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

Djalal Harouni
2013-10-02T18:48:51Z

On Wed, Oct 02, 2013 at 11:35:45AM -0700, Kees Cook wrote:
> On Wed, Oct 2, 2013 at 11:22 AM, Djalal Harouni <tixxdz@opendz.org> wrote:
> > 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
>
> I didn't mean maps should be 0400. The maps file is already handled
> differently (pinning mm at open time). I didn't think it was one of
> the problematic files.
Kees currently all these files can be used to leak data, except for
/proc/*/{mem,environ}

These are the only one that pin the mm at open time. But I'm not sure
that this solution will work for /proc/*/maps since they need vma info
which will be perhaps freed if task execv, Need to check it.

> Regardless, glibc uses /proc/self/maps, which would be fine here, right?
I did not touch /proc/self/maps and others, but I'm planning to fix them
if this solution is accepted.

I'll do the same thing as in /proc/*/stat for maps, let it be 0444, and
try to delay the check to ->read(). So during ->read() perform
ptrace_may_access() on currenct's cred and process_allow_access() on
file's opener cred. This should work.

> -Kees
>
> --
> Kees Cook
> Chrome OS Security

--
Djalal Harouni