Re: [PATCH 4/4] setns.2: Document the pid, user, and mount namespace support.

"Michael Kerrisk (man-pages)" [off-list ref] writes:

Hi Eric,

On Thu, Dec 27, 2012 at 6:40 PM, Eric W. Biederman
[off-list ref] wrote:

quoted

"Michael Kerrisk (man-pages)" [off-list ref] writes:

quoted

Hi Eric,

Some questions below.

A quick note.  Getting the permission checks correct has been a little
more interesting that I would have preferred.

I had to add a nsown_capable(CAP_SYS_ADMIN) check to all of the setns()
install methods except the user namespace.  Not a change in pre 3.8
behavior but a change to my patch, and possibly a documentation change
below.

quoted

On Tue, Nov 27, 2012 at 1:48 AM, Eric W. Biederman
[off-list ref] wrote:

quoted

Signed-off-by: "Eric W. Biederman" <redacted>
---
 man2/setns.2 |   41 +++++++++++++++++++++++++++++++++--------
 1 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/man2/setns.2 b/man2/setns.2
index 6aa01e1..63b04dc 100644
--- a/man2/setns.2
+++ b/man2/setns.2

@@ -48,6 +48,18 @@ must refer to a network namespace.
 .BR CLONE_NEWUTS
 .I fd
 must refer to a UTS namespace.
+.TP
+.BR CLONE_NEWPID
+.I fd
+must refer to a PID namespace.
+.TP
+.BR CLONE_NEWUSER
+.I fd
+must refer to a user namespace.
+.TP
+.BR CLONE_NEWNS
+.I fd
+must refer to a mount namespace.
 .PP
 Specifying
 .I nstype

@@ -63,6 +75,25 @@ and wants to ensure that the namespace is of a particular type.
 .IR fd
 if the file descriptor was opened by another process and, for example,
 passed to the caller via a UNIX domain socket.)
+
+The pid namespace is a little different.  Reassociating the calling
+thread with a pid namespace only changes the pid namespace that the
+child processes will be created in.
+
+Changing the pid namespace for child processes is only allowed if the
+pid namespace specified by
+.IR fd
+is a child pid namespace of the pid namespace of the current thread.

I assume "current thread" above should be "calling thread", right?

What I mean in "current" from a kernel perspective.

It should be just "caller".

Okay. Changed.

quoted

Threads must share a pid namespace so mentioning threads seems wrong.

quoted

quoted

+
+A multi-threaded process may not change user namespace with setns.  A
+process may not reassociate the thread with the current user
+namespace.

What do you mean by "the current user nsamesapce"?

fd = open("/proc/self/ns/user");
setns(fd) -> -EINVAL.

So from a userspace perspective I mean "the callers user namespace".

quoted

quoted

The process reassociating itself with a user namespace
+must have CAP_SYS_ADMIN privileges in the target user namespace.

+A process may not be reassociated with a new mount namespace if it is
+multi-threaded

I tried to verify the precdeing two lines from the kernel source, but
did not work out where this check is made. Where is it?

kernel/user_namespace.c:userns_install()
fs/namespace.c:mntns_install()

Thanks.

quoted

A couple of the security checks have been pushed down into a per
namespace context, because the exact check that makes sense depends on
the namespace.

quoted

quoted

or it does not possess both CAP_SYS_CHROOT privileges
+and CAP_SYS_ADMIN rights over the target mount namespace.

Could you please expand/clarify the preceding two lines. As they
stand, I don't really understand them.

Ugh.  The text is slightly wrong.

The code is:
        if (!ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN) ||
            !nsown_capable(CAP_SYS_CHROOT) ||
            !nsown_capable(CAP_SYS_ADMIN))
                return -EPERM;

Basically you aren't allowed change your mount namespace into
a mount namespace that doesn't see you as the all powerful root
able to mount and unmount filesystems.

You aren't allowed to change your mount namespace unless you possesses
CAP_SYS_CHROOT and CAP_SYS_ADMIN.

Okay -- reworded.

So, I've done some more reworking of the text, which now reads as
folows. Could you please check this (and see my questions below).

       CLONE_NEWPID  behaves somewhat differently from the other
       nstype values: reassociating the calling  thread  with  a
       PID  namespace  only changes the PID namespace that child
       processes of the caller will be created in; it  does  not
       change the PID namespace of the caller itself.

I reworked the preceding piece a lot. Is it correct still?

       Reassoci‐
       ating with a PID namespace is only  allowed  if  the  PID
       namespace  specified by fd is a descendant (child, grand‐
       child, etc.)

Is the preceding sentence correct? (You talked only of children in
your original patch, but I believe it's more general than that.)

Yes.  That is correct.

       PID namespace of the PID namespace  of  the
       caller.

       A  multi-threaded  process  may not change user namespace
       with setns().  A process may not reassociate  the  thread
       with  the caller's user namespace.

What does the last sentence above *mean*? I don't understand it.

So the set of checks are:

	/* Don't allow gaining capabilities by reentering
	 * the same user namespace.
	 */
	if (user_ns == current_user_ns())
		return -EINVAL;

	/* Threaded processes may not enter a different user namespace */
	if (atomic_read(&current->mm->mm_users) > 1)
		return -EINVAL;

	if (!ns_capable(user_ns, CAP_SYS_ADMIN))
		return -EPERM;

Rereading it looks like I was going fast and suffered from dropping
important words.

A  multi-threaded  process  may not change it's user namespace
with setns().

aka if you have threads setns for a user namespace will fail.


    A process may not change the user namespace to the caller's user
    namespace via setns.  This is important because changing to a
    user namespace via setns implies gaining all caps, and you should
    not be able to gain all caps over your current user namespace.

Hopefully that clears it up.

       A process reassociat‐
       ing itself with a user namespace must have  CAP_SYS_ADMIN
       privileges in the target user namespace.

       A process may not be reassociated with a new mount names‐
       pace if it is multi-threaded.  Changing the mount  names‐
       pace requires that the caller possess both CAP_SYS_CHROOT
       and CAP_SYS_ADMIN capabilities.

Re the last sentence: are those capabilities required in (1) the
target namespace, or (2) the source namespace, or (3) both? I suspect
(1), but please confirm.

CAP_SYS_ADMIN is required in the current user namespace.
CAP_SYS_ADMIN is required over the target mount namesapce.

CAP_SYS_CHROOT is required in the current user namespace.

Eric
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