LinuxLists.cc - [PATCH v11 21/48] ext4: Add richacl feature flag

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Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

On 2015-10-16 11:17, Andreas Gruenbacher wrote:
> From: "Aneesh Kumar K.V" <[email protected]>
>
> This feature flag selects richacl instead of posix acl support on the
> file system. In addition, the "acl" mount option is needed for enabling
> either of the two kinds of acls.
>
> Signed-off-by: Aneesh Kumar K.V <[email protected]>
> Signed-off-by: Andreas Gruenbacher <[email protected]>
> ---
> fs/ext4/ext4.h | 6 ++++--
> fs/ext4/super.c | 42 +++++++++++++++++++++++++++++++++---------
> 2 files changed, 37 insertions(+), 11 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index fd1f28b..b97a3b1 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -991,7 +991,7 @@ struct ext4_inode_info {
> #define EXT4_MOUNT_UPDATE_JOURNAL 0x01000 /* Update the journal format */
> #define EXT4_MOUNT_NO_UID32 0x02000 /* Disable 32-bit UIDs */
> #define EXT4_MOUNT_XATTR_USER 0x04000 /* Extended user attributes */
> -#define EXT4_MOUNT_POSIX_ACL 0x08000 /* POSIX Access Control Lists */
> +#define EXT4_MOUNT_ACL 0x08000 /* Access Control Lists */
> #define EXT4_MOUNT_NO_AUTO_DA_ALLOC 0x10000 /* No auto delalloc mapping */
> #define EXT4_MOUNT_BARRIER 0x20000 /* Use block barriers */
> #define EXT4_MOUNT_QUOTA 0x80000 /* Some quota option set */
> @@ -1582,6 +1582,7 @@ static inline int ext4_encrypted_inode(struct inode *inode)
> #define EXT4_FEATURE_INCOMPAT_LARGEDIR 0x4000 /* >2GB or 3-lvl htree */
> #define EXT4_FEATURE_INCOMPAT_INLINE_DATA 0x8000 /* data in inode */
> #define EXT4_FEATURE_INCOMPAT_ENCRYPT 0x10000
> +#define EXT4_FEATURE_INCOMPAT_RICHACL 0x20000
I would like to re-iterate, on both XFS and ext4, I _really_ think this
should be a ro_compat flag, and not an incompat one. If a person has
the ability to mount the FS (even if it's a read-only mount), then they
by definition have read access to the file or partition that the
filesystem is contained in, which means that any ACL's stored on the
filesystem are functionally irrelevant, and making this an incompat
feature will just complicate things further for people who have a
legitimate need to recover data.

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2015-10-16 17:41:30

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Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

On Fri, Oct 16, 2015 at 7:31 PM, Austin S Hemmelgarn
<[email protected]> wrote:
> I would like to re-iterate, on both XFS and ext4, I _really_ think this
> should be a ro_compat flag, and not an incompat one. If a person has the
> ability to mount the FS (even if it's a read-only mount), then they by
> definition have read access to the file or partition that the filesystem is
> contained in, which means that any ACL's stored on the filesystem are
> functionally irrelevant,

It is unfortunately not safe to make such a file system accessible to
other users, so the feature is not strictly read-only compatible.

Andreas

2015-10-16 18:27:57

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Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

On 2015-10-16 13:41, Andreas Gruenbacher wrote:
> On Fri, Oct 16, 2015 at 7:31 PM, Austin S Hemmelgarn
> <[email protected]> wrote:
>> I would like to re-iterate, on both XFS and ext4, I _really_ think this
>> should be a ro_compat flag, and not an incompat one. If a person has the
>> ability to mount the FS (even if it's a read-only mount), then they by
>> definition have read access to the file or partition that the filesystem is
>> contained in, which means that any ACL's stored on the filesystem are
>> functionally irrelevant,
>
> It is unfortunately not safe to make such a file system accessible to
> other users, so the feature is not strictly read-only compatible.
If it's not safe WRT data integrity, then the design needs to be
reworked, as that directly implies that isn't safe for even every day
usage on a writable filesystem.

If it's not safe WRT the ACL's being honored, then that really isn't
something we should be worrying about. POSIX ACL's have this issue, as
does mounting a filesystem on any system with a different
/etc/{passwd,shadow,group,gshadow} than the one that wrote the
permissions to the FS in the first place, and as such this is the type
of thing any sensible system administrator will already expect to be
dangerous, which means in turn that they will only do it if there is no
other choice.

