linux-kernel - Re: [PATCH v2 1/8] x86-64: Improve vsyscall emulation CS and RIP handling

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Date:	Tue, 12 Jul 2011 08:58:58 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Borislav Petkov <bp@...en8.de>, Andrew Lutomirski <luto@....edu>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>, John Stultz <johnstul@...ibm.com>,
	Borislav Petkov <bp@...64.org>
Subject: Re: [PATCH v2 1/8] x86-64: Improve vsyscall emulation CS and RIP handling

On Tue, Jul 12, 2011 at 3:18 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Mon, Jul 11, 2011 at 06:20:50PM -0400, Andrew Lutomirski wrote:
>> > I'm wondering: why don't you make this function return negative value on
>> > error, i.e. -EINVAL and the vsyscall number on success so that you can
>> > get rid of returning it through the arg pointer?
>> >
>> > Then at the callsite you can do:
>> >
>> >        vsyscall_nr = addr_to_vsyscall_nr(addr);
>> >        if (vsyscall_nr < 0)
>> >                warn_bad_vsyscall(...)
>>
>> Because I don't want a warning about ret being used without being initialized.
>
> not if you preinit it...

I kind of like that warning as a sanity check, and preiniting it
grates against my irrational desire to over-optimize :)

>
>> With the code in this patch, the compiler is smart enough to figure
>> out that either vsyscall_nr is 0, 1, or 2 or that the EINVAL branch is
>> taken.  I'll see if it works the other way.
>
> here's what i mean, I changed your patch a bit:

How about this:

static int addr_to_vsyscall_nr(unsigned long addr)
{
	int nr;

	if ((addr & ~0xC00UL) != VSYSCALL_START)
		return -EINVAL;

	nr = (addr & 0xC00UL) >> 10;
	if (nr >= 3)
		return -EINVAL;

	return nr;
}

...

	int vsyscall_nr;

...

	vsyscall_nr = addr_to_vsyscall_nr(regs->ip - 2);
	if (vsyscall_nr < 0) {
		warn_bad_vsyscall(KERN_WARNING, regs,
				  "illegal int 0xcc (exploit attempt?)");
		goto sigsegv;
	}

gcc 4.6 at least does not warn.


Also, IRQ disabling was still mismatched in the sigsegv path.  I'll
fix that as well.

--Andy
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