[manpages PATCH] capabilities.7: describe namespaced file capabilities
Michael Kerrisk (man-pages)
mtk.manpages at gmail.com
Fri Apr 13 19:26:14 UTC 2018
Hello Serge, Jann,
On 01/16/2018 06:26 PM, Jann Horn wrote:
> On Tue, Jan 9, 2018 at 7:52 PM, Serge E. Hallyn <serge at hallyn.com> wrote:
>> Update the capabilities(7) manpage with a description of the
>> new-ish namespaced file capability support.
>> A note on userspace tools: since the kernel will automatically
>> convert between v2 and v3 xattrs, and translate nsroot between
>> v3 xattrs, we can make do with the current getcap(8) and setcap(8)
>> tools. I.e. a user on the host can create a transient user namespace
>> with the appropriate mappings and run setcap(8) there. The kernel
>> will automatically write a v3 xattr with the transient namespace's
>> root user as nsroot.
After a long gap, I have come back to the task of working up
some text to describe file capability versioning and namespaced file
I still not convinced I've captured things correctly, and I still
have a few questions (see below). But first, here's the text that
I have so far (suggestions for improvements welcome). These changes
have already been pushed to the Git repo.
File capability mask versioning
To allow extensibility, the kernel supports a scheme to encode
a version number inside the security.capability extended
attribute that is used to implement file capabilities. These
version numbers are internal to the implementation, and not
directly visible to user-space applications. To date, the fol‐
lowing versions are supported:
This was the original file capability implementation,
which supported 32-bit masks for file capabilities.
VFS_CAP_REVISION_2 (since Linux 2.6.25)
This version allows for file capability masks that are
64 bits in size, and was necessary as the number of sup‐
ported capabilities grew beyond 32. The kernel trans‐
parently continues to support the execution of files
that have 32-bit version 1 capability masks, but when
adding capabilities to files that did not previously
have capabilities, or modifying the capabilities of
existing files, it automatically uses the version 2
scheme (or possibly the version 3 scheme, as described
VFS_CAP_REVISION_3 (since Linux 4.14)
Version 3 file capabilities are provided to support
namespaced file capabilities (described below).
As with version 2 file capabilities, version 3 capabil‐
ity masks are 64 bits in size. But in addition, the
root user ID of namespace is encoded in the secu‐
rity.capability extended attribute. (A namespace's root
user ID is the value that user ID 0 inside that names‐
pace maps to in the initial user namespace.)
["namespace root user ID" is my term for what Serge called nsroot.
I think it's a little more meaningful, but I am also open to suggestions
for a better term.]
Version 3 file capabilities are designed to coexist with
version 2 capabilities; that is, on a modern Linux sys‐
tem, there may be some files with version 2 capabilities
while others have version 3 capabilities.
Before Linux 4.14, the only kind of capability mask that could
be attached to a file was a VFS_CAP_REVISION_2 mask. Since
Linux 4.14, the version of the capability mask that is attached
to a file depends on the circumstances in which the secu‐
rity.capability extended attribute was created.
Starting with Linux 4.14, a security.capability extended
attribute is automatically created as (or converted to) a ver‐
sion 3 (VFS_CAP_REVISION_3) attribute if both of the following
(1) The thread writing the attribute resides in a noninitial
namespace. (More precisely: the thread resides in a user
namespace other than the one from which the underlying
filesystem was mounted.)
(2) The thread has the CAP_SETFCAP capability over the file
inode, meaning that (a) the thread has the CAP_SETFCAP
capability in its own user namespace; and (b) the UID and
GID of the file inode have mappings in the writer's user
│Does there also need to be some kind of credential │
│match between the file and the namespace creator │
When a VFS_CAP_REVISION_3 security.capability extended
attribute is created, the root user ID of the creating thread's
user namespace is saved in the extended attribute.
By contrast, creating a security.capability extended attribute
from a privileged (CAP_SETFCAP) thread that resides in the
namespace where the the underlying filesystem was mounted (this
normally means the initial user namespace) automatically
results in a version 2 (VFS_CAP_REVISION_2) attribute.
Note that a file can have either a version 2 or a version 3
security.capability extended attribute associated with it, but
not both: creation or modification of the security.capability
extended attribute will automatically modify the version
according to the circumstances in which the extended attribute
is created or modified.
Namespaced file capabilities
Traditional (i.e., version 2) file capabilities associate only
a set of capability masks with a binary executable file. When
a process executes a binary with such capabilities, it gains
the associated capabilities (within its user namespace) as per
the rules described above in "Transformation of capabilities
Because version 2 file capabilities confer capabilities to the
executing process regardless of which user namespace it resides
in, only privileged processes are permitted to associate capa‐
bilities with a file. Here, "privileged" means a process that
has the CAP_SETFCAP capability in the user namespace where the
filesystem was mounted (normally the initial user namespace).
