Re: [PATCH v2 0/9] procfs: protect /proc//* files with file->f_cred

Kees Cook
2013-10-02T18:07:44Z

On Wed, Oct 2, 2013 at 11:00 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Wed, Oct 2, 2013 at 10:48 AM, Kees Cook <keescook@chromium.org> wrote:
>> On Wed, Oct 2, 2013 at 9:51 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>>> On Wed, Oct 2, 2013 at 3:37 PM, Djalal Harouni <tixxdz@opendz.org> wrote:
>>>> On Tue, Oct 01, 2013 at 06:40:41PM -0700, Andy Lutomirski wrote:
>>>>> On 10/01/2013 01:26 PM, Djalal Harouni wrote:
>>>>> > /proc/<pid>/* entries varies at runtime, appropriate permission checks
>>>>> > need to happen during each system call.
>>>>> >
>>>>> > Currently some of these sensitive entries are protected by performing
>>>>> > the ptrace_may_access() check. However even with that the /proc file
>>>>> > descriptors can be passed to a more privileged process
>>>>> > (e.g. a suid-exec) which will pass the classic ptrace_may_access()
>>>>> > check. In general the ->open() call will be issued by an unprivileged
>>>>> > process while the ->read(),->write() calls by a more privileged one.
>>>>> >
>>>>> > Example of these files are:
>>>>> > /proc/*/syscall, /proc/*/stack etc.
>>>>> >
>>>>> > And any open(/proc/self/*) then suid-exec to read()/write() /proc/self/*
>>>>> >
>>>>> >
>>>>> > These files are protected during read() by the ptrace_may_access(),
>>>>> > however the file descriptor can be passed to a suid-exec which can be
>>>>> > used to read data and bypass ASLR. Of course this was discussed several
>>>>> > times on LKML.
>>>>>
>>>>> Can you elaborate on what it is that you're fixing? That is, can you
>>>>> give a concrete example of what process opens what file and passes the
>>>>> fd to what process?
>>>> Yes, the references were already given in this email:
>>>> https://lkml.org/lkml/2013/8/31/209
>>>>
>>>> This has been discussed several times on lkml:
>>>> https://lkml.org/lkml/2013/8/28/544
>>>>
>>>> https://lkml.org/lkml/2013/8/28/564 (check Kees's references)
>>>>
>>>>
>>>>> I'm having trouble following your description.
>>>> Process open a /proc file and pass the fd to a more privilaged process
>>>> that will pass the ptrace_may_access() check, while the original process
>>>> that opened that file should fail at the ptrace_may_access()
>>>
>>> So we're talking about two kinds of attacks, right?
>>
>> Correct.
>>
>>> Type 1: Unprivileged process does something like open("/proc/1/maps",
>>> O_RDONLY) and then passes the resulting fd to something privileged.
>>
>> ... and then leaks contents back to unprivileged process.
>>
>>> Type 2: Unprivileged process does something like
>>> open("/proc/self/maps", O_RDONLY) and then forks. The parent calls
>>> execve on something privileged.
>>
>> ... and then parent snoops on file contents for the privileged child.
>>
>> (Type 2 is solved currently, IIUC. Type 1 could be reduced in scope by
>> changing these file modes back to 0400.)
>>
>>> Can we really not get away with fixing type 1 by preventing these
>>> files from being opened in the first place and type 2 by revoking all
>>> of these fds when a privilege-changing exec happens?
>>
>> Type 1 can be done via exec as well. Instead of using a priv exec to
>> read an arbitrary process, read it could read its own.
>
> Right.
>
>>
>> I think revoking the fd would be great. Does that mechanism exist?
>
> There's this thing that never got merged.
>
> http://thread.gmane.org/gmane.linux.kernel/1523331
>
> But doing it more directly should be reasonably straightforward. Either:
>
> (a) when a process execs and privileges change, find all the old proc
> inodes, mark them dead, and unlink them, or
>
> (b) add self_exec_id to all the proc file private_data entries (or
> somewhere else). Then just make sure that they're unchanged. I think
> the bug last time around was because the self_exec_id and struct pid
> weren't being compared together.
>
> (a) is probably nicer. I don't know if it'll break things. Linus
> seemed to think that the Chrome sandbox was sensitive to this stuff,
> but I don't know why.

I agree, (a) seems much cleaner. Hm, I don't think Chrome does
anything with these sensitive files (maps, stack, syscall, etc). But
let's ask Julien. :)

Julien, do you see any problem with Chrome's sandbox behavior if these
proc files would be unavailable across privilege changes?

-Kees

--
Kees Cook
Chrome OS Security