Trying to rely on making this an incompat feature to 'enforce' the ACL's
is inherently flawed for two very specific reasons:
1. If the person theoretically trying to attack the system has write
access to the disk, they can flip the feature bit and get access anyway
(seriously, this takes maybe ten minutes of looking at the source code,
some simple math and a hex editor).
2. If the disk is read-only (or even if it's writable), they can just
forgo mounting the filesystem entirely and use any of a number of
existing tools to pull the data directly off of the disk.

As I said in a previous discussion about this, the three most likely
reasons for someone mounting a filesystem with this feature on a kernel
that doesn't support it are:
1. They've booted into a recovery environment (eg SystemRescueCD) to
attempt to recover data from the system itself (this usage implies
access to the hardware, and therefore the ACL's are inherently useless
for protecting the data anyway).
2. They've pulled the disk and hooked it up to a different system to
recover data from it (again, this implies access to the hardware, and
ACL's are inherently useless for protecting from this).
3. They're trying to bisect a kernel bug introduced at around the same
time that richacls went in (which means again they have hardware access
and ACL's are useless).
All that making this an incompat feature will do is make things harder
for these legitimate use cases, for any competent attacker it will only
be a minor inconvenience.

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2015-10-17 23:17:59

by Dave Chinner

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Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

On Fri, Oct 16, 2015 at 02:27:57PM -0400, Austin S Hemmelgarn wrote:
> On 2015-10-16 13:41, Andreas Gruenbacher wrote:
> >On Fri, Oct 16, 2015 at 7:31 PM, Austin S Hemmelgarn
> ><[email protected]> wrote:
> >>I would like to re-iterate, on both XFS and ext4, I _really_ think this
> >>should be a ro_compat flag, and not an incompat one. If a person has the
> >>ability to mount the FS (even if it's a read-only mount), then they by
> >>definition have read access to the file or partition that the filesystem is
> >>contained in, which means that any ACL's stored on the filesystem are
> >>functionally irrelevant,
> >
> >It is unfortunately not safe to make such a file system accessible to
> >other users, so the feature is not strictly read-only compatible.
> If it's not safe WRT data integrity, then the design needs to be
> reworked, as that directly implies that isn't safe for even every
> day usage on a writable filesystem.

This is exactly what we have *incompat feature flags for*: to
protect old code that doesn't know about potentially dangerous new
on-disk formats from trying to parse those formats and causing
unpredictable bad things from happening.

Austin, your arguments hold no weight because they are no different
to the considerations for any new on-disk feature: the user needs to
have both kernel and userspace support to recover filesystems that
go bad. If you are using a brand new fs/kernel feature, then it is
expected that you know that your DR processes take this into
account.

This is also why we XFS devs wait at least a year after new on-disk
features are merged into XFS before we consider turning them on by
default. i.e. to give distros and recovery utilities time to pick
up kernels and userspace pacakges that support the new feature
before the average user will encounter it....

Cheers,

Dave.
--
Dave Chinner
[email protected]

2015-10-19 13:12:34

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Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

