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Date:   Fri, 7 May 2021 16:57:55 -0700
From:   Kees Cook <keescook@chromium.org>
To:     James Bottomley <jejb@linux.ibm.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christopher Lameter <cl@linux.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Hildenbrand <david@redhat.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        Matthew Garrett <mjg59@srcf.ucam.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Michal Hocko <mhocko@suse.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: [PATCH v18 0/9] mm: introduce memfd_secret system call to create
 "secret" memory areas
Message-ID: <202105071620.E834B1FA92@keescook>
References: <20210303162209.8609-1-rppt@kernel.org>
 <20210505120806.abfd4ee657ccabf2f221a0eb@linux-foundation.org>
 <de27bfae0f4fdcbb0bb4ad17ec5aeffcd774c44b.camel@linux.ibm.com>
 <202105060916.ECDEC21@keescook>
 <9e1953a1412fad06a9f7988a280d2d9a74ab0464.camel@linux.ibm.com>
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On Thu, May 06, 2021 at 11:47:47AM -0700, James Bottomley wrote:
> On Thu, 2021-05-06 at 10:33 -0700, Kees Cook wrote:
> > On Thu, May 06, 2021 at 08:26:41AM -0700, James Bottomley wrote:
> [...]
> > > > I think that a very complete description of the threats which
> > > > this feature addresses would be helpful.  
> > > 
> > > It's designed to protect against three different threats:
> > > 
> > >    1. Detection of user secret memory mismanagement
> > 
> > I would say "cross-process secret userspace memory exposures" (via a
> > number of common interfaces by blocking it at the GUP level).
> > 
> > >    2. significant protection against privilege escalation
> > 
> > I don't see how this series protects against privilege escalation.
> > (It protects against exfiltration.) Maybe you mean include this in
> > the first bullet point (i.e. "cross-process secret userspace memory
> > exposures, even in the face of privileged processes")?
> 
> It doesn't prevent privilege escalation from happening in the first
> place, but once the escalation has happened it protects against
> exfiltration by the newly minted root attacker.

So, after thinking a bit more about this, I don't think there is
protection here against privileged execution. This feature kind of helps
against cross-process read/write attempts, but it doesn't help with
sufficiently privileged (i.e. ptraced) execution, since we can just ask
the process itself to do the reading:

$ gdb ./memfd_secret
...
ready: 0x7ffff7ffb000
Breakpoint 1, ...
(gdb) compile code unsigned long addr = 0x7ffff7ffb000UL; printf("%016lx\n", *((unsigned long *)addr));
55555555555555555

And since process_vm_readv() requires PTRACE_ATTACH, there's very little
difference in effort between process_vm_readv() and the above.

So, what other paths through GUP exist that aren't covered by
PTRACE_ATTACH? And if none, then should this actually just be done by
setting the process undumpable? (This is already what things like gnupg
do.)

So, the user-space side of this doesn't seem to really help. The kernel
side protection is interesting for kernel read/write flaws, though, in
the sense that the process is likely not being attacked from "current",
so a kernel-side attack would need to either walk the page tables and
create new ones, or spawn a new userspace process to do the ptracing.

So, while I like the idea of this stuff, and I see how it provides
certain coverages, I'm curious to learn more about the threat model to
make sure it's actually providing meaningful hurdles to attacks.

-- 
Kees Cook