From: Eric W. Biederman <hidden> Date: 2016-07-04 16:40:12
Jan Kara [off-list ref] writes:
On Sat 02-07-16 12:18:08, Eric W. Biederman wrote:
quoted
As well as in these patches the code is also available from:
git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git for-testing
It has been a long time in coming but recently in the userns tree the
superblock has been expanded with a s_user_ns field indicating the user
namespace that owns a superblock.
The s_user_ns owner of a superblock has three implications.
- Only kuids and kgids that map into s_user_ns are allowed to be sent to a
filesystem from the vfs.
- If the uid or gid on the filesystem does not map into s_user_ns i_uid
is set to INVALID_UID and i_gid is set to INVALID_GID.
- The scope of permission checks can be changed from global to a
capabilitiy check in s_user_ns.
OK, to check that I understand it right:
So the uids and gids that are stored on disk are still expected to be in
the initial id namespace, aren't they?
No.
The general expectation is that the ids on disk are store in s_user_ns.
Id's that don't map to the initial id namespace get stored in the
generic data structures as INVALID_UID and INVALID_GID.
In practice I don't expect anyone will set up a situation knowingly
where id's don't map, but the case has to be handled because mistakes
and malicious code happens.
Eric
From: Jan Kara <jack@suse.cz> Date: 2016-07-06 08:54:44
On Mon 04-07-16 11:27:46, Eric W. Biederman wrote:
Jan Kara [off-list ref] writes:
quoted
On Sat 02-07-16 12:18:08, Eric W. Biederman wrote:
quoted
As well as in these patches the code is also available from:
git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git for-testing
It has been a long time in coming but recently in the userns tree the
superblock has been expanded with a s_user_ns field indicating the user
namespace that owns a superblock.
The s_user_ns owner of a superblock has three implications.
- Only kuids and kgids that map into s_user_ns are allowed to be sent to a
filesystem from the vfs.
- If the uid or gid on the filesystem does not map into s_user_ns i_uid
is set to INVALID_UID and i_gid is set to INVALID_GID.
- The scope of permission checks can be changed from global to a
capabilitiy check in s_user_ns.
OK, to check that I understand it right:
So the uids and gids that are stored on disk are still expected to be in
the initial id namespace, aren't they?
No.
The general expectation is that the ids on disk are store in s_user_ns.
Id's that don't map to the initial id namespace get stored in the
generic data structures as INVALID_UID and INVALID_GID.
In practice I don't expect anyone will set up a situation knowingly
where id's don't map, but the case has to be handled because mistakes
and malicious code happens.
OK, thanks for explanation. But then the namespace the filesystem is
mounted with essentially becomes part of the on-disk format, doesn't it?
Because if someone mounts the media from a different namespace, suddently
the UID/GIDs may map to different users in initial user namespace and
consequences may be weird, right? Shouldn't it thus be somehow stored
together with the filesystem to make things more robust?
I don't remember the indented uses for user-ns mounts so I may be just
wrong. But my experience tells me that external data (such as user
namespace ID mappings in your case) that modify meaning of on-disk format
tend to cause maintenance difficulties in the long run... Because someone
*will* have the idea of migrating these fs images between containers /
machines and then they have to make sure mappings get migrated as well and
it all becomes cumbersome.
Honza
--
Jan Kara [off-list ref]
SUSE Labs, CR
On Wed, Jul 06, 2016 at 10:54:40AM +0200, Jan Kara wrote:
On Mon 04-07-16 11:27:46, Eric W. Biederman wrote:
quoted
Jan Kara [off-list ref] writes:
quoted
On Sat 02-07-16 12:18:08, Eric W. Biederman wrote:
quoted
As well as in these patches the code is also available from:
git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git for-testing
It has been a long time in coming but recently in the userns tree the
superblock has been expanded with a s_user_ns field indicating the user
namespace that owns a superblock.
The s_user_ns owner of a superblock has three implications.
- Only kuids and kgids that map into s_user_ns are allowed to be sent to a
filesystem from the vfs.
- If the uid or gid on the filesystem does not map into s_user_ns i_uid
is set to INVALID_UID and i_gid is set to INVALID_GID.
- The scope of permission checks can be changed from global to a
capabilitiy check in s_user_ns.
