linux-kernel - Re: [PATCH v2] x86/mm/pti: in pti_clone

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <7DB70388-174A-4948-9CAA-4AE582F6FB53@fb.com>
Date:   Sat, 24 Aug 2019 02:10:01 +0000
From:   Song Liu <songliubraving@...com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Joerg Roedel <jroedel@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v2] x86/mm/pti: in pti_clone_pgtable(), increase addr
 properly



> On Aug 23, 2019, at 5:59 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> On Wed, 21 Aug 2019, Thomas Gleixner wrote:
>> On Wed, 21 Aug 2019, Song Liu wrote:
>>>> On Aug 20, 2019, at 1:23 PM, Song Liu <songliubraving@...com> wrote:
>>>> 
>>>> Before 32-bit support, pti_clone_pmds() always adds PMD_SIZE to addr.
>>>> This behavior changes after the 32-bit support:  pti_clone_pgtable()
>>>> increases addr by PUD_SIZE for pud_none(*pud) case, and increases addr by
>>>> PMD_SIZE for pmd_none(*pmd) case. However, this is not accurate because
>>>> addr may not be PUD_SIZE/PMD_SIZE aligned.
>>>> 
>>>> Fix this issue by properly rounding up addr to next PUD_SIZE/PMD_SIZE
>>>> in these two cases.
>>> 
>>> After poking around more, I found the following doesn't really make 
>>> sense. 
>> 
>> I'm glad you figured that out yourself. Was about to write up something to
>> that effect.
>> 
>> Still interesting questions remain:
>> 
>>  1) How did you end up feeding an unaligned address into that which points
>>     to a 0 PUD?
>> 
>>  2) Is this related to Facebook specific changes and unlikely to affect any
>>     regular kernel? I can't come up with a way to trigger that in mainline
>> 
>>  3) As this is a user page table and the missing mapping is related to
>>     mappings required by PTI, how is the machine going in/out of user
>>     space in the first place? Or did I just trip over what you called
>>     nonsense?
> 
> And just because this ended in silence I looked at it myself after Peter
> told me that this was on a kernel with PTI disabled. Aside of that my built
> in distrust for debug war stories combined with fairy tale changelogs
> triggered my curiousity anyway.

I am really sorry that I was silent. Somehow I didn't see this in my inbox
(or it didn't show up until just now?). 

For this patch, I really messed up this with something else. The issue we
are seeing is that kprobe on CONFIG_KPROBES_ON_FTRACE splits PMD located 
at 0xffffffff81a00000. I sent another patch last night, but that might not
be the right fix either. 

I haven't started testing our PTI enabled kernel, so I am not sure whether
there is really an issue with the PTI code. 

Thanks,
Song