Re: Documenting ptrace access mode checking
From: Michael Kerrisk (man-pages)
Date: Wed Jun 22 2016 - 15:21:38 EST
Hi Jann,
On 06/21/2016 10:55 PM, Jann Horn wrote:
On Tue, Jun 21, 2016 at 11:41:16AM +0200, Michael Kerrisk (man-pages) wrote:
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.
Here's the new ptrace(2) text. Any comments, technical or terminological
fixes, other improvements, etc. are welcome.
As others have said, I'm surprised about seeing documentation about
kernel-internal constants in manpages - but I think it might be a good
thing to have there, given that people who look at ptrace(2) are likely
to be interested in low-level details.
I agree that it is a little surprising to add kernel-internal
constants in a man page. (There are precedents, but they are few.)
But see my reply to Kees. It's more than just explaining low level
details: there are various kinds of user-space behavior differences
(real vs filesystem credentials; permitted vs effective capabilities)
produced by the ptrace_may_access() checks, and those behaviors need
to be described and *somehow* labeled for cross-referencing from
other man pages.
[[
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 effecâ
tively the default before Linux 2.6.27.)
Since Linux 4.5, the above access mode checks may be combined
s/may/must/; otherwise __ptrace_may_access() will yell about the kernel
code being broken and deny access.
Good point. I changed "may" to "are". ("must" is not quite right to my
"user-space" ear; it might be misread as implying that the user-space
developer must do something.)
(ORed) with one of the following modifiers:
PTRACE_MODE_FSCREDS
Use the caller's filesystem UID and GID (see credenâ
tials(7)) or effective capabilities for LSM checks.
PTRACE_MODE_REALCREDS
Use the caller's real UID and GID or permitted capabiliâ
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.]
The algorithm employed for ptrace access mode checking deterâ
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.
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.
Might want to add a "for historical reasons" or so here.
Can you be a little more precise about "here", and maybe tell me why
you think it helps?
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.
Might want to also specify here (like below) that the caller needs to
have the capability relative to the user ns of the target.
Done.
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:
For people who are unaware of how the LSM API works, it might be good to
clarify that the commoncap LSM is *always* invoked; otherwise, it might
give the impression that using another LSM would replace it.
As we can see, I am one of those who are unaware of how the LSM API
works :-/.
(Also, are there other documents that refer to it as "default LSM"? I
think that that term is slightly confusing.)
No, that's a terminological confusion of my own making. Fixed now.
I changed this text to:
Various parts of the kernel-user-space API (not just ptrace(2)
operations), require so-called "ptrace access mode permissions"
which are gated by any enabled Linux Security Module (LSMs)âfor
example, SELinux, Yama, or Smackâand by the the commoncap LSM
(which is always invoked). 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:
BTW, can you point me at the piece(s) of kernel code that show that
"commoncap" is always invoked in addition to any other LSM that has
been installed?
a) If the access mode includes PTRACE_MODE_FSCREDS, then
use the caller's effective capability set in the followâ
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.
This also requires the caller and the target to be in the same user
namespace.
Thanks! Fixed.
 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).
(Actually, kcmp() just needs READ access - you described that accurately
earlier, but it's wrong here.)
D'oh! Thanks!
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).
That sounds great! :)
Thanks for the review!
Thanks,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/