Messages in this thread | | | Subject | Re: [PATCH v20 08/25] x86/mm: Introduce _PAGE_COW | From | "Yu, Yu-cheng" <> | Date | Wed, 10 Feb 2021 12:28:39 -0800 |
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On 2/10/2021 11:42 AM, Kees Cook wrote: > On Wed, Feb 10, 2021 at 09:56:46AM -0800, Yu-cheng Yu wrote: >> There is essentially no room left in the x86 hardware PTEs on some OSes >> (not Linux). That left the hardware architects looking for a way to >> represent a new memory type (shadow stack) within the existing bits. >> They chose to repurpose a lightly-used state: Write=0, Dirty=1. >> >> The reason it's lightly used is that Dirty=1 is normally set by hardware >> and cannot normally be set by hardware on a Write=0 PTE. Software must >> normally be involved to create one of these PTEs, so software can simply >> opt to not create them. >> >> In places where Linux normally creates Write=0, Dirty=1, it can use the >> software-defined _PAGE_COW in place of the hardware _PAGE_DIRTY. In other >> words, whenever Linux needs to create Write=0, Dirty=1, it instead creates >> Write=0, Cow=1, except for shadow stack, which is Write=0, Dirty=1. This >> clearly separates shadow stack from other data, and results in the >> following: >> >> (a) A modified, copy-on-write (COW) page: (Write=0, Cow=1) >> (b) A R/O page that has been COW'ed: (Write=0, Cow=1) >> The user page is in a R/O VMA, and get_user_pages() needs a writable >> copy. The page fault handler creates a copy of the page and sets >> the new copy's PTE as Write=0 and Cow=1. >> (c) A shadow stack PTE: (Write=0, Dirty=1) >> (d) A shared shadow stack PTE: (Write=0, Cow=1) >> When a shadow stack page is being shared among processes (this happens >> at fork()), its PTE is made Dirty=0, so the next shadow stack access >> causes a fault, and the page is duplicated and Dirty=1 is set again. >> This is the COW equivalent for shadow stack pages, even though it's >> copy-on-access rather than copy-on-write. >> (e) A page where the processor observed a Write=1 PTE, started a write, set >> Dirty=1, but then observed a Write=0 PTE. That's possible today, but >> will not happen on processors that support shadow stack. >> >> Define _PAGE_COW and update pte_*() helpers and apply the same changes to >> pmd and pud. > > I still find this commit confusing mostly due to _PAGE_COW being 0 > without CET enabled. Shouldn't this just get changed universally? Why > should this change depend on CET? >
For example, in...
static inline int pte_write(pte_t pte) { if (cpu_feature_enabled(X86_FEATURE_SHSTK)) return pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY); else return pte_flags(pte) & _PAGE_RW; }
There are four cases:
(a) RW=1, Dirty=1 -> writable (b) RW=1, Dirty=0 -> writable (c) RW=0, Dirty=0 -> not writable (d) RW=0, Dirty=1 -> shadow stack, or not-writable if !X86_FEATURE_SHSTK
Case (d) is ture only when shadow stack is enabled, otherwise it is not writable. With shadow stack feature, the usual dirty, copy-on-write PTE becomes RW=0, Cow=1.
We can get this changed universally, but all usual dirty, copy-on-write PTEs need the Dirty/Cow swapping, always. Is that desirable?
-- Yu-cheng
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