On 2015-10-17 19:17, Dave Chinner wrote:
> On Fri, Oct 16, 2015 at 02:27:57PM -0400, Austin S Hemmelgarn wrote:
>> On 2015-10-16 13:41, Andreas Gruenbacher wrote:
>>> On Fri, Oct 16, 2015 at 7:31 PM, Austin S Hemmelgarn
>>> <[email protected]> wrote:
>>>> I would like to re-iterate, on both XFS and ext4, I _really_ think this
>>>> should be a ro_compat flag, and not an incompat one. If a person has the
>>>> ability to mount the FS (even if it's a read-only mount), then they by
>>>> definition have read access to the file or partition that the filesystem is
>>>> contained in, which means that any ACL's stored on the filesystem are
>>>> functionally irrelevant,
>>>
>>> It is unfortunately not safe to make such a file system accessible to
>>> other users, so the feature is not strictly read-only compatible.
>> If it's not safe WRT data integrity, then the design needs to be
>> reworked, as that directly implies that isn't safe for even every
>> day usage on a writable filesystem.
>
> This is exactly what we have *incompat feature flags for*: to
> protect old code that doesn't know about potentially dangerous new
> on-disk formats from trying to parse those formats and causing
> unpredictable bad things from happening.
However, unless things have changed (I haven't had time to re-read the
patch-set yet), then the only change will be a new set of xattrs, and
that type of change _shouldn't_ break existing code that doesn't know
about them. Andreas really needs to explain _exactly_ why it isn't safe
to mount this on a kernel that doesn't support it and let other users
access it, and if the answer is 'because the ACL's won't be honored'
then that really isn't acceptable reason IMHO, because (as I outlined in
the previous e-mail) being able to mount the filesystem implies that
they have at least read access to the underlying storage, which means
that the ACL's in the filesystem are irrelevant as far as any competent
individual who is actively trying to illegitimately access the data is
concerned.
> Austin, your arguments hold no weight because they are no different
> to the considerations for any new on-disk feature: the user needs to
> have both kernel and userspace support to recover filesystems that
> go bad. If you are using a brand new fs/kernel feature, then it is
> expected that you know that your DR processes take this into
> account.
Except that the given argument from Andreas as to why it's an incompat
feature does not clarify whether it's to prevent breaking the existing
filesystem code (which I understand and agree is a proper usage for such
a flag), or to try and provide some thin facade of security when there
really is none (which is what the bit about 'and expose it to other
users' really sounds like to me). Yes the argument that I made which
you have replied to was admittedly shortsighted and didn't need to be
made to get the point that I was actually trying to make across, and I
sincerely apologize for that.

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2015-10-19 13:16:48

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Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

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2015-10-19 15:34:54

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Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

On Mon, Oct 19, 2015 at 3:16 PM, Austin S Hemmelgarn
<[email protected]> wrote:
> On 2015-10-16 13:41, Andreas Gruenbacher wrote:
>>
>> On Fri, Oct 16, 2015 at 7:31 PM, Austin S Hemmelgarn
>> <[email protected]> wrote:
>>>
>>> I would like to re-iterate, on both XFS and ext4, I _really_ think this
>>> should be a ro_compat flag, and not an incompat one. If a person has the
>>> ability to mount the FS (even if it's a read-only mount), then they by
>>> definition have read access to the file or partition that the filesystem
>>> is contained in, which means that any ACL's stored on the filesystem are
>>> functionally irrelevant,
>>
>> It is unfortunately not safe to make such a file system accessible to
>> other users, so the feature is not strictly read-only compatible.
>>
> OK, seeing as I wasn't particularly clear as to why I object to this in my
> other e-mail, let's try this again.
>
> Can you please explain exactly why it isn't safe to make such a filesystem
> accessible to other users?

See here: http://www.spinics.net/lists/linux-ext4/msg49541.html

Andreas

2015-10-19 16:19:56

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Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

On 2015-10-19 11:34, Andreas Gruenbacher wrote:
> On Mon, Oct 19, 2015 at 3:16 PM, Austin S Hemmelgarn
> <[email protected]> wrote:
>> On 2015-10-16 13:41, Andreas Gruenbacher wrote:
>>>
>>> On Fri, Oct 16, 2015 at 7:31 PM, Austin S Hemmelgarn
>>> <[email protected]> wrote:
>>>>
>>>> I would like to re-iterate, on both XFS and ext4, I _really_ think this
>>>> should be a ro_compat flag, and not an incompat one. If a person has the
>>>> ability to mount the FS (even if it's a read-only mount), then they by
>>>> definition have read access to the file or partition that the filesystem
>>>> is contained in, which means that any ACL's stored on the filesystem are
>>>> functionally irrelevant,
>>>
>>> It is unfortunately not safe to make such a file system accessible to
>>> other users, so the feature is not strictly read-only compatible.
>>>
>> OK, seeing as I wasn't particularly clear as to why I object to this in my
>> other e-mail, let's try this again.
>>
>> Can you please explain exactly why it isn't safe to make such a filesystem
>> accessible to other users?
>
> See here: http://www.spinics.net/lists/linux-ext4/msg49541.html
OK, so to clarify, this isn't 'safe' because:
1. The richacls that exist on the filesystem won't be enforced.
2. Newly created files will have no ACL's set.

