Re: [PATCH] x86/asm/entry/64: better check for canonical address

[Andy Lutomirski]

On Mon, Mar 30, 2015 at 7:27 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> On 03/26/2015 07:45 PM, Andy Lutomirski wrote:
>> On Thu, Mar 26, 2015 at 5:42 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
>>> This change makes the check exact (no more false positives
>>> on kernel addresses).
>>>
>>> It isn't really important to be fully correct here -
>>> almost all addresses we'll ever see will be userspace ones,
>>> but OTOH it looks to be cheap enough:
>>> the new code uses two more ALU ops but preserves %rcx,
>>> allowing to not reload it from pt_regs->cx again.
>>> On disassembly level, the changes are:
>>>
>>> cmp %rcx,0x80(%rsp) -> mov 0x80(%rsp),%r11; cmp %rcx,%r11
>>> shr $0x2f,%rcx      -> shl $0x10,%rcx; sar $0x10,%rcx; cmp %rcx,%r11
>>> mov 0x58(%rsp),%rcx -> (eliminated)
>>>
>>> Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
>>> CC: Borislav Petkov <bp@xxxxxxxxx>
>>> CC: x86@xxxxxxxxxx
>>> CC: linux-kernel@xxxxxxxxxxxxxxx
>>> ---
>>>
>>> Andy, I'd undecided myself on the merits of doing this.
>>> If you like it, feel free to take it in your tree.
>>> I trimmed CC list to not bother too many people with this trivial
>>> and quite possibly "useless churn"-class change.
>>
>> I suspect that the two added ALU ops are free for all practical
>> purposes, and the performance of this path isn't *that* critical.
>>
>> If anyone is running with vsyscall=native because they need the
>> performance, then this would be a big win.  Otherwise I don't have a
>> real preference.  Anyone else have any thoughts here?
>>
>> Let me just run through the math quickly to make sure I believe all the numbers:
>>
>> Canonical addresses either start with 17 zeros or 17 ones.
>>
>> In the old code, we checked that the top (64-47) = 17 bits were all
>> zero.  We did this by shifting right by 47 bits and making sure that
>> nothing was left.
>>
>> In the new code, we're shifting left by (64 - 48) = 16 bits and then
>> signed shifting right by the same amount, this propagating the 17th
>> highest bit to all positions to its left.  If we get the same value we
>> started with, then we're good to go.
>>
>> So it looks okay to me.
>
>
> So please take it into your tree :)
>

Will do, but not until later this week because I'm on vacation and I'm
allocating about ten minutes to using the computer :)  Or maybe Ingo
will beat me.

-- 
Andy Lutomirski
AMA Capital Management, LLC
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