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.
What ACL would "task->cwd" use?
Options:
(1.a) Use the one calculated during the original chdir() call.
(1.b) Navigate "up" task->cwd building an ACL backwards.
(1.c) $CAN_YOU_THINK_OF_SOMETHING_ELSE_HERE
1.a
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.
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.
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.
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.
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?
And yes, this has been exploited before, although not often as
chroot()-ed uid=0 daemons aren't all that common.
So, pray tell, when this code runs and you do the "chroot" call, what
ACL do you think should get stuck on "cwd"? It doesn't reference
anything available relative to the chroot.
Same root abuse, same result. The acl on the cwd would still be exactly
what it was before the chroot.
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.
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?
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