by Andreas Gruenbacher

[permalink] [raw]

Subject: [PATCH v11 01/48] vfs: Add IS_ACL() and IS_RICHACL() tests

The vfs does not apply the umask for file systems that support acls. The
test used for this used to be called IS_POSIXACL(). Switch to a new
IS_ACL() test to check for either posix acls or richacls instead. Add a new
MS_RICHACL flag and IS_RICHACL() test for richacls alone. The IS_POSIXACL()
test is still needed by nfsd.

Signed-off-by: Andreas Gruenbacher <[email protected]>
Reviewed-by: J. Bruce Fields <[email protected]>
---
fs/Kconfig | 3 +++
fs/namei.c | 8 ++++----
include/linux/fs.h | 12 ++++++++++++
include/uapi/linux/fs.h | 3 ++-
4 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/fs/Kconfig b/fs/Kconfig
index da3f32f..bff2879 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -56,6 +56,9 @@ endif # BLOCK
config FS_POSIX_ACL
def_bool n

+config FS_RICHACL
+ def_bool n
+
config EXPORTFS
tristate

diff --git a/fs/namei.c b/fs/namei.c
index 33e9495..224ecf1 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2798,7 +2798,7 @@ static int atomic_open(struct nameidata *nd, struct dentry *dentry,
}

mode = op->mode;
- if ((open_flag & O_CREAT) && !IS_POSIXACL(dir))
+ if ((open_flag & O_CREAT) && !IS_ACL(dir))
mode &= ~current_umask();

excl = (open_flag & (O_EXCL | O_CREAT)) == (O_EXCL | O_CREAT);
@@ -2982,7 +2982,7 @@ static int lookup_open(struct nameidata *nd, struct path *path,
/* Negative dentry, just create the file */
if (!dentry->d_inode && (op->open_flag & O_CREAT)) {
umode_t mode = op->mode;
- if (!IS_POSIXACL(dir->d_inode))
+ if (!IS_ACL(dir->d_inode))
mode &= ~current_umask();
/*
* This write is needed to ensure that a
@@ -3553,7 +3553,7 @@ retry:
if (IS_ERR(dentry))
return PTR_ERR(dentry);

- if (!IS_POSIXACL(path.dentry->d_inode))
+ if (!IS_ACL(path.dentry->d_inode))
mode &= ~current_umask();
error = security_path_mknod(&path, dentry, mode, dev);
if (error)
@@ -3622,7 +3622,7 @@ retry:
if (IS_ERR(dentry))
return PTR_ERR(dentry);

- if (!IS_POSIXACL(path.dentry->d_inode))
+ if (!IS_ACL(path.dentry->d_inode))
mode &= ~current_umask();
error = security_path_mkdir(&path, dentry, mode);
if (!error)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 72d8a84..4efa435 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1781,6 +1781,12 @@ struct super_operations {
#define IS_IMMUTABLE(inode) ((inode)->i_flags & S_IMMUTABLE)
#define IS_POSIXACL(inode) __IS_FLG(inode, MS_POSIXACL)

+#ifdef CONFIG_FS_RICHACL
+#define IS_RICHACL(inode) __IS_FLG(inode, MS_RICHACL)
+#else
+#define IS_RICHACL(inode) 0
+#endif
+
#define IS_DEADDIR(inode) ((inode)->i_flags & S_DEAD)
#define IS_NOCMTIME(inode) ((inode)->i_flags & S_NOCMTIME)
#define IS_SWAPFILE(inode) ((inode)->i_flags & S_SWAPFILE)
@@ -1794,6 +1800,12 @@ struct super_operations {
(inode)->i_rdev == WHITEOUT_DEV)

