linux-kernel - Re: [PATCH v5 18/39] mm: Handle faultless write upgrades for shstk

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 for Android: free password hash cracker in your pocket
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <6a38779c1539c2bcfeb6bc8251ed04aa9b06802e.camel@intel.com>
Date:   Sat, 28 Jan 2023 00:51:00 +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>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.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>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "pavel@....cz" <pavel@....cz>, "oleg@...hat.com" <oleg@...hat.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "bp@...en8.de" <bp@...en8.de>,
        "jamorris@...ux.microsoft.com" <jamorris@...ux.microsoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Schimpe, Christina" <christina.schimpe@...el.com>,
        "mike.kravetz@...cle.com" <mike.kravetz@...cle.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "john.allen@....com" <john.allen@....com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "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>
CC:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v5 18/39] mm: Handle faultless write upgrades for shstk

On Fri, 2023-01-27 at 17:12 +0100, David Hildenbrand wrote:
> On 26.01.23 21:19, Edgecombe, Rick P wrote:
> > On Thu, 2023-01-26 at 09:46 +0100, David Hildenbrand wrote:
> > > On 26.01.23 01:59, Edgecombe, Rick P wrote:
> > > > On Wed, 2023-01-25 at 10:43 -0800, Rick Edgecombe wrote:
> > > > > Thanks for your comments and ideas here, I'll give the:
> > > > > pte_t pte_mkwrite(struct vm_area_struct *vma, pte_t pte)
> > > > > ...solution a try.
> > > > 
> > > > Well, it turns out there are some pte_mkwrite() callers in
> > > > other
> > > > arch's
> > > > that operate on kernel memory and don't have a VMA. So it
> > > > needed a
> > > > new
> > > 
> > > Why not pass in NULL as VMA then and document the semantics? The
> > > less
> > > similarly named but slightly different functions, the better :)
> > 
> > Hmm. The x86 and generic versions should probably have the same
> > semantics, so then if you pass a NULL, it would do a regular
> > pte_mkwrite() I guess?
> > 
> > I see another benefit of requiring the vma argument, such that raw
> > pte_mkwrite()s are less likely to appear in core MM code. But I
> > think
> > the NULL is awkward because it's not obvious, to me at least, what
> > the
> > implications of that should be.
> > 
> > So it will be confusing to read in the NULL cases for the other
> > archs.
> > We also have some warnings to catch miss cases in the PTE tear down
> > code, so the scenario of new code accidentally marking shadow stack
> > PTEs as writable is not totally unchecked.
> > 
> > The three functions that do slightly different things are:
> > 
> > pte_mkwrite():
> > Makes a PTE conventionally writable, only takes a PTE. Very clear
> > that
> > it is a low level helper and what it does.
> > 
> > maybe_mkwrite():
> > Might make a PTE writable if the VMA allows it.
> > 
> > pte_mkwrite_vma():
> > Makes a PTE writable in a specific way depending on the VMA
> > 
> > I wonder if the name pte_mkwrite_vma() is maybe just not clear
> > enough.
> > It takes a VMA, yes, but what does it do with it?
> > 
> > What if it was called pte_mkwrite_type() instead? Some arch's have
> > additional types of writable memory and this function creates them.
> > Of
> > course they also have the normal type of writable memory, and
> > pte_mkwrite() creates that like usual. Doesn't it seem more
> > readable?
> 
> The issue is, the more variants we provide the easier it is to make 
> mistakes and introduce new buggy code.
> 
> It's tempting to simply use pte_mkwrite() and call it a day, where 
> people actually should use pte_mkwrite_vma().
> 
> Then, they at least have to investigate what to do about the second
> VMA 
> parameter.

Ok, I'll give it a spin. So far it looks ok. The downside is the giant
tree-wide pte_mkwrite() signature change, but once that is over with
there are other advantages. Like getting rid of maybe_mkwrite()'s
awareness of shadow stack so the logic is more centralized. Please let
me know if you don't feel comfortable with a suggested-by credit tag.

Thanks,
Rick