On Sat, May 30, 2020 at 9:07 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > On Sat, May 30, 2020 at 03:08:37PM +0100, Al Viro wrote: > > On Fri, May 29, 2020 at 07:43:10PM -0700, Kees Cook wrote: > > > > > Can anyone clarify the expected failure mode from SCM_RIGHTS? Can we > > > move the put_user() after instead? I think cleanup would just be: > > > replace_fd(fd, NULL, 0) > > > > Bollocks. > > > > Repeat after me: descriptor tables can be shared. There is no > > "cleanup" after you've put something there. > > Right -- this is what I was trying to ask about, and why I didn't like > the idea of just leaving the fd in the table on failure. But yeah, there > is a race if the process is about to fork or something. > > So the choice here is how to handle the put_user() failure: > > - add the put_user() address to the new helper, as I suggest in [1]. > (exactly duplicates current behavior) > - just leave the fd in place (not current behavior: dumps a fd into > the process without "agreed" notification). > - do a double put_user (once before and once after), also in [1]. > (sort of a best-effort combo of the above two. and SCM_RIGHTS is > hardly fast-pth). > > -Kees > > [1] https://lore.kernel.org/linux-api/202005282345.573B917@keescook/ > > -- > Kees Cook I'm going to suggest we stick to the approach of doing[1]: 1. Allocate FD 2. put_user 3. "Receive" and install file into FD That is the only way to preserve the current behaviour in which userspace is notified about *every* FD that is received via SCM_RIGHTS. The scm_detach_fds code as it reads today does effectively what is above, in that the fd is not installed until *after* the put user. Therefore if put_user gets an EFAULT or ENOMEM, it falls through to the MSG_CTRUNC bit. The approach suggested[2] has a "change" in behaviour, in that (all in file_receive): 1. Allocate FD 2. Receive file 3. put_user Based on what Al Viro said, I don't think we can simply add step #4, being "just" uninstall the FD. [1]: https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg2179418.html [2]: https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg2179453.html
