low 6 bits I guess---yes, that would make sense and it would have also > fixed the bug. No, that wouldn't have fixed this specific bug. In this case, the issue was CPUID.8008H:AL == 0; masking off bits 7:6 wouldn't have changed anything. And, masking bits 7:6 is architecturally wrong. Both the SDM and APM state that bits 7:0 contain the number of PA bits. KVM could reject guest.MAXPA > host.MAXPA, but arbitrarily dropping bits would be wrong.
Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid
On 12/01/21 00:01, Sean Christopherson wrote: Perhaps cpuid_query_maxphyaddr() should just look at the low 5 bits of CPUID.8008H:EAX? The low 6 bits I guess---yes, that would make sense and it would have also fixed the bug. (Nevertheless it's a good idea to make rsvd_bits more robust as well, as in the commit that Sean referenced. Paolo
Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid
It looks like userspace can possibly induce this by providing guest CPUID information with a "physical address width" of 64 in leaf 0x8008. Perhaps cpuid_query_maxphyaddr() should just look at the low 5 bits of CPUID.8008H:EAX? Better would be to return an error for out-of-range values, but I understand that the kvm community's stance is that, in general, guest CPUID information should not be validated by kvm. On Tue, Dec 22, 2020 at 12:36 AM syzbot wrote: > > Hello, > > syzbot found the following issue on: > > HEAD commit:5e60366d Merge tag 'fallthrough-fixes-clang-5.11-rc1' of g.. > git tree: upstream > console output: https://syzkaller.appspot.com/x/log.txt?x=11c7046b50 > kernel config: https://syzkaller.appspot.com/x/.config?x=db720fe37a6a41d8 > dashboard link: https://syzkaller.appspot.com/bug?extid=e87846c48bf72bc85311 > compiler: gcc (GCC) 10.1.0-syz 20200507 > userspace arch: i386 > > Unfortunately, I don't have any reproducer for this issue yet. > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > Reported-by: syzbot+e87846c48bf72bc85...@syzkaller.appspotmail.com > > > UBSAN: shift-out-of-bounds in arch/x86/kvm/mmu.h:52:16 > shift exponent 64 is too large for 64-bit type 'long long unsigned int' > CPU: 1 PID: 11156 Comm: syz-executor.1 Not tainted 5.10.0-syzkaller #0 > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS > Google 01/01/2011 > Call Trace: > __dump_stack lib/dump_stack.c:79 [inline] > dump_stack+0x107/0x163 lib/dump_stack.c:120 > ubsan_epilogue+0xb/0x5a lib/ubsan.c:148 > __ubsan_handle_shift_out_of_bounds.cold+0xb1/0x181 lib/ubsan.c:395 > rsvd_bits arch/x86/kvm/mmu.h:52 [inline] > kvm_vcpu_after_set_cpuid.cold+0x35/0x3a arch/x86/kvm/cpuid.c:181 > kvm_vcpu_ioctl_set_cpuid+0x28e/0x970 arch/x86/kvm/cpuid.c:273 > kvm_arch_vcpu_ioctl+0x1091/0x2d70 arch/x86/kvm/x86.c:4699 > kvm_vcpu_ioctl+0x7b9/0xdb0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3386 > kvm_vcpu_compat_ioctl+0x1a2/0x340 > arch/x86/kvm/../../../virt/kvm/kvm_main.c:3430 > __do_compat_sys_ioctl+0x1d3/0x230 fs/ioctl.c:842 > do_syscall_32_irqs_on arch/x86/entry/common.c:78 [inline] > __do_fast_syscall_32+0x56/0x80 arch/x86/entry/common.c:137 > do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:160 > entry_SYSENTER_compat_after_hwframe+0x4d/0x5c > RIP: 0023:0xf7fe8549 > Code: b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 10 08 03 74 d8 01 00 00 00 00 > 00 00 00 00 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 > 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90 > RSP: 002b:f55e20cc EFLAGS: 0296 ORIG_RAX: 0036 > RAX: ffda RBX: 0005 RCX: 4008ae8a > RDX: 20c0 RSI: RDI: > RBP: R08: R09: > R10: R11: R12: > R13: R14: R15: > > > > --- > This report is generated by a bot. It may contain errors. > See https://goo.gl/tpsmEJ for more information about syzbot. > syzbot engineers can be reached at syzkal...@googlegroups.com. > > syzbot will keep track of this issue. See: > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid
On Mon, Jan 11, 2021, Jim Mattson wrote:
> It looks like userspace can possibly induce this by providing guest
> CPUID information with a "physical address width" of 64 in leaf
> 0x8008.
