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

Djalal Harouni
2013-10-02T18:22:14Z

On Wed, Oct 02, 2013 at 10:48:55AM -0700, Kees Cook 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.)
Kees for 0400 on /proc/*/maps, it was reported that it could break glibc

Even with 0444 this series will catch it, take a look at /proc/*/stat
example, we just delay the check that is suposed to be done during
->open() into ->read(), if the cred change of course

--
Djalal Harouni