Hi Kees,
On 06/21/2016 10:29 PM, Kees Cook wrote:
On Tue, Jun 21, 2016 at 12:55 PM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
Adding Oleg just because he seems to do most of the ptrace related
maintenance these days.
"Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> writes:
Hi Jann, Stephen, et al.
Jann, since you recently committed a patch in this area, and Stephen,
since you committed 006ebb40d3d much further back in time, I wonder if
you might help me by reviewing the text below that I propose to add to
the ptrace(2) man page, in order to document "ptrace access mode
checking" that is performed in various parts of the kernel-user-space
interface. Of course, I welcome input from anyone else as well.
Your text matches my understand of this code. :)
Thanks for reviewing the text!
Here's the new ptrace(2) text. Any comments, technical or terminological
fixes, other improvements, etc. are welcome.
[[
Ptrace access mode checking
Various parts of the kernel-user-space API (not just ptrace(2)
operations), require so-called "ptrace access mode permissions"
which are gated by Linux Security Modules (LSMs) such as
SELinux, Yama, Smack, or the default LSM. Prior to Linux
2.6.27, all such checks were of a single type. Since Linux
2.6.27, two access mode levels are distinguished:
PTRACE_MODE_READ
For "read" operations or other operations that are less
dangerous, such as: get_robust_list(2); kcmp(2); reading
/proc/[pid]/auxv, /proc/[pid]/environ, or
/proc/[pid]/stat; or readlink(2) of a /proc/[pid]/ns/*
file.
PTRACE_MODE_ATTACH
For "write" operations, or other operations that are
more dangerous, such as: ptrace attaching
(PTRACE_ATTACH) to another process or calling
process_vm_writev(2). (PTRACE_MODE_ATTACH was effec‐
tively the default before Linux 2.6.27.)
Since Linux 4.5, the above access mode checks may be combined
(ORed) with one of the following modifiers:
PTRACE_MODE_FSCREDS
Use the caller's filesystem UID and GID (see creden‐
tials(7)) or effective capabilities for LSM checks.
PTRACE_MODE_REALCREDS
Use the caller's real UID and GID or permitted capabili‐
ties for LSM checks. This was effectively the default
before Linux 4.5.
Because combining one of the credential modifiers with one of
the aforementioned access modes is typical, some macros are
defined in the kernel sources for the combinations:
PTRACE_MODE_READ_FSCREDS
Defined as PTRACE_MODE_READ | PTRACE_MODE_FSCREDS.
PTRACE_MODE_READ_REALCREDS
Defined as PTRACE_MODE_READ | PTRACE_MODE_REALCREDS.
PTRACE_MODE_ATTACH_FSCREDS
Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS.
PTRACE_MODE_ATTACH_REALCREDS
Defined as PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS.
One further modifier can be ORed with the access mode:
PTRACE_MODE_NOAUDIT (since Linux 3.3)
Don't audit this access mode check.
[I'd quite welcome some text to explain "auditing" here.]
AKA don't let the audit subsystem know. Which tends to
generate audit records capable is called.
The algorithm employed for ptrace access mode checking deter‐
mines whether the calling process is allowed to perform the
corresponding action on the target process, as follows:
1. If the calling thread and the target thread are in the same
thread group, access is always allowed.
This test only exsits because the LSMs historically and I suspect
continue to be broken and deny a process the ability to ptrace itself.
Well, it's not that the LSMs are broken, it's that self-inspection is
a short-circuited "allow". The LSMs aren't involved.
2. If the access mode specifies PTRACE_MODE_FSCREDS, then for
the check in the next step, employ the caller's filesystem
user ID and group ID (see credentials(7)); otherwise (the
access mode specifies PTRACE_MODE_REALCREDS, so) use the
caller's real user ID and group ID.
3. Deny access if neither of the following is true:
· The real, effective, and saved-set user IDs of the target
match the caller's user ID, and the real, effective, and
saved-set group IDs of the target match the caller's
group ID.
· The caller has the CAP_SYS_PTRACE capability.
4. Deny access if the target process "dumpable" attribute has
a value other than 1 (SUID_DUMP_USER; see the discussion of
PR_SET_DUMPABLE in prctl(2)), and the caller does not have
the CAP_SYS_PTRACE capability in the user namespace of the
target process.
5. The kernel LSM security_ptrace_access_check() interface is
invoked to see if ptrace access is permitted. The results
depend on the LSM. The implementation of this interface in
the default LSM performs the following steps:
a) If the access mode includes PTRACE_MODE_FSCREDS, then
use the caller's effective capability set in the follow‐
ing check; otherwise (the access mode specifies
PTRACE_MODE_REALCREDS, so) use the caller's permitted
capability set.
b) Deny access if neither of the following is true:
· The caller's capabilities are a proper superset of the
target process's permitted capabilities.
· The caller has the CAP_SYS_PTRACE capability in the
target process's user namespace.
Note that the default LSM does not distinguish between
PTRACE_MODE_READ and PTRACE_MODE_ATTACH.
6. If access has not been denied by any of the preceding
steps, then access is allowed.
]]
There are accompanying changes to various pages that refer to
the new text in ptrace(2), so that, for example, kcmp(2) adds:
Permission to employ kcmp() is governed by ptrace access mode
PTRACE_MODE_ATTACH_REALCREDS checks against both pid1 and pid2;
see ptrace(2).
and proc.5 has additions such as:
/proc/[pid]/auxv (since 2.6.0-test7)
...
Permission to access this file is governed by a ptrace
access mode PTRACE_MODE_READ_FSCREDS check; see
ptrace(2).
/proc/[pid]/cwd
...
Permission to dereference or read (readlink(2)) this
symbolic link is governed by a ptrace access mode
PTRACE_MODE_READ_FSCREDS check; see ptrace(2).
Hmm.
When I gave this level of detail about the user namespace permission
checks you gave me some flack, because it was not particularly
comprehensible to the end users. I think you deserve the same feedback.
How do we say this in a way that does not describes a useful way to
think about it. I read this and I know a lot of what is going on and my
mind goes numb.
If the goal is to document the kernel behavior, then the details
around the various MODE flags makes sense. If this is just about
userspace-visible behavior, I would agree: the MODE flag details may
be overkill (since they are not exposed to userspace).
The problem is that this is about user-space-visible behavior, but in
order to explain that behavior at what I believe is a sufficient level
of detail (e.g., to differentiate the various types of checks that
are performed for various system calls and pseudofile accesses),
one needs (1) to discuss the MODE flag details as implemented in
the kernel, and (2) to have a shorthand way to refer to the various
cases from other pages. It's not absolutely necessary to name the
flags for (1), but using the flag names is certainly a handy
shorthand for (2).
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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