Mmm, slow-as-dirt hotel wireless. What fun...
On Aug 15, 2007, at 18:14:44, Phillip Susi 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.
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
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.
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?
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.
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.
In Linux the ONLY distinction between "relative" and "absolute" paths
is that the "absolute" path begins with a magic slash which implies
that you start at the hidden "root" fd the kernel manages.
More detail on problems with the "building the ACL from scratch" part:
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!
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.
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.
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.
Cheers,
Kyle Moffett
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