Re: Review of ptrace Yama ptrace_scope description
From: Michael Kerrisk (man-pages)
Date: Tue Jun 28 2016 - 02:11:46 EST
Hi Jann,
On 06/25/2016 04:30 PM, Jann Horn wrote:
On Sat, Jun 25, 2016 at 09:30:43AM +0200, Michael Kerrisk (man-pages) wrote:
Hi Kees,
So, last year, I added some documentation to ptrace(2) to describe
the Yama ptrace_scope file. I don't think I asked you for review
at the time, but in the light of other changes to the ptrace(2)
page, it occurred to me that it might be a good idea to ask you
to check the text below to see if anything is missing or could be
improved. Might you have a moment for that?
/proc/sys/kernel/yama/ptrace_scope
On systems with the Yama Linux Security Module (LSM) installed
(i.e., the kernel was configured with CONFIG_SECURITY_YAMA),
the /proc/sys/kernel/yama/ptrace_scope file (available since
Linux 3.4) can be used to restrict the ability to trace a
process with ptrace(2) (and thus also the ability to use tools
such as strace(1) and gdb(1)). The goal of such restrictions
is to prevent attack escalation whereby a compromised process
can ptrace-attach to other sensitive processes (e.g., a GPG
agent or an SSH session) owned by the user in order to gain
additional credentials and thus expand the scope of the attack.
More precisely, the Yama LSM limits two types of operations:
* Any operation that performs a ptrace access mode
PTRACE_MODE_ATTACH checkâfor example, ptrace()
PTRACE_ATTACH. (See the "Ptrace access mode checking" disâ
cussion above.)
* ptrace() PTRACE_TRACEME.
A process that has the CAP_SYS_PTRACE capability can update the
/proc/sys/kernel/yama/ptrace_scope file with one of the followâ
ing values:
0 ("classic ptrace permissions")
No additional restrictions on operations that perform
PTRACE_MODE_ATTACH checks (beyond those imposed by the
commoncap and other LSMs).
The use of PTRACE_TRACEME is unchanged.
1 ("restricted ptrace") [default value]
When performing an operation that requires a
PTRACE_MODE_ATTACH check, the calling process must have
a predefined relationship with the target process. By
default, the predefined relationship is that the target
process must be a child of the caller.
A target process can employ the prctl(2) PR_SET_PTRACER
operation to declare a different PID that is allowed to
perform PTRACE_MODE_ATTACH operations on the target.
See the kernel source file Documentation/secuâ
rity/Yama.txt for further details.
The use of PTRACE_TRACEME is unchanged.
(namespaced) CAP_SYS_PTRACE is also sufficient here.
Both here and in the "admin-only attach" case, it is IMO important to
note that creating a user namespace effectively removes the Yama
protection because the owner of a namespace, when accessing its
contents from outside, is relatively capable.
This means that when a process tries to use namespaces to sandbox
itself, it inadvertently makes itself more accessible.
(This could probably be worked around in the kernel, but such a
workaround would likely not be default, but rather opt-in via a new
flag for clone() and unshare() or so.)
Tanks for catching this!
So I've made that section of text:
A process that has the CAP_SYS_PTRACE capability can update the
/proc/sys/kernel/yama/ptrace_scope file with one of the following
values:
0 ("classic ptrace permissions")
No additional restrictions on operations that perform
PTRACE_MODE_ATTACH checks (beyond those imposed by the comâ
moncap and other LSMs).
The use of PTRACE_TRACEME is unchanged.
1 ("restricted ptrace") [default value]
When performing an operation that requires a
PTRACE_MODE_ATTACH check, the calling process must either
have the CAP_SYS_PTRACE capability in the user namespace of
the target process or it have a predefined relationship
with the target process. By default, the predefined relaâ
tionship is that the target process must be a child of the
caller.
A target process can employ the prctl(2) PR_SET_PTRACER
operation to declare a different PID that is allowed to
perform PTRACE_MODE_ATTACH operations on the target. See
the kernel source file Documentation/security/Yama.txt for
further details.
The use of PTRACE_TRACEME is unchanged.
2 ("admin-only attach")
Only processes with the CAP_SYS_PTRACE capability in the
user namespace of the target process may perform
PTRACE_MODE_ATTACH operations or trace children that employ
PTRACE_TRACEME.
3 ("no attach")
No process may perform PTRACE_MODE_ATTACH operations or
trace children that employ PTRACE_TRACEME.
Once this value has been written to the file, it cannot be
changed.
With respect to values 1 and 2, note that creating a user namesâ
pace effectively removes the Yama protection, because the owner of
a namespace, when accessing its members from outside, has
CAP_SYS_PTRACE within the namespace. This means that when a
process tries to use namespaces to sandbox itself, it inadverâ
tently weakens the protections offered by the Yama LSM.
Okay?
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/