It was actually the opposite, where userspace provides '0' and caused '63 - 0 +
1'
to overflow. KVM controls the upper bound, and rsvd_bits() explicitly handles
'end < start', so an absurdly large maxpa is handled correctly.
Aleady fixed by Paolo in commit 2f80d502d627 ("KVM: x86: fix shift out of bounds
reported by UBSAN").
> Perhaps cpuid_query_maxphyaddr() should just look at the low 5 bits of
> CPUID.8008H:EAX? Better would be to return an error for
> out-of-range values, but I understand that the kvm community's stance
> is that, in general, guest CPUID information should not be validated
> by kvm.
And rob Paolo of his crazy^Wbrilliant bit math shenanigans? :-D
UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid
Hello, syzbot found the following issue on: HEAD commit:5e60366d Merge tag 'fallthrough-fixes-clang-5.11-rc1' of g.. git tree: upstream console output: https://syzkaller.appspot.com/x/log.txt?x=11c7046b50 kernel config: https://syzkaller.appspot.com/x/.config?x=db720fe37a6a41d8 dashboard link: https://syzkaller.appspot.com/bug?extid=e87846c48bf72bc85311 compiler: gcc (GCC) 10.1.0-syz 20200507 userspace arch: i386 Unfortunately, I don't have any reproducer for this issue yet. IMPORTANT: if you fix the issue, please add the following tag to the commit: Reported-by: syzbot+e87846c48bf72bc85...@syzkaller.appspotmail.com UBSAN: shift-out-of-bounds in arch/x86/kvm/mmu.h:52:16 shift exponent 64 is too large for 64-bit type 'long long unsigned int' CPU: 1 PID: 11156 Comm: syz-executor.1 Not tainted 5.10.0-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: __dump_stack lib/dump_stack.c:79 [inline] dump_stack+0x107/0x163 lib/dump_stack.c:120 ubsan_epilogue+0xb/0x5a lib/ubsan.c:148 __ubsan_handle_shift_out_of_bounds.cold+0xb1/0x181 lib/ubsan.c:395 rsvd_bits arch/x86/kvm/mmu.h:52 [inline] kvm_vcpu_after_set_cpuid.cold+0x35/0x3a arch/x86/kvm/cpuid.c:181 kvm_vcpu_ioctl_set_cpuid+0x28e/0x970 arch/x86/kvm/cpuid.c:273 kvm_arch_vcpu_ioctl+0x1091/0x2d70 arch/x86/kvm/x86.c:4699 kvm_vcpu_ioctl+0x7b9/0xdb0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3386 kvm_vcpu_compat_ioctl+0x1a2/0x340 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3430 __do_compat_sys_ioctl+0x1d3/0x230 fs/ioctl.c:842 do_syscall_32_irqs_on arch/x86/entry/common.c:78 [inline] __do_fast_syscall_32+0x56/0x80 arch/x86/entry/common.c:137 do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:160 entry_SYSENTER_compat_after_hwframe+0x4d/0x5c RIP: 0023:0xf7fe8549 Code: b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 10 08 03 74 d8 01 00 00 00 00 00 00 00 00 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90 RSP: 002b:f55e20cc EFLAGS: 0296 ORIG_RAX: 0036 RAX: ffda RBX: 0005 RCX: 4008ae8a RDX: 20c0 RSI: RDI: RBP: R08: R09: R10: R11: R12: R13: R14: R15: --- This report is generated by a bot. It may contain errors. See https://goo.gl/tpsmEJ for more information about syzbot. syzbot engineers can be reached at syzkal...@googlegroups.com. syzbot will keep track of this issue. See: https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
