Re: new flag: AT_NO_JUMPS

From: Linus Torvalds
Date: Thu May 04 2017 - 23:49:56 EST

On Thu, May 4, 2017 at 7:47 PM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> Thread 1 starts an AT_BENEATH path walk using an O_PATH fd
> pointing to /srv/www/; the path given to the syscall is
> "bar/../../../../etc/passwd". The path walk enters the "bar" directory.
> Thread 2 moves /srv/www/ to
> /srv/www/
> Thread 1 processes the rest of the path ("../../../../etc/passwd"), never
> hitting /srv/www/ 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.