This limitation renders file capabilities useless for certain
use cases. For example, in user-namespaced containers, it can
be desirable to be able to create a binary that confers capa‐
bilities only to processes executed inside that container, but
not to processes that are executed outside the container.
Linux 4.14 added so-called namespaced file capabilities to sup‐
port such use cases. Namespaced file capabilities are recorded
as version 3 (i.e., VFS_CAP_REVISION_3) security.capability
extended attributes. Such an attribute is automatically cre‐
ated when a process that resides in a noninitial user namespace
associates (setxattr(2)) file capabilities with a file whose
user ID matches the user ID of the creator of the namespace.
In this case, the kernel records not just the capability masks
in the extended attribute, but also the namespace root user ID.
For further details, see File capability mask versioning,
As with a binary that has VFS_CAP_REVISION_2 file capabilities,
a binary with VFS_CAP_REVISION_3 file capabilities confers
capabilities to a process during execve(). However, capabili‐
ties are conferred only if the binary is executed by a process
that resides in a user namespace whose UID 0 maps to the root
user ID that is saved in the extended attribute, or when exe‐
cuted by a process that resides in descendant of such a names‐
The following is Serge's original patch, with some questions
>> Signed-off-by: Serge Hallyn <shallyn at cisco.com>
>> man7/capabilities.7 | 44 ++++++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 44 insertions(+)
>> diff --git a/man7/capabilities.7 b/man7/capabilities.7
>> index 166eaaf..76e7e02 100644
>> --- a/man7/capabilities.7
>> +++ b/man7/capabilities.7
>> @@ -936,6 +936,50 @@ if we specify the effective flag as being enabled for any capability,
>> then the effective flag must also be specified as enabled
>> for all other capabilities for which the corresponding permitted or
>> inheritable flags is enabled.
>> +Until 4.13, only VFS_CAP_REVISION_2 xattrs were supported. These store only
>> +the capabilities to be applied to the file, with no record of the writer's
>> +credentials. Therefore only privileged users can be trusted to write them, and
>> +.BR CAP_SETFCAP
>> +over the user namespace which mounted the filesystem (usually the initial user
>> +namespace) is required. This makes it impossible to write file capabilities
>> +from a user namespaced container, which causes some package updates to fail.
>> +In order to support setting file capabilities in containers, the
>> +kernel must be able to identify whether the task executing the
>> +file will be constrained to a subset of the resources over which
>> +the writer of the file capabilities has privilege. To this end,
>> +since 4.13, VFS_CAP_REVISION_3 capabilities store the user ID
>> +of the root user in the writer's namespace ("nsroot"). Hence the writer only
>> +.IP 1.
>> +.BR CAP_SETFCAP
>> +over the file inode, meaning the writing task must have
>> +.BR CAP_SETFCAP
>> +over a user namespace into which the inode's owning user ID is mapped.
>> +.IP 2.
>> +.BR CAP_SETFCAP
>> +over the writer's own user namespace.
> I think that the following would be clearer (but technically
> equivalent): "Hence the writer only requires CAP_SETFCAP over the file
> inode, meaning that the writing task must have CAP_SETFCAP in its own
> user namespace and the UID and GID of the file inode must be mapped in
> the writing task's user namespace.".
I've tried to capture that idea in my text above. Was I successful?
>> +A VFS_CAP_REVISION_3 file capability will take effect only when run in a user namespace
>> +whose UID 0 maps to the saved "nsroot", or a descendant of such a namespace.
>> +Users with the required privilege may use
>> +.BR setxattr(2)
>> +to request either a VFS_CAP_REVISION_2 or VFS_CAP_REVISION_3 write.
>> +The kernel will automatically convert a VFS_CAP_REVISION_2 to a
>> +VFS_CAP_REVISION_3 extended attribute with the "nsroot"
>> +set to the root user in the writer's user namespace, or, if a VFS_CAP_REVISION_3
>> +extended attribute is specified, then the kernel will map the
>> +specified root user ID (which must be a valid user ID mapped in the caller's
>> +user namespace) into the initial user namespace.
> Really, "into the initial user namespace"? That may be true for the
> kernel-internal representation, but the on-disk representation is the
> mapping into the user namespace that contains the mount namespace into
> which the file system was mounted, right? This would become observable
> when a file system is mounted in a different namespace than before, or
> when working with FUSE in a namespace.
>> +.BR getxattr(2)
>> +results will be converted and simplified to show a VFS_CAP_REVISION_2
>> +extended attribute, if a VFS_CAP_REVISION_3 applies to the caller's
>> +namespace, or to map the VFS_CAP_REVISION_3 root user ID into the
>> +caller's namespace.
I haven't captured that last paragraph in my text. I'm not sure I
understand the idea being presented. Serge, could you elaborate?
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