Re: [Qemu-devel] [PATCH] 9pfs: fix vulnerability in openat_dir() and local_unlinkat_common()

[Eric Blake <eblake@redhat.com>]

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On 03/03/2017 11:25 AM, Greg Kurz wrote:
> We should pass O_NOFOLLOW otherwise openat() will follow symlinks and make
> QEMU vulnerable.
> 
> O_PATH was used as an optimization: the fd returned by openat_dir() is only
> passed to openat() actually, so we don't really need to reach the underlying
> filesystem.
> 
> O_NOFOLLOW | O_PATH isn't an option: if name is a symlink, openat() will
> return a fd, forcing us to do some other syscall to detect we have a
> symlink. Also, O_PATH doesn't exist in glibc 2.13 and older.

But the very next use of openat(fd, ) should fail with EBADF if fd is
not a directory, so you don't need any extra syscalls.  I agree that we
_need_ O_NOFOLLOW, but I'm not yet convinced that we must avoid O_PATH
where it works.

I'm in the middle of writing a test program to probe kernel behavior and
demonstrate (at least to myself) whether there are scenarios where
O_PATH makes it possible to open something where omitting it did not,
while at the same time validating that O_NOFOLLOW doesn't cause problems
if a symlink-fd is returned instead of a directory fd, based on our
subsequent use of that fd in a *at call.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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