It is worth noting that these are also issues with any kind of access
control mechanism. Using your logic, all LSM's need to set separate
incompat feature flags in filesystems they are being used on, as should
POSIX ACLs, and for that matter so should Samba in many circumstances,
and any NFS system not using idmapping or synchronized/centralized user
databases. Now, if the SELinux (or SMACK, or TOMOYO) people had taken
this approach, then I might be inclined to not complain (at least not to
you, I'd be complaining to them about this rather poor design choice),
but that is not the case, because (I assume) they realized that all this
provides is a false sense of security.

Issue 1, as I have said before, is functionally irrelevant for anyone
who actually knows what they are doing; all you need for ext* is one of
the myriad of programs for un-deleting files on such a filesystem (such
as ext4magic or extundelete, and good luck convincing them to not allow
being used when this flag is set), for BTRFS you just need the regular
filesystem administration utilities ('btrfs restore' works wonders, and
that one will _never_ honor any kind of permissions, because it's for
disaster recovery), and while I don't know of any way to do this with
XFS, that is only because I don't use XFS myself and have not had the
need to provide tech support for anyone who does. If somebody
absolutely _needs_ a guarantee that the acls will be enforced, they need
to be using whole disk encryption, not just acls, and even that can't
provide such a guarantee.

As for issue 2, that can be solved by making it a read-only compatible
flag, which is what I was suggesting be done in the first place. The
only situation I can think of that this would cause an issue for is if
the filesystem was not cleanly unmounted, and the log-replay doesn't set
the ACL's, but mounting an uncleanly unmounted filesystem that has
richacls on a kernel without support should fall into one of the
following 2 cases more than 99% of the time:
1. The system crashed hard, and the regular kernel is un-bootable for
some reason, in this case you're at the point of disaster recovery,
should not be exposing _anything_ to a multi-user environment, and
probably care a lot more about being able to get the system running
again than about not accidentally creating a file with a missing ACL.
2. The filesystem was maliciously stolen in some way (either the
hardware was acquired, or more likely, someone got an image of a still
mounted filesystem), in which case all of my statements above regarding
issue 1 apply.

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2015-10-19 17:33:19

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Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

On Mon, Oct 19, 2015 at 6:19 PM, Austin S Hemmelgarn
<[email protected]> wrote:
> On 2015-10-19 11:34, Andreas Gruenbacher wrote:
>>
>> On Mon, Oct 19, 2015 at 3:16 PM, Austin S Hemmelgarn
>> <[email protected]> wrote:
>>>
>>> On 2015-10-16 13:41, Andreas Gruenbacher wrote:
>>>>
>>>>
>>>> On Fri, Oct 16, 2015 at 7:31 PM, Austin S Hemmelgarn
>>>> <[email protected]> wrote:
>>>>>
>>>>>
>>>>> I would like to re-iterate, on both XFS and ext4, I _really_ think this
>>>>> should be a ro_compat flag, and not an incompat one. If a person has
>>>>> the
>>>>> ability to mount the FS (even if it's a read-only mount), then they by
>>>>> definition have read access to the file or partition that the
>>>>> filesystem
>>>>> is contained in, which means that any ACL's stored on the filesystem
>>>>> are
>>>>> functionally irrelevant,
>>>>
>>>>
>>>> It is unfortunately not safe to make such a file system accessible to
>>>> other users, so the feature is not strictly read-only compatible.
>>>>
>>> OK, seeing as I wasn't particularly clear as to why I object to this in
>>> my
>>> other e-mail, let's try this again.
>>>
>>> Can you please explain exactly why it isn't safe to make such a
>>> filesystem
>>> accessible to other users?
>>
>>
>> See here: http://www.spinics.net/lists/linux-ext4/msg49541.html
>
> OK, so to clarify, this isn't 'safe' because:
> 1. The richacls that exist on the filesystem won't be enforced.
> 2. Newly created files will have no ACL's set.
>
> It is worth noting that these are also issues with any kind of access
> control mechanism. Using your logic, all LSM's need to set separate
> incompat feature flags in filesystems they are being used on, as should
> POSIX ACLs, and for that matter so should Samba in many circumstances, and
> any NFS system not using idmapping or synchronized/centralized user
> databases. Now, if the SELinux (or SMACK, or TOMOYO) people had taken this
> approach, then I might be inclined to not complain (at least not to you, I'd
> be complaining to them about this rather poor design choice), but that is
> not the case, because (I assume) they realized that all this provides is a
> false sense of security.

