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

[Kees Cook]

On Wed, Oct 2, 2013 at 11:22 AM, Djalal Harouni <tixxdz@xxxxxxxxxx> 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@xxxxxxxxxxxxxx> wrote:
>> > On Wed, Oct 2, 2013 at 3:37 PM, Djalal Harouni <tixxdz@xxxxxxxxxx> 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.

Regardless, glibc uses /proc/self/maps, which would be fine here, right?

-Kees

-- 
Kees Cook
Chrome OS Security
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