linux-kernel - Re: [PATCH v5 10/39] x86/mm: Introduce _PAGE

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: <bbc4f4df98ec798ae15e5daa6b5ceab41bcc66f9.camel@intel.com>
Date:   Mon, 23 Jan 2023 20:56:36 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "david@...hat.com" <david@...hat.com>,
        "bsingharora@...il.com" <bsingharora@...il.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Syromiatnikov, Eugene" <esyr@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "nadav.amit@...il.com" <nadav.amit@...il.com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "dethoma@...rosoft.com" <dethoma@...rosoft.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "kcc@...gle.com" <kcc@...gle.com>, "pavel@....cz" <pavel@....cz>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Schimpe, Christina" <christina.schimpe@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "mike.kravetz@...cle.com" <mike.kravetz@...cle.com>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "jamorris@...ux.microsoft.com" <jamorris@...ux.microsoft.com>,
        "john.allen@....com" <john.allen@....com>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "gorcunov@...il.com" <gorcunov@...il.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v5 10/39] x86/mm: Introduce _PAGE_COW

Trying to answer both questions to this patch on this one.

On Mon, 2023-01-23 at 10:28 +0100, David Hildenbrand wrote:
> > +/*
> > + * Normally COW memory can result in Dirty=1,Write=0 PTEs. But in
> > the case
> > + * of X86_FEATURE_USER_SHSTK, the software COW bit is used, since
> > the
> > + * Dirty=1,Write=0 will result in the memory being treated as
> > shadow stack
> > + * by the HW. So when creating COW memory, a software bit is used
> > + * _PAGE_BIT_COW. The following functions pte_mkcow() and
> > pte_clear_cow()
> > + * take a PTE marked conventionally COW (Dirty=1) and transition
> > it to the
> > + * shadow stack compatible version of COW (Cow=1).
> > + */
> 
> TBH, I find that all highly confusing.
> 
> Dirty=1,Write=0 does not indicate a COW page reliably. You could
> have 
> both, false negatives and false positives.
> 
> False negative: fork() on a clean anon page.
> 
> False positives: wrpotect() of a dirty anon page.
> 
> 
> I wonder if it really has to be that complicated: what you really
> want 
> to achieve is to disallow "Dirty=1,Write=0" if it's not a shadow
> stack 
> page, correct?

The other thing is to save that the PTE is/was Dirty=1 somewhere (for
non-shadow stack memory). A slightly different but related thing. But
losing that information would would introduce differences for
pte_dirty() between when shadow stack was enabled or not. GUP/COW
doesn't need this anymore but there are lots of other places it gets
checked.

Perhaps following your GUP changes, _PAGE_COW is just now the wrong
name for it. _PAGE_SAVED_DIRTY maybe?