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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>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Roman Gushchin <guro@fb.com>, Shakeel Butt <shakeelb@google.com>,
	Shuah Khan <shuah@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Tycho Andersen <tycho@tycho.ws>, Will Deacon <will@kernel.org>,
	linux-api@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org
Subject: Re: [PATCH v18 0/9] mm: introduce memfd_secret system call to create "secret" memory areas
Date: Fri, 7 May 2021 16:57:55 -0700	[thread overview]
Message-ID: <202105071620.E834B1FA92@keescook> (raw)
In-Reply-To: <9e1953a1412fad06a9f7988a280d2d9a74ab0464.camel@linux.ibm.com>

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

  reply	other threads:[~2021-05-07 23:58 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-03 16:22 [PATCH v18 0/9] mm: introduce memfd_secret system call to create "secret" memory areas Mike Rapoport
2021-03-03 16:22 ` [PATCH v18 1/9] mm: add definition of PMD_PAGE_ORDER Mike Rapoport
2021-03-03 16:22 ` [PATCH v18 2/9] mmap: make mlock_future_check() global Mike Rapoport
2021-03-03 16:22 ` [PATCH v18 3/9] riscv/Kconfig: make direct map manipulation options depend on MMU Mike Rapoport
2021-03-03 16:22 ` [PATCH v18 4/9] set_memory: allow set_direct_map_*_noflush() for multiple pages Mike Rapoport
2021-03-03 16:22 ` [PATCH v18 5/9] set_memory: allow querying whether set_direct_map_*() is actually enabled Mike Rapoport
2021-03-03 16:22 ` [PATCH v18 6/9] mm: introduce memfd_secret system call to create "secret" memory areas Mike Rapoport
2021-03-03 16:22 ` [PATCH v18 7/9] PM: hibernate: disable when there are active secretmem users Mike Rapoport
2021-03-03 16:22 ` [PATCH v18 8/9] arch, mm: wire up memfd_secret system call where relevant Mike Rapoport
2021-03-03 16:22 ` [PATCH v18 9/9] secretmem: test: add basic selftest for memfd_secret(2) Mike Rapoport
2021-05-05 19:08 ` [PATCH v18 0/9] mm: introduce memfd_secret system call to create "secret" memory areas Andrew Morton
2021-05-06 15:26   ` James Bottomley
2021-05-06 16:45     ` David Hildenbrand
2021-05-06 17:05       ` James Bottomley
2021-05-06 17:24         ` David Hildenbrand
2021-05-06 23:16         ` Nick Kossifidis
2021-05-07  7:35           ` David Hildenbrand
2021-05-06 17:33     ` Kees Cook
2021-05-06 18:47       ` James Bottomley
2021-05-07 23:57         ` Kees Cook [this message]
2021-05-10 18:02         ` Mike Rapoport

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