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gpg-agent should disable ptrace
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Description

gpg-agent makes no attempt to disable using ptrace on Linux to extract sensitive
information - such as pass phrases.

This can be observed with the following command and grepping through the
resulting core file for recently entered passphrases:
gcore pidof gpg-agent

While ptrace can be disabled by installing gpg-agent setguid, it is recommended
to [also] add the following code (from openssh) early in the main routine to
disable it regardless (you will also need the appropriate autoconf foo to check
for the prctl header and prctl function - grep the openssh source for prctl and
ptrace for more details as well as a regression test):

#if defined(HAVE_SYS_PRCTL_H)
#include <sys/prctl.h> /* For prctl() and PR_SET_DUMPABLE */
#endif

...main()...
#if defined(HAVE_PRCTL) && defined(PR_SET_DUMPABLE)
/* Disable ptrace on Linux without sgid bit */
prctl(PR_SET_DUMPABLE, 0);
#endif

This does not prevent the root user from using ptrace on the agent, but it would
prevent another process of the same user from casually running ptrace on an
already running gpg-agent.

Thanks,
-Ian

Event Timeline

werner claimed this task.

You can't protect a user from himself. gpg-agent is a user process and not a
system wide daemon.

I recognise that gpg-agent is a user process - if it wasn't this issue wouldn't
apply at all.
And naturally this won't protect the user from themselves entirely - why if they
wanted, they could even start gpg-agent from gdb and skip the prctl call and
after entering his passphrase could then dump it from gdb. Or maybe they could
use an alternate "gpg-agent" that does not disable ptrace. Or they could wrap
gpg-agent and disable the call with LD_PRELOAD. Hell, if they wanted they could
probably even post their private keys unencrypted on a public webserver.

But what about an already running agent process? Why should the flash plugin in
firefox be allowed to generate a core dump of the agent and grab the passphrase
from there? Why should someone else be able to walk over the user's unlocked
terminal and enter gcore pidof gpg-agent and walk away with the user's passphrase?
Of course the best solution in a perfect world is better user education (lock ur
screen plz kthxbai) and ensuring that the rest of his/her system is secure. But
we all know that the world is not perfect and occasionally someone forgets to
lock their screen, or someone discovers that the lockout on gnome screensaver
can be trivially bypassed by repeatedly hitting the return key until an X error
is generated terminating the screensaver, but surely this simple extra step
could only help matters?

gpg-agent won't create a core dump; see disable_core_dump(). However it is
still possible to read the memory of a process you own using ptrace or
/proc/PID/mem.

We could now start and try to harden certain ways of attacking a user's account
from the user's account or with physical access. That will lead us to nowhere
on common Unix system because it is not designed to prevent such attacks.
There are selfs filled with literature on this problem. As a starter look for
capability based systems.

This is nothing for a bug tracker - if you want to continue this discussion,
please use a mailing list.

fwiw, i'd be fine with enabling protection against ptrace for gpg-agent, even if
that doesn't fix all the other ways a process can be attacked from another process.

if there are multiple holes, we should plug the ones we can plug, to increase
the pressure to make the other ones fixable.

werner renamed this task from gpg-agent allows ptrace to gpg-agent should disable ptrace.Aug 4 2015, 12:42 PM
werner added a project: Feature Request.
werner removed a project: Bug Report.

dkg: The problem is that the underlying architecture is broken. Unix's stock
permission model is about protecting users from other users, not protecting
processes from other processes. Thus, I don't think it makes any sense to
complicate the code by implementing these effectively useless protections.

FWIW, a widely used practical system that does a much better job at this is
Android. Android runs every program under its own uid. We could do the same
thing with gpg-agent. In fact, this is currently possible with a little help
from ssh. Unfortunately, this requires a fair amount of work by the user to set
up. In particular, the user needs to create a secondary account. It would be
nice if distributions provided a simply way for an unprivileged user to allocate
additional uids, but this is probably a lot of work.

I agree with you that the standard UNIX model is generally insufficient here.

