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From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jim Mattson <jmattson@google.com>,
	syzbot <syzbot+e87846c48bf72bc85311@syzkaller.appspotmail.com>,
	Borislav Petkov <bp@alien8.de>, "H . Peter Anvin" <hpa@zytor.com>,
	Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	the arch/x86 maintainers <x86@kernel.org>
Subject: Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid
Date: Wed, 13 Jan 2021 08:46:06 -0800	[thread overview]
Message-ID: <X/8jzqzDV35mAnIF@google.com> (raw)
In-Reply-To: <d3cc2a46-6b8b-cf7c-66f0-22fe4c05465e@redhat.com>

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;

      reply	other threads:[~2021-01-13 16:47 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-22  8:36 syzbot
2021-01-11 22:44 ` Jim Mattson
2021-01-11 23:01   ` Sean Christopherson
2021-01-12  8:06     ` Paolo Bonzini
2021-01-12 16:53       ` Sean Christopherson
2021-01-13 14:20         ` Paolo Bonzini
2021-01-13 16:46           ` Sean Christopherson [this message]

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