Interaction user namespace, /proc/1 ownership & cap_set

Daniel P. Berrange berrange at redhat.com
Mon Jul 1 16:19:46 UTC 2013


On Mon, Jul 01, 2013 at 05:16:25PM +0100, Daniel P. Berrange wrote:
> I'm struggling debugging a strange problem with interaction between user
> namespaces, cap_set and ownership of files in /proc/1/
> 
> I'm using a modified version (attached to this mail) of the demo program
> userns_child_exec.c linked on https://lwn.net/Articles/532593/
> 
>   $ gcc -lcap -Wall -o userns_child_exec userns_child_exec.c 
> 
> First normal execution appears to work just fine (as root):
> 
>   $ ./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash
>   Launching child init
>   # umount /proc/sys/fs/binfmt_misc
>   # umount /proc/sys/fs/binfmt_misc
>   # umount /proc/fs/nfsd
>   # umount /proc
>   # mount -t proc proc /proc/
>   # ls -al /proc/1/environ 
>   -r--------. 1 root root 0 Jul  1 17:04 /proc/1/environ
> 
> 
> My modification adds support for a '-c' arg to call the program to use
> cap_set() from libcap.so in order to remove the CAP_SYS_MODULE capability.
> 
> If I run the program with the '-c' arg present, then the files in
> the /proc/1/ directory all end up owned by nfsnobody.nfsbody
> 
>   $ ./userns_child_exec -c -p -m -U -M '0 1000 1' -G '0 1000 1' bash
>   Launching child init
>   # umount /proc/sys/fs/binfmt_misc
>   # umount /proc/sys/fs/binfmt_misc
>   # umount /proc/fs/nfsd
>   # umount /proc
>   # mount -t proc proc /proc/
>   # ls -al /proc/1/environ 
>   -r--------. 1 nfsnobody nfsnobody 0 Jul  1 17:01 /proc/1/environ
> 
> Why on earth would calling 'cap_set()' to drop a capability cause
> the user/group ownership of files in /proc/1/ to change ?
> 
> Any child processes launched from this point get correct ownership
> on their /proc/NNN files - only /proc/1/ seems to be affected.
> 
> Via strace, we can see the libcap code only calls 3 syscalls:
> 
> capget({_LINUX_CAPABILITY_VERSION_3, 0}, NULL) = 0
> capget({_LINUX_CAPABILITY_VERSION_3, 0}, {CAP_CHOWN|CAP_DAC_OVERRIDE|CAP_DAC_READ_SEARCH|CAP_FOWNER|CAP_FSETID|CAP_KILL|CAP_SETGID|CAP_SET
> UID|CAP_SETPCAP|CAP_LINUX_IMMUTABLE|CAP_NET_BIND_SERVICE|CAP_NET_BROADCAST|CAP_NET_ADMIN|CAP_NET_RAW|CAP_IPC_LOCK|CAP_IPC_OWNER|CAP_SYS_MO
> DULE|CAP_SYS_RAWIO|CAP_SYS_CHROOT|CAP_SYS_PTRACE|CAP_SYS_PACCT|CAP_SYS_ADMIN|CAP_SYS_BOOT|CAP_SYS_NICE|CAP_SYS_RESOURCE|CAP_SYS_TIME|CAP_S
> YS_TTY_CONFIG|CAP_MKNOD|CAP_LEASE|CAP_AUDIT_WRITE|CAP_AUDIT_CONTROL|CAP_SETFCAP, CAP_CHOWN|CAP_DAC_OVERRIDE|CAP_DAC_READ_SEARCH|CAP_FOWNER