OK, to check that I understand it right:
So the uids and gids that are stored on disk are still expected to be in
the initial id namespace, aren't they?
No.
The general expectation is that the ids on disk are store in s_user_ns.
Id's that don't map to the initial id namespace get stored in the
generic data structures as INVALID_UID and INVALID_GID.
quoted
In practice I don't expect anyone will set up a situation knowingly
where id's don't map, but the case has to be handled because mistakes
and malicious code happens.
OK, thanks for explanation. But then the namespace the filesystem is
mounted with essentially becomes part of the on-disk format, doesn't it?
Because if someone mounts the media from a different namespace, suddently
the UID/GIDs may map to different users in initial user namespace and
consequences may be weird, right? Shouldn't it thus be somehow stored
together with the filesystem to make things more robust?
I don't remember the indented uses for user-ns mounts so I may be just
wrong. But my experience tells me that external data (such as user
namespace ID mappings in your case) that modify meaning of on-disk format
tend to cause maintenance difficulties in the long run... Because someone
*will* have the idea of migrating these fs images between containers /
machines and then they have to make sure mappings get migrated as well and
it all becomes cumbersome.
The intended use case for this is containers, with the idea being that I
as a user will get the same behavior in the container as I would in
init_user_ns without needing any userspace modifications to achieve
that.
So if I have a filesystem that contains uid 0 and I mount it in my
container, I should see uid 0. If I mount the same bits in another
container with a different uid mapping I should also see uid 0.
If I mkfs a new filesystem in my container then mount it, the root
directory of the fs is owned by uid 0 in my container without any
modifications to mkfs.
I'd argue that this makes it easier to migrate a disk between containers
because the ids in the disk show up the same within the container
regardless of the id mapping. If someone wants to mount a filesystem in
one container and also access it in another container with a completely
different id mapping, well I don't think that's ever going to work well.
Seth
From: Jan Kara <jack@suse.cz> Date: 2016-07-06 14:23:07
On Wed 06-07-16 08:54:46, Seth Forshee wrote:
On Wed, Jul 06, 2016 at 10:54:40AM +0200, Jan Kara wrote:
quoted
On Mon 04-07-16 11:27:46, Eric W. Biederman wrote:
I don't remember the indented uses for user-ns mounts so I may be just
wrong. But my experience tells me that external data (such as user
namespace ID mappings in your case) that modify meaning of on-disk format
tend to cause maintenance difficulties in the long run... Because someone
*will* have the idea of migrating these fs images between containers /
machines and then they have to make sure mappings get migrated as well and
it all becomes cumbersome.
The intended use case for this is containers, with the idea being that I
as a user will get the same behavior in the container as I would in
init_user_ns without needing any userspace modifications to achieve
that.
So if I have a filesystem that contains uid 0 and I mount it in my
container, I should see uid 0. If I mount the same bits in another
container with a different uid mapping I should also see uid 0.
If I mkfs a new filesystem in my container then mount it, the root
directory of the fs is owned by uid 0 in my container without any
modifications to mkfs.
I'd argue that this makes it easier to migrate a disk between containers
because the ids in the disk show up the same within the container
regardless of the id mapping. If someone wants to mount a filesystem in
one container and also access it in another container with a completely
different id mapping, well I don't think that's ever going to work well.
OK, I see how this is supposed to work. However you assume here that both
containers have the same set of valid UIDs, don't you? If that is not the
case, the mounted image will not be usable in the other container, right?
Honza
--
Jan Kara [off-list ref]
SUSE Labs, CR
On Wed, Jul 06, 2016 at 04:22:55PM +0200, Jan Kara wrote:
On Wed 06-07-16 08:54:46, Seth Forshee wrote:
quoted
On Wed, Jul 06, 2016 at 10:54:40AM +0200, Jan Kara wrote:
quoted
On Mon 04-07-16 11:27:46, Eric W. Biederman wrote:
I don't remember the indented uses for user-ns mounts so I may be just
wrong. But my experience tells me that external data (such as user
namespace ID mappings in your case) that modify meaning of on-disk format
tend to cause maintenance difficulties in the long run... Because someone
*will* have the idea of migrating these fs images between containers /
machines and then they have to make sure mappings get migrated as well and
it all becomes cumbersome.
The intended use case for this is containers, with the idea being that I
as a user will get the same behavior in the container as I would in
init_user_ns without needing any userspace modifications to achieve
that.
