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

Kees Cook
2013-10-02T19:43:36Z

On Wed, Oct 2, 2013 at 11:48 AM, Djalal Harouni <tixxdz@opendz.org> wrote:
> 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.

Ah, yes, you're totally right. I had misremembered.

-Kees

--
Kees Cook
Chrome OS Security