LSMs reside above the filesystem level. Let's take SELinux as an
example. It has its own consistency check mechanism (relabeling). Fsck
could check the syntax of SELinux labels, but it couldn't do anything
sensible about corrupted labels, and syntactically correct labels also
don't mean much. A relabeling run to verify or restory the appropriate
policy would still be necessary to verify that labels are semantically
correct, and for that, the filesystem needs to be mounted in the right
place in the filesystem hierarchy.

TOMOYO and AppArmor are not based on inode labels at all.

LSMs usually also just provide an extra layer of security; when turned
off, the basic security mechanisms still in effect will make sure that
the system works just like before. (There are configurations like MLS
where that is not the case, but those are uncommon.)

ACLs are quite different from that. They can be checked statically by
fsck. They are a basic security concept, and when turned off, there is
no guarantee that the system will still be safe.

> Issue 1, as I have said before, is functionally irrelevant for anyone who
> actually knows what they are doing; all you need for ext* is one of the
> myriad of programs for un-deleting files on such a filesystem (such as
> ext4magic or extundelete, and good luck convincing them to not allow being
> used when this flag is set), for BTRFS you just need the regular filesystem
> administration utilities ('btrfs restore' works wonders, and that one will
> _never_ honor any kind of permissions, because it's for disaster recovery),
> and while I don't know of any way to do this with XFS, that is only because
> I don't use XFS myself and have not had the need to provide tech support for
> anyone who does. If somebody absolutely _needs_ a guarantee that the acls
> will be enforced, they need to be using whole disk encryption, not just
> acls, and even that can't provide such a guarantee.
>
> As for issue 2, that can be solved by making it a read-only compatible flag,
> which is what I was suggesting be done in the first place. The only
> situation I can think of that this would cause an issue for is if the
> filesystem was not cleanly unmounted, and the log-replay doesn't set the
> ACL's, but mounting an uncleanly unmounted filesystem that has richacls on a
> kernel without support should fall into one of the following 2 cases more
> than 99% of the time:
> 1. The system crashed hard, and the regular kernel is un-bootable for some
> reason, in this case you're at the point of disaster recovery, should not be
> exposing _anything_ to a multi-user environment, and probably care a lot
> more about being able to get the system running again than about not
> accidentally creating a file with a missing ACL.
> 2. The filesystem was maliciously stolen in some way (either the hardware
> was acquired, or more likely, someone got an image of a still mounted
> filesystem), in which case all of my statements above regarding issue 1
> apply.

Please spare me with all that nonsense. Compared to mount options,
filesystem feature flags in this case simplify things (you don't have
to specify whether a filesystem contains POSIX ACLs or richacls), and
they prevent administrator errors: when a filesystem mounts, it is
safe to use; when it doesn't, it is not. That's all there is to it.

Andreas

2015-10-19 16:39:15

by Andreas Dilger

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Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

On Oct 19, 2015, at 10:19 AM, Austin S Hemmelgarn <[email protected]> wrote:
>
> On 2015-10-19 11:34, Andreas Gruenbacher wrote:
>> On Mon, Oct 19, 2015 at 3:16 PM, Austin S Hemmelgarn
>> <[email protected]> wrote:
>>> On 2015-10-16 13:41, Andreas Gruenbacher wrote:
>>>>
>>>> On Fri, Oct 16, 2015 at 7:31 PM, Austin S Hemmelgarn
>>>> <[email protected]> wrote:
>>>>>
>>>>> I would like to re-iterate, on both XFS and ext4, I _really_ think this
>>>>> should be a ro_compat flag, and not an incompat one. If a person has the
>>>>> ability to mount the FS (even if it's a read-only mount), then they by
>>>>> definition have read access to the file or partition that the filesystem
>>>>> is contained in, which means that any ACL's stored on the filesystem are
>>>>> functionally irrelevant,
>>>>
>>>> It is unfortunately not safe to make such a file system accessible to
>>>> other users, so the feature is not strictly read-only compatible.
>>>>
>>> OK, seeing as I wasn't particularly clear as to why I object to this in my
>>> other e-mail, let's try this again.
>>>
>>> Can you please explain exactly why it isn't safe to make such a filesystem
>>> accessible to other users?
>>
>> See here: http://www.spinics.net/lists/linux-ext4/msg49541.html
> OK, so to clarify, this isn't 'safe' because:
> 1. The richacls that exist on the filesystem won't be enforced.
> 2. Newly created files will have no ACL's set.
>
> It is worth noting that these are also issues with any kind of access control mechanism. Using your logic, all LSM's need to set separate incompat feature flags in filesystems they are being used on, as should POSIX ACLs, and for that matter so should Samba in many circumstances, and any NFS system not using idmapping or synchronized/centralized user databases. Now, if the SELinux (or SMACK, or TOMOYO) people had taken this approach, then I might be inclined to not complain (at least not to you, I'd be complaining to them about this rather poor design choice), but that is not the case, because (I assume) they realized that all this provides is a false sense of security.

