Commit 13c164b1a186 - regression for LSMs/SELinux?

[Al Viro]

On Mon, Sep 21, 2020 at 06:09:22PM +0200, Christoph Hellwig wrote:
> [adding Linus and Al]
> 
> On Mon, Sep 21, 2020 at 04:51:35PM +0200, Ondrej Mosnacek wrote:
> > Hi folks,
> > 
> > It seems that after commit 13c164b1a186 ("autofs: switch to
> > kernel_write") there is now an extra LSM permission required (for the
> > current task to write to the automount pipe) for processes accessing
> > some yet-to-to-be mounted directory on which an autofs mount is set
> > up. The call chain is:
> > [...]
> > autofs_wait() ->
> > autofs_notify_daemon() ->
> > autofs_write() ->
> > kernel_write() ->
> > rw_verify_area() ->
> > security_file_permission()
> > 
> > The bug report that led me to this commit is at [1].
> > 
> > Technically, this is a regression for LSM users, since this is a
> > kernel-internal operation and an LSM permission for the current task
> > shouldn't be required. Can this patch be reverted? Perhaps
> > __kernel_{read|write}() could instead be renamed to kernel_*_nocheck()
> > so that the name is more descriptive?
> 
> So we obviously should not break existing user space and need to fix
> this ASAP.  The trivial "fix" would be to export __kernel_write again
> and switch autofs to use it.  The other option would be a FMODE flag
> to bypass security checks, only to be set if the callers ensures
> they've been valided (i.e. in autofs_prepare_pipe).
> 
> Any opinions?

Reexport for now.  Incidentally, what is LSM doing rejecting writes
into a pipe?