Re: new ...at() flag: AT_NO_JUMPS

On Thu, May 4, 2017 at 9:39 PM, Al Viro [off-list ref] wrote:

On Thu, May 04, 2017 at 08:46:49PM -0700, Linus Torvalds wrote:

quoted

On Thu, May 4, 2017 at 7:47 PM, Jann Horn [off-list ref] wrote:

quoted

Thread 1 starts an AT_BENEATH path walk using an O_PATH fd
pointing to /srv/www/example.org/foo; the path given to the syscall is
"bar/../../../../etc/passwd". The path walk enters the "bar" directory.
Thread 2 moves /srv/www/example.org/foo/bar to
/srv/www/example.org/bar.
Thread 1 processes the rest of the path ("../../../../etc/passwd"), never
hitting /srv/www/example.org/foo in the process.

I'm not really familiar with the VFS internals, but from a coarse look
at the patch, it seems like it wouldn't block this?

I think you're right.

I guess it would be safe for the RCU case due to the sequence number
check, but not the non-RCU case.

        Yes and no...  FWIW, to exclude that it would suffice to have
mount --rbind /src/www/example.org/foo /srv/www/example.org/foo done first.
Then this kind of race will end up with -ENOENT due to path_connected()
logics in follow_dotdot_rcu()/follow_dotdot().  I'm not sure about the
intended applications, though - is that thing supposed to be used along with
some horror like seccomp, or...?

How hard would it be for the kernel to prevent this on its own?
Asking users to do the mount --rbind seems like it's asking for users
to forget to do it.