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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Kees Cook <keescook@chromium.org>
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Date:   Tue, 12 May 2020 15:31:57 -0500
In-Reply-To: <202005121218.ED0B728DA@keescook> (Kees Cook's message of "Tue,
        12 May 2020 12:25:24 -0700")
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Subject: Re: [PATCH 3/5] exec: Remove recursion from search_binary_handler
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Kees Cook <keescook@chromium.org> writes:

> On Tue, May 12, 2020 at 01:42:53PM -0500, Eric W. Biederman wrote:
>> Kees Cook <keescook@chromium.org> writes:
>> > Should binfmt_misc do the install, or can the consuming binfmt do it?
>> > i.e. when binfmt_elf sees bprm->execfd, does it perform the install
>> > instead?
>> 
>> I am still thinking about this one, but here is where I am at.  At a
>> practical level passing the file descriptor of the script to interpreter
>> seems like something we should encourage in the long term.  It removes
>> races and it is cheaper because then the interpreter does not have to
>> turn around and open the script itself.
>
> Yeah, this does sounds pretty good, though I have concerns about doing
> it for a process that isn't expecting it. I've seen a lot of bad code
> make assumptions about initial fd numbers. :(

Yes.  That is definitely a concern.

>> Strictly speaking binfmt_misc should not need to close the file
>> descriptor in binfmt_misc because we have already unshared the files
>> struct and reset_files_struct should handle restoring it.
>
> If I get what you mean, I agree. The error case is fine.
>
>> Calling fd_install in binfmt_misc still seems wrong, as that exposes
>> the new file descriptor to user space with the old creds.
>
> I haven't dug into the details here -- is there a real risk here? The
> old creds are what opened the file originally for the exec. Are you
> thinking about executable-but-not-readable files?

I am thinking about looking in proc/<pid>/fd and maybe opening those
files.  That access is gated by ptrace_may_access which is gated
by the process credentials. So I know strictly speaking it is wrong.

I think you are correct that it would only allow access to a file that
could be accessed another way.  Even execveat at a quick glance appears
to go through the orinary permission checks of open.

The current code is definitely a maintenance pitfall as it install state
into the process early.

>> It is possible although unlikely for userspace to find the file
>> descriptor without consulting AT_EXECFD so just to be conservative I
>> think we should install the file descriptor in begin_new_exec even if
>> the next interpreter does not support AT_EXECFD.
>
> I think universally installing the fd needs to be a distinct patch --
> it's going to have a lot of consequences, IMO. We can certainly deal
> with them, but I don't think it should be part of this clean-up series.

I meant generically installing the fd not universally installing it.

>> I am still working on how to handle recursive binfmts but I suspect it
>> is just a matter of having an array of struct files in struct
>> linux_binprm.
>
> If install is left if binfmt_misc, then the recursive problem goes away,
> yes?

I don't think leaving the install in binfmt_misc is responsible at this
point.

Eric