On Sun, Apr 28, 2024 at 2:15 PM stsp [off-list ref] wrote:

28.04.2024 23:19, Andy Lutomirski пишет:

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Doesn't this have the same problem
that was pointed to me? Namely (explaining
my impl first), that if someone puts the cred
fd to an unaware process's fd table, such
process can't fully drop its privs. He may not
want to access these dirs, but once its hacked,
the hacker will access these dirs with the
creds came from an outside.

This is not a real problem. If I have a writable fd for /etc/shadow or
an fd for /dev/mem, etc, then I need close them to fully drop privs.

But isn't that becoming a problem once
you are (maliciously) passed such fds via
exec() or SCM_RIGHTS? You may not know
about them (or about their creds), so you
won't close them. Or?

Wait, who's the malicious party?  Anyone who can open a directory has,
at the time they do so, permission to do so.  If you send that fd to
someone via SCM_RIGHTS, all you accomplish is that they now have the
fd.

In my scenario, the malicious party attacks an *existing* program that
opens an fd for purposes that it doesn't think are dangerous.  And
then it gives the fd *to the malicious program* by whatever means
(could be as simple as dropping privs then doing dlopen).  Then the
malicious program does OA2_INHERIT_CREDS and gets privileges it
shouldn't have.

But if the *whole point* of opening the fd was to capture privileges
and preserve them across a privilege drop, and the program loads
malicious code after dropping privs, then that's a risk that's taken
intentionally.  This is like how, if you do curl
http://whatever.com/foo.sh | bash, you are granting all kinds of
permissions to unknown code.

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My solution was to close such fds on
exec and disallowing SCM_RIGHTS passage.

I don't see what problem this solves.

That the process that received them,
doesn't know they have O_CRED_ALLOW
within. So it won't deduce to close them
in time.

Hold on -- what exactly are you talking about?  A process does
recvmsg() and doesn't trust the party at the other end.  Then it
doesn't close the received fd.  Then it does setuid(getuid()).  Then
it does dlopen or exec of an untrusted program.

Okay, so the program now has a completely unknown fd.  This is already
part of the thread model.  It could be a cred-capturing fd, it could
be a device node, it could be a socket, it could be a memfd -- it
could be just about anything.  How do any of your proposals or my
proposals cause an actual new problem here?

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This is fundamental to the whole model. If I stick a FAT formatted USB
drive in the system and mount it, then any process that can find its
way to the mountpoint can write to it.  And if I open a dirfd, any
process with that dirfd can write it.  This is old news and isn't a
problem.

But IIRC O_DIRECTORY only allows O_RDONLY.
I even re-checked now, and O_DIRECTORY|O_RDWR
gives EISDIR. So is it actually true that
whoever has dir_fd, can write to it?

If the filesystem grants that UID permission to write, then it can write.