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

Andy Lutomirski
2013-10-02T16:51:45Z

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?

Type 1: Unprivileged process does something like open("/proc/1/maps",
O_RDONLY) and then passes the resulting fd to something privileged.

Type 2: Unprivileged process does something like
open("/proc/self/maps", O_RDONLY) and then forks. The parent calls
execve on something privileged.

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?

I'm not objecting to your patches so much as thinking that read(2) has
no business looking at current->cred *at all*. But maybe that ship
has already sailed.

--Andy

>
>
>> --Andy
>>
>
> --
> Djalal Harouni
> http://opendz.org

--
Andy Lutomirski
AMA Capital Management, LLC