Perhaps the distros could weigh in with mechanisms to facilitate
secondary-account creation for agents and the like. I've opened
https://bugs.debian.org/794667 about this.

However, i don't think the weak UNIX permissions model is a reason to avoid a
small piece of code like that offered by DarkStarSword below. Closing off one
avenue of attack is still worthwhile, even if other avenues remain. As werner
said, gpg-agent won't create a coredump (even though other avenues of attack are
possible).

Is there something about the complexity of prctl(PR_SET_DUMPABLE, 0); that makes
it undesirable?

So a single syscall to demonstrably reduce the attack surface really complicates
things does it?

News to me.

It does not reduce the attack surface. And yes, it complicates things
because you can't anymore debug the process without changing the code
or using other tricks (aka attack). I have not tested the SELinux
feature for quite some time but gnupg supports SELinux if configured
with --enable-selinux-support. If you want some protection better use
that.

dkg: We disable core dumps for the simple reason that we do not want
to see core files on disk. Disk sectors have a longer lifetime than a
process and a user session - thus avoiding core files is a real world
threat mitigation.

Without this I can do gcore pidof gpg-agent and QUITE CLEARLY see my
passphrase in the produced dump:

ian@draal~ [i]> gcore (pidof gpg-agent)
0x00007fb8f8849293 in __select_nocancel () at

../sysdeps/unix/syscall-template.S:81

81      ../sysdeps/unix/syscall-template.S: No such file or directory.
warning: target file /proc/1560/cmdline contained unexpected null characters
warning: Memory read failed for corefile section, 8192 bytes at 0x7ffce0a12000.
Saved corefile core.1560
0x00007f2dd583c293 in __select_nocancel () at

../sysdeps/unix/syscall-template.S:81

81      ../sysdeps/unix/syscall-template.S: No such file or directory.
warning: target file /proc/1540/cmdline contained unexpected null characters
warning: Memory read failed for corefile section, 8192 bytes at 0x7ffccfbe3000.
Saved corefile core.1540
ian@draal~ [i]> strings core.1560 | grep pass
passwd
This is my ultra secure passphrase - I definitely expect any program that

manages this to take reasonable steps to keep it safe even if I accidentally
leave my screen unlocked one day!

Invalid passphrase; please try again
You need a passphrase to unlock the secret key for user:%0A"test123

<test@test.com>"%0A2048-bit RSA key, ID DE3A7EAB, created 2015-08-06 (main key
ID F459B571)%0A

    SETERROR Invalid passphrase; please try again
    his is my ultra secure passphrase - IQ
    ian@draal~ [i]>

With this one line change I cannot do the above - that's the definition of
reducing the attack surface last time I checked.

You absolutely can still use gdb to debug it - you just have to start it under
gdb as opposed to attaching to an existing process, or attach the debugger as
root. You could also just disable the syscall in a debug build.

This identified another bug: To be prepared for FIPS evaluation,
gpg-agent does not store the cached passphrases in the clear but
encrypts them in memory. Right this is security by obscurity but if
we ever have a way to store that key in a secured RAM (e.g. TPM, ARM
TrustZone) we can indeed limit the time a passphrase is available in
the clear to the period it is really needed. This all seems to work
but your tests shows that libassuan does not clear its internal line
buffers so that you can actually find the passphrase in the core
file. I just pushed a fix for this.

IIRC, FIPS mode in Linux inhibits all access to process memory system
wide. Changing this just a for a single user process does not make
much sense.

Further, being able to attach to a running processing is one of the
best debug methods we have. Giving up on this without for a perceived
extra protection is not going to work. There are too many ways to get
the passphrase using other ways. Linux can't protect a user to get
data belonging to him. Iff gpg-agent were a system daemon things
would be different and extra protection would make sense as a
fallback.

Glad I was able to help get one bug fixed at least :)

The in-memory encryption will definitely help in this scenario (that is, a
casual attack by e.g. a colleague or another student having a laugh as their
friend left their screen unlocked... real attack - I should know, I've done it,
and had it done to me, as had several of my friends back in uni (to be fair -
that was on Firefox password manager, but it could just as easily have been
gpg-agent)... This is not about stopping a motivated attacker with physical
access to the system as they could always subvert the system in other ways e.g.
adding a shell alias to run a trojaned gpg-agent instead of the real thing,
install a key logger, etc).

