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

Andy Lutomirski
2013-10-02T18:00:50Z

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.

--Andy