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

Djalal Harouni
2013-10-02T18:26:50Z

On Wed, Oct 02, 2013 at 11:00:26AM -0700, Andy Lutomirski 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
Will take a look at it.

> (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.
The bug was about self_exec_id not beeing unique. self_exec_id stuff
must be unique during life time as it's done currently in grsecurity
with exec_id.

--
Djalal Harouni