linux-kernel - Re: [PATCH v2 2/3] x86/traps: Print non-canonical address on #GP

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Date:   Wed, 20 Nov 2019 05:56:07 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v2 2/3] x86/traps: Print non-canonical address on #GP

> Is there a specific concern you have about the instruction decoder? As
> far as I can tell, all the paths of insn_get_addr_ref() only work if
> the instruction has a mod R/M byte according to the instruction
> tables, and then figures out the address based on that. While that
> means that there's a wide variety of cases in which we won't be able
> to figure out the address, I'm not aware of anything specific that is
> likely to lead to false positives.

First there will be a lot of cases you'll just print 0, even
though 0 is canonical if there is no operand.

Then it might be that the address is canonical, but triggers
#GP anyways (e.g. unaligned SSE)

Or it might be the wrong address if there is an operand,
there are many complex instructions that reference something
in memory, and usually do canonical checking there.

And some other odd cases. For example when the instruction length
exceeds 15 bytes. I know there is fuzzing for the instruction
decoder, but it might be worth double checking it handles
all of that correctly. I'm not sure how good the fuzzer's coverage
is.

At a minimum you should probably check if the address is
actually non canonical. Maybe that's simple enough and weeds out
most cases.

-Andi