So if I have a filesystem that contains uid 0 and I mount it in my
container, I should see uid 0. If I mount the same bits in another
container with a different uid mapping I should also see uid 0.
If I mkfs a new filesystem in my container then mount it, the root
directory of the fs is owned by uid 0 in my container without any
modifications to mkfs.
I'd argue that this makes it easier to migrate a disk between containers
because the ids in the disk show up the same within the container
regardless of the id mapping. If someone wants to mount a filesystem in
one container and also access it in another container with a completely
different id mapping, well I don't think that's ever going to work well.
OK, I see how this is supposed to work. However you assume here that both
containers have the same set of valid UIDs, don't you? If that is not the
case, the mounted image will not be usable in the other container, right?
It's possible of course. I'd expect anyone wanting to use this in
practice to set up their containers with appropriate mappings. Full OS
style containers will at least need some minimal uid mapping to work
properly.
Seth
From: James Bottomley <James.Bottomley@HansenPartnership.com> Date: 2016-07-06 15:23:52
On Wed, 2016-07-06 at 16:22 +0200, Jan Kara wrote:
On Wed 06-07-16 08:54:46, Seth Forshee wrote:
quoted
On Wed, Jul 06, 2016 at 10:54:40AM +0200, Jan Kara wrote:
quoted
On Mon 04-07-16 11:27:46, Eric W. Biederman wrote:
I don't remember the indented uses for user-ns mounts so I may be
just wrong. But my experience tells me that external data (such
as user namespace ID mappings in your case) that modify meaning
of on-disk format tend to cause maintenance difficulties in the
long run... Because someone *will* have the idea of migrating
these fs images between containers / machines and then they have
to make sure mappings get migrated as well and it all becomes
cumbersome.
The intended use case for this is containers, with the idea being
that I as a user will get the same behavior in the container as I
would in init_user_ns without needing any userspace modifications
to achieve that.
So if I have a filesystem that contains uid 0 and I mount it in my
container, I should see uid 0. If I mount the same bits in another
container with a different uid mapping I should also see uid 0.
If I mkfs a new filesystem in my container then mount it, the root
directory of the fs is owned by uid 0 in my container without any
modifications to mkfs.
I'd argue that this makes it easier to migrate a disk between
containers because the ids in the disk show up the same within the
container regardless of the id mapping. If someone wants to mount a
filesystem in one container and also access it in another container
with a completely different id mapping, well I don't think that's
ever going to work well.
OK, I see how this is supposed to work. However you assume here that
both containers have the same set of valid UIDs, don't you? If that
is not the case, the mounted image will not be usable in the other
container, right?
You can always set it up wrongly is the rule of containers. Because
the virtualizations are so granular, there are many possible
configurations which don't make sense in the real world.
The main use case for this is operating system images. For them we
have a set of known UID/GIDs in the image (usually 0-1000 plus the
nobody/nogroups for both). Using this scheme, we'd set up the
container in a userns that mapped all these ids to something
unprivileged and then set up a s_user_ns to do the same for the mount
location of the image meaning that the unprivileged container can now
manipulate the image.
There are several self contained proposals on linux-fsdevel for doing
this, like shifts, which is what I'm currently using to manipulate
images, so for me what it does is allows me to get rid of all the
credential shifting when performing operations on the underlying
filesystem. In fact, I think it pretty much allows me to get rid of a
lot of the upper/lower filesystem distinction in shiftfs and I'd get
quotas and other stuff I ignored for free.
However, any of the other uid/gid shifting proposals can also use this
as the engine.
The point here is, this patch set is simply mechanism; it requires a
glue layer (like shiftfs, fuse or the vfs remapping proposal) to
activate it. The activation decides how much exposure to the
underlying filesystem there is, so with shiftfs, there's none, it's a
purely volatile system crafted for chosen images. However, it's fully
possible to come up with an activation where the filesystem would
decide (through some on disk format information) to declare the image
to be safely remapped in this uid/gid range and then we could allow it
to be mounted unprivileged (without a capability check) into a user_ns
that matched the mapping. This latter is a bit of a fantasy since
container images are currently little more than tar files and we have
no extant way to connect them to linux fs formats, but once the
possibility exists, whose to say this won't change?
James