I would tend to agree here. Anyone who can mount the filesystem on a kernel
without RichACL support can do whatever they want, so at most having a
RO_COMPAT flag would serve as a reminder for accidental problems by a
sysadmin not in the know. Using an INCOMPAT flag is just asking for major
headaches when someone needs to recover their filesystem on an old kernel
but doesn't provide any added safety.

Cheers, Andreas

> Issue 1, as I have said before, is functionally irrelevant for anyone who actually knows what they are doing; all you need for ext* is one of the myriad of programs for un-deleting files on such a filesystem (such as ext4magic or extundelete, and good luck convincing them to not allow being used when this flag is set), for BTRFS you just need the regular filesystem administration utilities ('btrfs restore' works wonders, and that one will _never_ honor any kind of permissions, because it's for disaster recovery), and while I don't know of any way to do this with XFS, that is only because I don't use XFS myself and have not had the need to provide tech support for anyone who does. If somebody absolutely _needs_ a guarantee that the acls will be enforced, they need to be using whole disk encryption, not just acls, and even that can't provide such a guarantee.
>
> As for issue 2, that can be solved by making it a read-only compatible flag, which is what I was suggesting be done in the first place. The only situation I can think of that this would cause an issue for is if the filesystem was not cleanly unmounted, and the log-replay doesn't set the ACL's, but mounting an uncleanly unmounted filesystem that has richacls on a kernel without support should fall into one of the following 2 cases more than 99% of the time:
> 1. The system crashed hard, and the regular kernel is un-bootable for some reason, in this case you're at the point of disaster recovery, should not be exposing _anything_ to a multi-user environment, and probably care a lot more about being able to get the system running again than about not accidentally creating a file with a missing ACL.
> 2. The filesystem was maliciously stolen in some way (either the hardware was acquired, or more likely, someone got an image of a still mounted filesystem), in which case all of my statements above regarding issue 1 apply.
>

Cheers, Andreas

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2015-10-19 18:45:09

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Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

