linux-kernel - Re: [PATCH 4/7] x86/vsyscall: Setup vsyscall to compromise LASS protection

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Date:   Wed, 11 Jan 2023 18:49:39 -0800
From:   Sohil Mehta <sohil.mehta@...el.com>
To:     "Chen, Yian" <yian.chen@...el.com>, <linux-kernel@...r.kernel.org>,
        <x86@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        "Tony Luck" <tony.luck@...el.com>, Paul Lai <paul.c.lai@...el.com>
Subject: Re: [PATCH 4/7] x86/vsyscall: Setup vsyscall to compromise LASS
 protection

>> The existing documentation here is incorrect. The default vsyscall 
>> mode is actually xonly. This has been so since:
>> commit 625b7b7f79c6 (x86/vsyscall: Change the default vsyscall mode to 
>> xonly)
>>
> Yes, you are right. but this patch can overwrite and correct existing 
> one. I am assuming we don't need to correct the existing document first 
> before update it for LASS.
> 

We should fix this independent of the LASS enabling. I sent a patch 
earlier today to address it. I apologize, I missed cc'ing you.

https://lore.kernel.org/lkml/20230111193211.1987047-1-sohil.mehta@intel.com/

>>> +            In newer versions of Intel platforms that come with
>>
>> Words such as "newer" in the kernel start losing meaning very quickly. 
>> Also, this comment looks out of place in between the vsyscall 
>> sub-options.
>>
>>> +            LASS(Linear Address Space separation) protection,
>>> +            vsyscall is disabled by default. Enabling vsyscall
>>> +            via the parameter overrides LASS protection.
>>> +
> Sure, I will take out this part change.

Actually, having some text here might be ok. I mistook it to be placed 
between the sub-options. But avoid merging it with the previous 
paragraph as is the case right now.

>> Instead of doing this dance, can we provide a simplified behavior to 
>> the user/admin and move the decision making to compile time?
>>
> Current strategy is to disable vsyscall by default only for LASS capable 
> platforms. So that the dynamic decision is likely a necessary.
> 

Making this dynamic and platform dependent would make things hard to 
debug and isolate. It would be a perfect recipe for "But, it works on my 
system!" type of issues.

Let's see what others have to say.

-Sohil