On Aug 16, 2007, at 11:09:16, Phillip Susi wrote:
Kyle Moffett wrote:
Let me repeat myself here: Algorithmically you fundamentally
CANNOT implement inheritance-based ACLs without one of the
following (although if you have some other algorithm in mind, I'm
listening):
(A) Some kind of recursive operation *every* time you change an
inheritable permission
(B) A unified "starting point" from which you begin *every*
access-control lookup (or one "starting point" per useful semantic
grouping, like a namespace).
The "(A)" is presently done in userspace and that's what you want
to avoid. As to (B), I will attempt to prove below that you
cannot implement "(B)" without breaking existing assumptions and
restricting a very nice VFS model.
No recursion is needed because only one acl exists, so that is the
only one you need to update. At least on disk. Any cached acls in
memory of descendant objects would need updated, but the number of
those should be relatively small. The starting point would be the
directory you start the lookup from. That may be the root, or it
may be some other directory that you have a handle to, and thus,
already has its effective acl computed.
Problem 1: "updating cached acls of descendent objects": How do you
find out what a 'descendent object' is? Answer: You can't without
recursing through the entire in-memory dentry tree. Such recursion
is lock-intensive and has poor performance. Furthermore, you have to
do the entire recursion as an atomic operation; other cross-directory
renames or ACL changes would invalidate your results halfway through
and cause race conditions.
Oh, and by the way, the kernel has no real way to go from a dentry to
a (process, fd) pair. That data simply is not maintained because it
is unnecessary and inefficent to do so. Without that data you
*can't* determine what is "dependent". Furthermore, even if you
could it still wouldn't work because you can't even tell which path
the file was originally opened via. Say you run:
mount --bind /mnt/cdrom /cdrom
umount /mnt/cdrom
Now any process which had a cwd or open directory handle in "/cdrom"
is STILL USING THE ACLs from when it was mounted as "/mnt/cdrom". If
you have the same volume bind-mounted in two places you can't easily
distinguish between them. Caching permission data at the vfsmount
won't even help you because you can move around vfsmounts as long as
they are in subdirectories:
mkdir -p /a/b/foo
mount -t tmpfs tmpfs /a/b/foo
mv /a/b /quux
umount /quux/foo
At this point you would also have to look at vfsmounts during your
recursive traversal and update their cached ACLs too.
Problem 2: "Some other directory that you have a handle to": When
you are given this relative path and this cwd ACL, how do you
determine the total ACL of the parent directory:
path: ../foo/bar
cached cwd total-ACL:
root rwx (inheritable)
bob rwx (inheritable)
somegroup rwx (inheritable)
jane rwx
".." partial-ACL
root +rwx (inheritable)
somegroup +rx (inheritable)
Answer: you can't. For example, if "/" had the permission 'root
+rwx (inheritable)', and nothing else had subtractive permissions,
then the "root +rwx (inheritable)" in the parent dir would be a no-
op, but you can't tell that without storing a complete parent
directory history.
Now assume that I "mkdir /foo && set-some-inheritable-acl-on /foo &&
mv /home /foo/home". Say I'm running all sorts of X apps and GIT and
a number of other programs and have some conservative 5k FDs open on /
home. This is actually something I've done before (without the
ACLs), albeit accidentally. With your proposal, the kernel would
first have to identify all of the thousands of FDs with cached ACL
data across a very large cache-hot /home directory. For each FD, it
would have to store an updated copy of the partial-ACL states down
its entire path. Oh, and you can't do any other ACL or rename
operations in the entire subtree while this is going on, because that
would lead to the first update reporting incorrect results and racing
with the second. You are also extremely slow, deadlock-prone, and
memory hungry, since you have to take an enormous pile of dentry
locks while doing the recursion. Nobody can even open files with
relative paths while this is going on because the cached ACLs are in
an intermediate and inconsistent state: they're updated but the
directory isn't in its new position yet.
Unsolvable problems with each option:
(1.a.I)
You just broke all sorts of chrooted daemons. When I start bind
in its chroot jail, it does the following:
chdir("/private/bind9");
chroot(".");
setgid(...);
setuid(...);
The "/private" directory is readable only by root, since root is
the only one who will be navigating you into these chroots for any
reason. You only switch UID/GID after the chroot() call, at which
point you are inside of a sub-context and your cwd is fully
accessible. If you stick an inheritable ACL on "/private", then
the "cwd" ACL will not allow access by anybody but root and my
bind won't be able to read any config files.
If you want the directory to be root accessible but the files
inside to have wider access then you set the acl on the directory
to have one ace granting root access to the directory, and one ace
that is inheritable granting access to bind. This latter ace does
not apply to the directory itself, only to its children.
This is completely opposite the way that permissions currently
operate in Linux. When I am chrooted, I don't care about the
permissions of *anything* outside of the chroot, because it simply
doesn't exist. Furthermore you still don't answer the "computing ACL
of parent directory requires lots of space" problem.
You also break relative paths and directory-moving. Say a process
does chdir("/foo/bar"). Now the ACL data in "cwd" is appropriate
for /foo/bar. If you later chdir("../quux"), how do you unapply
the changes made when you switched into that directory? For
inheritable ACLs, you can't "unapply" such an ACL state change
unless you save state for all the parent directories, except...
What happens when you are in "/foo/bar" and another process does
"mv /foo/bar /foobar/quux"? Suddenly any "cwd" ACL data you have
is completely invalid and you have to rebuild your ACLs from
scratch. Moreover, if the directory you are in was moved to a
portion of the filesystem not accessible from your current
namespace then how do you deal with it?