/*
+ * IS_ACL() tells the VFS to not apply the umask
+ * and use check_acl for acl permission checks when defined.
+ */
+#define IS_ACL(inode) __IS_FLG(inode, MS_POSIXACL | MS_RICHACL)
+
+/*
* Inode state bits. Protected by inode->i_lock
*
* Three bits determine the dirty state of the inode, I_DIRTY_SYNC,
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 9b964a5..6ac6bc9 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -81,7 +81,7 @@ struct inodes_stat_t {
#define MS_VERBOSE 32768 /* War is peace. Verbosity is silence.
MS_VERBOSE is deprecated. */
#define MS_SILENT 32768
-#define MS_POSIXACL (1<<16) /* VFS does not apply the umask */
+#define MS_POSIXACL (1<<16) /* Supports POSIX ACLs */
#define MS_UNBINDABLE (1<<17) /* change to unbindable */
#define MS_PRIVATE (1<<18) /* change to private */
#define MS_SLAVE (1<<19) /* change to slave */
@@ -91,6 +91,7 @@ struct inodes_stat_t {
#define MS_I_VERSION (1<<23) /* Update inode I_version field */
#define MS_STRICTATIME (1<<24) /* Always perform atime updates */
#define MS_LAZYTIME (1<<25) /* Update the on-disk [acm]times lazily */
+#define MS_RICHACL (1<<26) /* Supports richacls */

/* These sb flags are internal to the kernel */
#define MS_NOSEC (1<<28)
--
2.5.0

2015-10-16 17:31:40

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 23:22:55

by Dave Chinner

[permalink] [raw]

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 13:12:45

by Austin S Hemmelgarn

[permalink] [raw]

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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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 17:33:21

by Andreas Gruenbacher

[permalink] [raw]

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 17:42:05

by Andreas Dilger

[permalink] [raw]

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 18:45:50

On 2015-10-19 16:20, Andreas Gruenbacher wrote:
> 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.
Except that mounting a filesystem that wasn't created on the system
mounting it always has that exact same risk, because (AFAIK) all
Linux/BSD/Other UNIX filesystems store GID's and UID's as numbers, not
names. Perfect example of this, take a minimal install of some Linux
distribution, install a couple of packages that add new UID's, and mount
the root filesystem on a completely different distribution (say mount a
Debian root filesystem on a Gentoo box), any of the new UID's from the
first system will almost certainly not map to the same user on the
second system (in fact, you can also do this with two Gentoo systems
where the packages were installed in different orders). This is a
really basic security risk that any seasoned sysadmin should know, and
most new ones find out about rather quickly.

Also, based on established context of _every_ other feature with an
incompat feature flag in both ext4 and XFS, 'safe' means that you can
get the correct file contents off of the filesystem,and don't run the
risk of crashing the system, not that you have no risk of compromising
security.
> 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.
Yes, and this is why any sane filesystem will spit a warning out through
dmesg when a mount is forced read-only because of incompatible features.
If someone is actively mounting such a filesystem read-only (that is,
they've specified 'ro' in the mount options), then it's relatively safe
for the kernel to assume they are doing so for some very specific reason
and/or already know about the data safety implications.
>
>> 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.
Except there will be a lot of people who think they know better but
really don't. Such people will do this anyway, and by modifying the
filesystem run a bigger risk of shooting themselves in the foot (because
they'll almost certainly mount the filesystem read-write, and then end
up turning on richacls again). You're not removing the gun, you're just
hiding a potentially bigger one somewhere else.

It's a separate issue entirely however whether or not you absolutely
need to know that the richacls have the correct syntax in a recovery
situation. Fsck doesn't parse SELinux labels, (AFAIK) doesn't parse
filecaps attributes (bad filecaps syntax will only make things have
fewer privileges, but it will cause user visible brokenness), (again,
AFAIK) doesn't validate POSIX ACL's, and absolutely doesn't check IMA or
EVM hashes. IMA and EVM hashes being wrong _will_ cause almost any
system actually using them they way they are intended to fail to boot,
and incorrect SELinux labeling will make many systems not boot
correctly, and both situations are much worse for a significant majority
of users than a (security leak due to a bad ACL.
>
> 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.
While it's something that 'should' be the case, there are probably quite
a few people who will not realize this until they're already in a
situation that they need to recover data. On top of that some people
will likely assume that they just need richacl support in userspace for
their recovery environment.
>
>> 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.
While that may be the case, there will be people who assume that because
it's an incompat feature, their ACL's will _always_ be enforced. Such
people should admittedly not be allowed to run systems with any real
world security requirements, but that mentality is something that still
needs to be considered, and this is arguably making it easier for them
to shoot themselves in the foot.

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