On 2015-10-19 13:33, Andreas Gruenbacher wrote:
> On Mon, Oct 19, 2015 at 6:19 PM, Austin S Hemmelgarn
> <[email protected]> wrote:
>> On 2015-10-19 11:34, Andreas Gruenbacher wrote:
>>> On Mon, Oct 19, 2015 at 3:16 PM, Austin S Hemmelgarn
>>> <[email protected]> wrote:
>>>> On 2015-10-16 13:41, Andreas Gruenbacher wrote:
>>>>> On Fri, Oct 16, 2015 at 7:31 PM, Austin S Hemmelgarn
>>>>> <[email protected]> wrote:
>>>>>> I would like to re-iterate, on both XFS and ext4, I _really_ think this
>>>>>> should be a ro_compat flag, and not an incompat one. If a person has
>>>>>> the
>>>>>> ability to mount the FS (even if it's a read-only mount), then they by
>>>>>> definition have read access to the file or partition that the
>>>>>> filesystem
>>>>>> is contained in, which means that any ACL's stored on the filesystem
>>>>>> are
>>>>>> functionally irrelevant,
>>>>> It is unfortunately not safe to make such a file system accessible to
>>>>> other users, so the feature is not strictly read-only compatible.
>>>> OK, seeing as I wasn't particularly clear as to why I object to this in
>>>> my
>>>> other e-mail, let's try this again.
>>>> Can you please explain exactly why it isn't safe to make such a
>>>> filesystem
>>>> accessible to other users?
>>> See here: http://www.spinics.net/lists/linux-ext4/msg49541.html
>> OK, so to clarify, this isn't 'safe' because:
>> 1. The richacls that exist on the filesystem won't be enforced.
>> 2. Newly created files will have no ACL's set.
>>
>> It is worth noting that these are also issues with any kind of access
>> control mechanism. Using your logic, all LSM's need to set separate
>> incompat feature flags in filesystems they are being used on, as should
>> POSIX ACLs, and for that matter so should Samba in many circumstances, and
>> any NFS system not using idmapping or synchronized/centralized user
>> databases. Now, if the SELinux (or SMACK, or TOMOYO) people had taken this
>> approach, then I might be inclined to not complain (at least not to you, I'd
>> be complaining to them about this rather poor design choice), but that is
>> not the case, because (I assume) they realized that all this provides is a
>> false sense of security.
>
> LSMs reside above the filesystem level. Let's take SELinux as an
> example. It has its own consistency check mechanism (relabeling). Fsck
> could check the syntax of SELinux labels, but it couldn't do anything
> sensible about corrupted labels, and syntactically correct labels also
> don't mean much. A relabeling run to verify or restory the appropriate
> policy would still be necessary to verify that labels are semantically
> correct, and for that, the filesystem needs to be mounted in the right
> place in the filesystem hierarchy.
>
> TOMOYO and AppArmor are not based on inode labels at all.
Apologies for being unintentionally over-inclusive WRT LSM's, and also
for forgetting that TOMOYO doesn't use inode labels.
> LSMs usually also just provide an extra layer of security; when turned
> off, the basic security mechanisms still in effect will make sure that
> the system works just like before. (There are configurations like MLS
> where that is not the case, but those are uncommon.)
Um, actually no, even without MLS (or MCS), there is no guarantee
whatsoever that the system will work 'just' like before (I've actually
seen systems break on any (or even in one case all) of the transitions
between enforcing/permissive/off for SELinux, even assuming that
relabeling is done properly).
> ACLs are quite different from that. They can be checked statically by
> fsck. They are a basic security concept, and when turned off, there is
> no guarantee that the system will still be safe.
LSM's hook into the VFS code right alongside the regular permissions
checks. They are intended to supplement the regular UNIX DAC based
permissions. Richacls (and POSIX ACLs) also hook into the regular
permissions checks, and also are intended to supplement the regular UNIX
DAC based permissions. Fsck only checks the syntax of regular UNIX DAC
based permissions (it doesn't verify that the listed UID/GID are
actually valid in userspace, nor does it check for semantically
nonsensical permissions modes like 007 or 000), and it really can't
properly check anything more than that on ACL's either. Based on all of
this, richacls and LSM's which mediate filesystem accesses are on
exactly the same level with the exception that LSM's usually have way
more functionality beyond just using ACL's to control file access, LSM's
are usually MAC based while ACL's are DAC based, and the fact that ACL's
are (usually) not dependent on where in the filesystem hierarchy they
are found.

On top of that, there is no guarantee that a system will still be safe
when you turn SELinux (or other LSM's) off either (in fact, for some
configurations, it can be mathematically proven that the system will not
be safe if you turn off SELinux).
>> Issue 1, as I have said before, is functionally irrelevant for anyone who
>> actually knows what they are doing; all you need for ext* is one of the
>> myriad of programs for un-deleting files on such a filesystem (such as
>> ext4magic or extundelete, and good luck convincing them to not allow being
>> used when this flag is set), for BTRFS you just need the regular filesystem
>> administration utilities ('btrfs restore' works wonders, and that one will
>> _never_ honor any kind of permissions, because it's for disaster recovery),
>> and while I don't know of any way to do this with XFS, that is only because
>> I don't use XFS myself and have not had the need to provide tech support for
>> anyone who does. If somebody absolutely _needs_ a guarantee that the acls
>> will be enforced, they need to be using whole disk encryption, not just
>> acls, and even that can't provide such a guarantee.
>>
>> As for issue 2, that can be solved by making it a read-only compatible flag,
>> which is what I was suggesting be done in the first place. The only
>> situation I can think of that this would cause an issue for is if the
>> filesystem was not cleanly unmounted, and the log-replay doesn't set the
>> ACL's, but mounting an uncleanly unmounted filesystem that has richacls on a
>> kernel without support should fall into one of the following 2 cases more
>> than 99% of the time:
>> 1. The system crashed hard, and the regular kernel is un-bootable for some
>> reason, in this case you're at the point of disaster recovery, should not be
>> exposing _anything_ to a multi-user environment, and probably care a lot
>> more about being able to get the system running again than about not
>> accidentally creating a file with a missing ACL.
>> 2. The filesystem was maliciously stolen in some way (either the hardware
>> was acquired, or more likely, someone got an image of a still mounted
>> filesystem), in which case all of my statements above regarding issue 1
>> apply.
> Please spare me with all that nonsense. Compared to mount options,
> filesystem feature flags in this case simplify things (you don't have
> to specify whether a filesystem contains POSIX ACLs or richacls), and
> they prevent administrator errors: when a filesystem mounts, it is
> safe to use; when it doesn't, it is not. That's all there is to it.
You're ignoring what I'm actually saying. I've said absolutely nothing
about needing to use mount options at all, and I'm not arguing against
using filesystem feature flags, I'm arguing for using them sensibly in a
way that does not present a false sense of security.

