On 2019-05-06, Jann Horn <jannh@xxxxxxxxxx> wrote: > On Mon, May 6, 2019 at 6:56 PM Aleksa Sarai <cyphar@xxxxxxxxxx> wrote: > > The need to be able to scope path resolution of interpreters became > > clear with one of the possible vectors used in CVE-2019-5736 (which > > most major container runtimes were vulnerable to). > > > > Naively, it might seem that openat(2) -- which supports path scoping -- > > can be combined with execveat(AT_EMPTY_PATH) to trivially scope the > > binary being executed. Unfortunately, a "bad binary" (usually a symlink) > > could be written as a #!-style script with the symlink target as the > > interpreter -- which would be completely missed by just scoping the > > openat(2). An example of this being exploitable is CVE-2019-5736. > > > > In order to get around this, we need to pass down to each binfmt_* > > implementation the scoping flags requested in execveat(2). In order to > > maintain backwards-compatibility we only pass the scoping AT_* flags. > > > > To avoid breaking userspace (in the exceptionally rare cases where you > > have #!-scripts with a relative path being execveat(2)-ed with dfd != > > AT_FDCWD), we only pass dfd down to binfmt_* if any of our new flags are > > set in execveat(2). > > This seems extremely dangerous. I like the overall series, but not this patch. > > > @@ -1762,6 +1774,12 @@ static int __do_execve_file(int fd, struct filename *filename, > > > > sched_exec(); > > > > + bprm->flags = flags & (AT_XDEV | AT_NO_MAGICLINKS | AT_NO_SYMLINKS | > > + AT_THIS_ROOT); > [...] > > +#define AT_THIS_ROOT 0x100000 /* - Scope ".." resolution to dirfd (like chroot(2)). */ > > So now what happens if there is a setuid root ELF binary with program > interpreter "/lib64/ld-linux-x86-64.so.2" (like /bin/su), and an > unprivileged user runs it with execveat(..., AT_THIS_ROOT)? Is that > going to let the unprivileged user decide which interpreter the > setuid-root process should use? From a high-level perspective, opening > the interpreter should be controlled by the program that is being > loaded, not by the program that invoked it. I went a bit nuts with openat_exec(), and I did end up adding it to the ELF interpreter lookup (and you're completely right that this is a bad idea -- I will drop it from this patch if it's included in the next series). The proposed solutions you give below are much nicer than this patch so I can drop it and work on fixing those issues separately. > In my opinion, CVE-2019-5736 points out two different problems: > > The big problem: The __ptrace_may_access() logic has a special-case > short-circuit for "introspection" that you can't opt out of; this > makes it possible to open things in procfs that are related to the > current process even if the credentials of the process wouldn't permit > accessing another process like it. I think the proper fix to deal with > this would be to add a prctl() flag for "set whether introspection is > allowed for this process", and if userspace has manually un-set that > flag, any introspection special-case logic would be skipped. We could do PR_SET_DUMPABLE=3 for this, I guess? > An additional problem: /proc/*/exe can be used to open a file for > writing; I think it may have been Andy Lutomirski who pointed out some > time ago that it would be nice if you couldn't use /proc/*/fd/* to > re-open files with more privileges, which is sort of the same thing. This is something I'm currently working on a series for, which would boil down to some restrictions on how re-opening of file descriptors works through procfs. However, execveat() of a procfs magiclink is a bit hard to block -- there is no way for userspace to to represent a file being "open for execute" so they are all "open for execute" by default and blocking it outright seems a bit extreme (though I actually hope to eventually add the ability to mark an O_PATH as "open for X" to resolveat(2) -- hence why I've reserved some bits). (Thinking more about it, there is an argument that I should include the above patch into this series so that we can block re-opening of fds opened through resolveat(2) without explicit flags from the outset.) -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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