Yes, if /foo/quux is not already cached in memory, you would have
to walk the tree to build its acl. /foo should already be cached
in memory so this work is minimal. Is this so horrible of a problem?
As for moving, it is handled the same way as any other event that
makes cwd go away, such as deleting it or revoking your access; cwd
is now invalid.
No, you aren't getting it: YOUR CWD DOES NOT GO AWAY WHEN YOU MOVE
IT OR UMOUNT -L IT. NEITHER DO OPEN DIRECTORY HANDLES. Sorry for
yelling but this is the crux of the point I am trying to make. Any
permissions system which cannot handle a *completely* discontiguous
filesystem space cannot work on Linux; end of story. The primary
reason behind that is all sorts of filesystem operations are
internally discontiguous because it makes them much more efficient.
By attempting to "force" the VFS to pretend like everything is
contiguous you are going to break horribly in a thousand different
corner cases that simply don't exist at the moment.
For example:
NS1 has the / root dir of /dev/sdb1 mounted on /mnt
NS2 has the /bar subdir of /dev/sdb1 mounted on /mnt
Your process is in NS2 and does chdir("/mnt/quux"). A user in NS1
does: "mv /mnt/bar/quux /mnt/quux". Now your "cwd" is in a
directory on a filesystem you have mounted, but it does not
correspond *AT ALL* to any path available from your namespace.
Which would be no different than if they just deleted the entire
thing. Your cwd no longer exists.
No, your cwd still exists and is full of files. You can still
navigate around in it (same with any open directory handle). You can
still open files, chdir, move files, etc. There isn't even a way for
the process in NS1 to tell the processes in NS2 that its directories
were rearranged, so even a simple "NS1# mv /mnt/bar/a/somedir /mnt/
bar/b/somedir" is not going to work.
Another example:
Your process has done dirfd=open("/media/cdrom/somestuff") when
the admin does "umount -l /media/cdrom". You still have the CD-
ROM open and accessible but IT HAS NO PATH. It isn't even mounted
in *any* namespace, it's just kind of dangling waiting for its
last users to go away. You can still do fchdir(dirfd), openat
(dirfd, "foo/bar", ...), open("./foo"), etc.
What's this got to do with acls? If you are asking what effect the
umount thas on the acls of the cdrom, the answer is none. The acls
are on the disc and nothing on the disc has changed.
But you said above "Yes, if /foo/quux is not already cached in
memory, then you would have to walk the tree to build it's ACL". Now
assume that instead of "/foo/quux", you are one directory deep in the
now-unmounted CDROM and you try to open "../baz/quux". In order to
get at the ACL of the parent directory it has to have an absolute
path somewhere, but at that point it doesn't.
No, this is correct because in the root directory "/", the ".."
entry is just another link to the root directory. So the absolute
path "/../../../../../.." is just a fancy name for the root
directory. The above jail-escape-as-root exploit is possible
because it is impossible to determine whether a directory is or is
not a subentry of another directory without an exhaustive search.
So when your "cwd" points to a path outside of the chroot, the one
special case in the code for the "root" directory does not ever
match and you can "chdir" all the way up to the real root. You
can even do an fstat() after every iteration to figure out whether
you're there or not!
Ohh, I see... yes... that is a very clever way for root to misuse
chroot(). What does it have to do with this discussion?
What it "has to do" is it is part of the Linux ABI and as such you
can't just break it because it's "inconvenient" for inheritable
ACLs. You also can't make a previously O(1) operation take lots of
time, as that's also considered "major breakage".
With this you just got into the big-ugly-nasty-recursive-behavior
again. Say I untar 20 kernel source trees and then have my
program open all 1000 available FDs to various directories in the
kernel source tree. Now I run 20 copies of this program, one for
each tree, still well within my ulimits even on a conservative
box. Now run "mv dir_full_of_kernel_sources some/new/dir". The
only thing you can do to find all of the FDs is to iterate down
the entire subdirectory tree looking for open files and updating
their contexts one-by-one. Except you have 20,000 directory FDs
to update. Ouch.
Ok, so you found a pedantic corner case that is slow. So? And it
is still going to be faster than chmod -R.ee
"Pedantic corner case"? You could do the same thing even *WITHOUT*
all the processes holding open FDs, you would still have to iterate
over the entire in-cache portion of the subtree in order to verify
that there are no open FDs on it. Yet again you would also run into
the problem that we don't have *ANY* dentry-to-filehandle mapping in
the kernel.
To sum up, when doing access control the only values you can
safely and efficiently get at are:
(A) The dentry/inode
(B) The superblock
(C) *Maybe* the vfsmount if those patches get accepted
Any access control model which tries to poke other values is just
going to have a shitload of corner cases where it just falls over.
If by falls over you mean takes some time, then yes.... so what?
Converting a previously O(1) operation into an O(number-of-subdirs)
operation is also known as "a major regression which we don't do a
release till we get it fixed". For boxes where O(number-of-subdirs)
numbers in the millions that would make it slow to a painful crawl.
By the way, I'm done with this discussion since you don't seem to be
paying attention at all. Don't bother replying unless you've
actually written testable code you want people on the list to look
at. I'll eat my own words if you actually come up with an algorithm
which works efficiently without introducing regressions.
Cheers,
Kyle Moffett
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