Re: [PATCH v3 0/5] Add support for RESOLVE_MAYEXEC

[Christian Heimes <christian@python.org>]

On 29/04/2020 00.01, Jann Horn wrote:
> On Tue, Apr 28, 2020 at 11:21 PM Florian Weimer <fw@deneb.enyo.de> wrote:
>> * Jann Horn:
>>
>>> Just as a comment: You'd probably also have to use RESOLVE_MAYEXEC in
>>> the dynamic linker.
>>
>> Absolutely.  In typical configurations, the kernel does not enforce
>> that executable mappings must be backed by files which are executable.
>> It's most obvious with using an explicit loader invocation to run
>> executables on noexec mounts.  RESOLVE_MAYEXEC is much more useful
>> than trying to reimplement the kernel permission checks (or what some
>> believe they should be) in userspace.
> 
> Oh, good point.
> 
> That actually seems like something Mickaël could add to his series? If
> someone turns on that knob for "When an interpreter wants to execute
> something, enforce that we have execute access to it", they probably
> also don't want it to be possible to just map files as executable? So
> perhaps when that flag is on, the kernel should either refuse to map
> anything as executable if it wasn't opened with RESOLVE_MAYEXEC or
> (less strict) if RESOLVE_MAYEXEC wasn't used, print a warning, then
> check whether the file is executable and bail out if not?
> 
> A configuration where interpreters verify that scripts are executable,
> but other things can just mmap executable pages, seems kinda
> inconsistent...

+1

I worked with Steve Downer on Python PEP 578 [1] that added audit hooks
and PyFile_OpenCode() to CPython. A PyFile_OpenCode() implementation
with RESOLVE_MAYEXEC will hep to secure loading of Python code. But
Python also includes a wrapper of libffi. ctypes or cffi can load native
code from either shared libraries with dlopen() or execute native code
from mmap() regions. For example SnakeEater [2] is a clever attack that
abused memfd_create syscall and proc filesystem to execute code.

A consistent security policy must also ensure that mmap() PROT_EXEC
enforces the same restrictions as RESOLVE_MAYEXEC. The restriction
doesn't have be part of this patch, though.

Christian

[1] https://www.python.org/dev/peps/pep-0578/
[2] https://github.com/nullbites/SnakeEater/blob/master/SnakeEater2.py