> |CAP_FSETID|CAP_KILL|CAP_SETGID|CAP_SETUID|CAP_SETPCAP|CAP_LINUX_IMMUTABLE|CAP_NET_BIND_SERVICE|CAP_NET_BROADCAST|CAP_NET_ADMIN|CAP_NET_RA
> W|CAP_IPC_LOCK|CAP_IPC_OWNER|CAP_SYS_MODULE|CAP_SYS_RAWIO|CAP_SYS_CHROOT|CAP_SYS_PTRACE|CAP_SYS_PACCT|CAP_SYS_ADMIN|CAP_SYS_BOOT|CAP_SYS_N
> ICE|CAP_SYS_RESOURCE|CAP_SYS_TIME|CAP_SYS_TTY_CONFIG|CAP_MKNOD|CAP_LEASE|CAP_AUDIT_WRITE|CAP_AUDIT_CONTROL|CAP_SETFCAP, 0}) = 0
> capset({_LINUX_CAPABILITY_VERSION_3, 0}, {CAP_CHOWN|CAP_DAC_OVERRIDE|CAP_DAC_READ_SEARCH|CAP_FOWNER|CAP_FSETID|CAP_KILL|CAP_SETGID|CAP_SETUID|CAP_SETPCAP|CAP_LINUX_IMMUTABLE|CAP_NET_BIND_SERVICE|CAP_NET_BROADCAST|CAP_NET_ADMIN|CAP_NET_RAW|CAP_IPC_LOCK|CAP_IPC_OWNER|CAP_SYS_RAWIO|CAP_SYS_CHROOT|CAP_SYS_PTRACE|CAP_SYS_PACCT|CAP_SYS_ADMIN|CAP_SYS_BOOT|CAP_SYS_NICE|CAP_SYS_RESOURCE|CAP_SYS_TIME|CAP_SYS_TTY_CONFIG|CAP_MKNOD|CAP_LEASE|CAP_AUDIT_WRITE|CAP_AUDIT_CONTROL|CAP_SETFCAP, CAP_CHOWN|CAP_DAC_OVERRIDE|CAP_DAC_READ_SEARCH|CAP_FOWNER|CAP_FSETID|CAP_KILL|CAP_SETGID|CAP_SETUID|CAP_SETPCAP|CAP_LINUX_IMMUTABLE|CAP_NET_BIND_SERVICE|CAP_NET_BROADCAST|CAP_NET_ADMIN|CAP_NET_RAW|CAP_IPC_LOCK|CAP_IPC_OWNER|CAP_SYS_RAWIO|CAP_SYS_CHROOT|CAP_SYS_PTRACE|CAP_SYS_PACCT|CAP_SYS_ADMIN|CAP_SYS_BOOT|CAP_SYS_NICE|CAP_SYS_RESOURCE|CAP_SYS_TIME|CAP_SYS_TTY_CONFIG|CAP_MKNOD|CAP_LEASE|CAP_AUDIT_WRITE|CAP_AUDIT_CONTROL|CAP_SETFCAP, 0}) = 0
> 
> though, for added fun, when running the demo program via strace
> the problem does not appear :-(
> 
> 
> 
> On a slightly related topic, I've noticed that it is not possible to
> invoke prctl(PR_CAPBSET_DROP) to clear the bounding set for processes
> inside a container. The kernel code uses capable() instead of ns_capable().
> Is this intended, or a missing conversion ?
> 
> Indeed, even ignoring namespaces for a minute, I'm curious as to why
> CAP_SETPCAP is required at all for PR_CAPBSET_DROP ?  Is it really
> a security risk to allow a non-privileged user to remove bits from
> the bounding set ? For KVM I'd like to be able to use PR_CAPBSET_DROP
> to prevent a compromised KVM process from using any setuid program to
> re-gain any kind of capabilities.  Similarly I think a container admin
> may well wish to make use of PR_CAPBSET_DROP to lock down applications
> there.


Opps, I should have mentioned that I'm using 3.9.4 kernel. Basically the
Fedora 3.9.4-303 build, but with CONFIG_XFS_FS=n and CONFIG_USER_NS=y
set in the Kconfig.

Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|


More information about the Containers mailing list