I'm still not super happy that a casual attacker could walk away with a core
file containing the encrypted passphrase and the key to decrypt it. What started
as a casual attack for a laugh could later transform into a more serious attack
given that they can hold onto this information indefinitely. I'm not a motivated
attacker, but that would tempt the hell out of me if I was even slightly so
inclined. I should know, because I've been in a similar situation in the past
where I obtained an unshadowed passwd file (through a purely casual attack when
I was looking up a friend's uid and discovered the passwd file was not
shadowed... so of course I made a copy). I could have left it alone, but it
tempted the hell out of me and I ended up running john over it for two straight
weeks (never did much with the result, but that's not the point)!

I would hope that the developers of any security product learns to think like an
attacker.

I must say I am deeply troubled by the priority seeming to be on the ease of
debugging a security product which has the sole purpose of keeping a passphrase
safe. As the saying goes security is always a trade-off, but given that
gpg-agent is a security product and not a word processor, this particular trade
off does not sit right with me. I would expect the development team to have root
access on their own systems, which avoids the issue as the root user can always
attach a debugger with or without this change - is there truly a reason that
they need to attach to a running gpg-agent on a system they don't have root on?
And what about the thousands of gpg-agents running on other systems in the wild
that should never need to attach a debugger (and if they do... sudo)?

The information about FIPS mode and SELinux is good to know, at least for people
running distributions that support and enable them by default. But from what I
can gather FIPS mode is a RHEL only feature (I may be wrong - I'm not all that
familiar with it), and SELinux is still either not enabled, or in permissive
mode in many distributions by default (including Debian and Ubuntu).

As I mentioned in the original report, an alternative way to protect the memory
of gpg-agent is to install it with the setgid bit set (ssh-agent does both the
prctl() and setgid for example - now there's some developers I applaud). Unlike
SELinux and FIPS mode this works in every distribution and has been supported
for donkeys years.

I'm going to introduce the prctl(SET_DUMPABLE, 0) change to main in
agent/gpg-agent.c in the debian 2.1.x series as of 2.1.7-1, using the patch i'm
attaching here.

I make no representations that this solves all possible memory leakages, but it
does address one specific and relatively straightforward attack.

As to Werner's legitimate concerns about making debugging harder, there remain
at least two options: ptrace as the superuser, and launching gpg-agent itself
under gdb directly.

If this experiment proves disastrous somehow (i'm not seeing how), we can always
revert the patch.

I am closing this.

BTW: I can't share DarkStarSword's fear about prioritizing ease of debugging
over security - I would never do that for a real security problem; Neal and me
both explained why this proposed fix does can't help against an attack.

werner added a project: Won't Fix.

so far, the proposed mechanisms for getting at gpg-agent's memory from a peer
process running as the same user are:

a) ptrace (e.g. via /usr/bin/gcore or /usr/bin/strace)
b) /proc/$PID/mem, which is owned by the user and mode 0600

DarkStarSword's patch effectively closes (a) (by rejecting ptrace connections)
and appears on my GNU/Linux system to close (b) as well: /proc/$PID/mem is
root-owned when the patch is applied instead of being user-owned.

Are there other channels for per-process memory access that we should be
thinking about?

I agree with Werner and Neal that the UNIX model is probably insufficient to
close all the holes easily, but i also don't think that's a good reason to avoid
closing those holes we can close.

If there are other ways that another process by the same user can get at the
RAM, please point them out and i'll look into ways to address them too.

In the meantime, i'll also look into ways to facilitate running the process as a
separate user account entirely.

c) Run gpg-agent under gdb
d) Run a modified gpg-agent (rm ~/S.gpg-agent; my-gpg-agent --daemon)
e) Hook into the tty and use pinentry-curses
f) scp ~/.gnupg/private-keys-v1.d/* mybox: and sniff the passphrase.