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Message-ID: <f4c90626-51d8-5551-5b77-baaff81f16bb@linux.intel.com>
Date: Wed, 18 Jul 2018 17:06:33 -0700
From: Dave Hansen <dave.hansen@...ux.intel.com>
To: Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org, linux-mm@...ck.org,
linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
Arnd Bergmann <arnd@...db.de>,
Andy Lutomirski <luto@...capital.net>,
Balbir Singh <bsingharora@...il.com>,
Cyrill Gorcunov <gorcunov@...il.com>,
Florian Weimer <fweimer@...hat.com>,
"H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
Jonathan Corbet <corbet@....net>,
Kees Cook <keescook@...omiun.org>,
Mike Kravetz <mike.kravetz@...cle.com>,
Nadav Amit <nadav.amit@...il.com>,
Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
Peter Zijlstra <peterz@...radead.org>,
"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for
shadow stack
>>> -static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
>>> +static inline bool can_follow_write(pte_t pte, unsigned int flags,
>>> + struct vm_area_struct *vma)
>>> {
>>> - return pte_write(pte) ||
>>> - ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
>>> + if (!is_shstk_mapping(vma->vm_flags)) {
>>> + if (pte_write(pte))
>>> + return true;
>> Let me see if I can say this another way.
>>
>> The bigger issue is that these patches change the semantics of
>> pte_write(). Before these patches, it meant that you *MUST* have this
>> bit set to write to the page controlled by the PTE. Now, it means: you
>> can write if this bit is set *OR* the shadowstack bit combination is set.
>
> Here, we only figure out (1) if the page is pointed by a writable PTE; or
> (2) if the page is pointed by a RO PTE (data or SHSTK) and it has been
> copied and it still exists. We are not trying to
> determine if the
> SHSTK PTE is writable (we know it is not).
Please think about the big picture. I'm not just talking about this
patch, but about every use of pte_write() in the kernel.
>> That's the fundamental problem. We need some code in the kernel that
>> logically represents the concept of "is this PTE a shadowstack PTE or a
>> PTE with the write bit set", and we will call that pte_write(), or maybe
>> pte_writable().
>>
>> You *have* to somehow rectify this situation. We can absolutely no
>> leave pte_write() in its current, ambiguous state where it has no real
>> meaning or where it is used to mean _both_ things depending on context.
>
> True, the processor can always write to a page through a shadow stack
> PTE, but it must do that with a CALL instruction. Can we define a
> write operation as: MOV r1, *(r2). Then we don't have any doubt on
> pte_write() any more.
No, we can't just move the target. :)
You can define it this way, but then you also need to go to every spot
in the kernel that calls pte_write() (and _PAGE_RW in fact) and audit it
to ensure it means "mov ..." and not push.
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