Making the comparability flag read-only will exactly solve the second
issue that you outlined for more than 99% of all use cases (and the
remaining cases are provably irrelevant), still indicate clearly to the
FS code which type of ACL is in use on the FS, and remove the entirely
false sense of security WRT the first issue you outlined.

Making it an incompatible flag will likely cause headaches for some
legitimate users, and at most delay competent hackers by a few seconds
to a few minutes, and script kiddies by a few hours, and is really no
better than security by obscurity (and from a purely logistical
standpoint, that's _all_ it is) in that it actively tries to hide the
fact that someone having read access to the storage the filesystem is on
can bypass the ACL's.

To reiterate, if someone can call mount() on a filesystem, and mount()
does not return -EPERM, then even if mount() returns a different error,
they still have the ability to completely bypass all permissions and
ACL's in that filesystem, because they have the ability to read the
entire filesystem directly.

The _only_ way to properly protect against people bypassing the ACL's is
to use full disk encryption and lock down root access on the system, and
even that can't completely prevent it from happening.

Attachments:

2015-10-19 20:20:43

[permalink] [raw]

Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag

On Mon, Oct 19, 2015 at 8:45 PM, Austin S Hemmelgarn
<[email protected]> wrote:
> On 2015-10-19 13:33, Andreas Gruenbacher wrote:
>> Please spare me with all that nonsense. Compared to mount options,
>> filesystem feature flags in this case simplify things (you don't have
>> to specify whether a filesystem contains POSIX ACLs or richacls), and
>> they prevent administrator errors: when a filesystem mounts, it is
>> safe to use; when it doesn't, it is not. That's all there is to it.
>
> You're ignoring what I'm actually saying. I've said absolutely nothing
> about needing to use mount options at all, and I'm not arguing against using
> filesystem feature flags, I'm arguing for using them sensibly in a way that
> does not present a false sense of security.

We could be on a multi-user system, and the user mounting the
filesystem may not be the only user on the system. When a filesystem
can be mounted read-only, it should be safe to use read-only. It is
not safe in general to use such a filesystem read-only, so an
incompatible feature flag which prevents such unsafe mounting is more
approporiate than a read-only incompatible feature flag.

Mounting a filesystem read-only doesn't mean that the filesystem is
being recovered, it is perfectly legal to mount a filesystem read-only
for other reasons. I don't want to give people using read-only
filesystems the false sense that everything is okay.

> Making it an incompatible flag will likely cause headaches for some
> legitimate users,

Indeed. It will also make it less likely for users to accidentally
shoot themselves in the foot. If someone knows better, they can clear
the feature flag.

When recovering a broken system that contains richacl filesystems, you
really want to have richacl support in the rescue system as well.
Otherwise, you won't be able to fsck those filesystems.

> and at most delay competent hackers by a few seconds to a
> few minutes, and script kiddies by a few hours, and is really no better than
> security by obscurity (and from a purely logistical standpoint, that's _all_
> it is) in that it actively tries to hide the fact that someone having read
> access to the storage the filesystem is on can bypass the ACL's.
>
> To reiterate, if someone can call mount() on a filesystem, and mount() does
> not return -EPERM, then even if mount() returns a different error, they
> still have the ability to completely bypass all permissions and ACL's in
> that filesystem, because they have the ability to read the entire filesystem
> directly.
>
> The _only_ way to properly protect against people bypassing the ACL's is to
> use full disk encryption and lock down root access on the system, and even
> that can't completely prevent it from happening.

That's all completely beside the point. I'm not talking about
preventing attacks at all, just basic administrative workflows.

Andreas

2015-10-20 12:33:54