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From: ebiederm@xmission.com (Eric W. Biederman)
To: Jann Horn <jann@thejh.net>
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Date: Sun, 30 Oct 2016 23:14:04 -0500
In-Reply-To: <20161030190915.GD2558@pc.thejh.net> (Jann Horn's message of
        "Sun, 30 Oct 2016 20:09:15 +0100")
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Subject: Re: [PATCH v2 8/8] Documentation: add security/ptrace_checks.txt
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Jann Horn <jann@thejh.net> writes:

> On Sat, Oct 01, 2016 at 08:16:00PM -0700, Krister Johansen wrote:
>> On Fri, Sep 23, 2016 at 10:40:38PM +0200, Jann Horn wrote:
>> > +=====================
>> > +FILESYSTEM DEBUG APIS
>> > +=====================
>> > +
>> > +The pid / tgid entries in procfs contain various entries that allow debugging
>> > +access to a process. Interesting entries are:
>> > +
>> > + - auxv permits an ASLR bypass
>> > + - cwd can permit bypassing filesystem restrictions in some cases
>> > + - environ can leak secret tokens
>> > + - fd can permit bypassing filesystem restrictions or leak access to things like
>> > +   pipes
>> > + - maps permits an ASLR bypass
>> > + - mem grants R+W access to process memory
>> > + - stat permits an ASLR bypass
>> > +
>> > +Of these, all use both a normal filesystem DAC check (where the file owner is
>> > +the process owner for a dumpable process, root for a nondumpable process) and a
>> > +ptrace_may_access() check; however, the DAC check may be modified, and the
>> > +ptrace_may_access() is performed under PTRACE_FSCREDS, meaning that instead of
>> > +the caller's ruid, rgid and permitted capabilities, the fsuid, fsgid and
>> > +effective capabilities are used, causing the case where a daemon drops its euid
>> > +prior to accessing a file for the user to be treated correctly for this check.
>> 
>> Thanks for writing this up.
>> 
>> Is it worth mentioning some of the less obvious aspects of how user
>> namespaces interact with the filesystem debug APIs?  Of particular note:
>> a nondumpable process will always be assigned the global root ids.
>> Checks against capabilities for procfs require that the uid and gid have
>> a mapping in the current namepsace.   That's enforced through
>> capable_wrt_inode_uidgid().
>
> Yeah, makes sense. Added that. Thanks!

That will actually be changing for 4.10.  mm->user_ns allows me to use
the user namespace id 0 if that id is mapped.

Eric