exec: Compute file based creds only once

Move the computation of creds from prepare_binfmt into begin_new_exec
so that the creds need only be computed once.  This is just code
reorganization no semantic changes of any kind are made.

Moving the computation is safe.  I have looked through the kernel and
verified none of the binfmts look at bprm->cred directly, and that
there are no helpers that look at bprm->cred indirectly.  Which means
that it is not a problem to compute the bprm->cred later in the
execution flow as it is not used until it becomes current->cred.

A new function bprm_creds_from_file is added to contain the work that
needs to be done.  bprm_creds_from_file first computes which file
bprm->executable or most likely bprm->file that the bprm->creds
will be computed from.

The funciton bprm_fill_uid is updated to receive the file instead of
accessing bprm->file.  The now unnecessary work needed to reset the
bprm->cred->euid, and bprm->cred->egid is removed from brpm_fill_uid.
A small comment to document that bprm_fill_uid now only deals with the
work to handle suid and sgid files.  The default case is already
heandled by prepare_exec_creds.

The function security_bprm_repopulate_creds is renamed
security_bprm_creds_from_file and now is explicitly passed the file
from which to compute the creds.  The documentation of the
bprm_creds_from_file security hook is updated to explain when the hook
is called and what it needs to do.  The file is passed from
cap_bprm_creds_from_file into get_file_caps so that the caps are
computed for the appropriate file.  The now unnecessary work in
cap_bprm_creds_from_file to reset the ambient capabilites has been
removed.  A small comment to document that the work of
cap_bprm_creds_from_file is to read capabilities from the files
secureity attribute and derive capabilities from the fact the
user had uid 0 has been added.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

include/linux/lsm_hooks.h

@@ -44,18 +44,18 @@
  *	request libc enable secure mode.
  *	@bprm contains the linux_binprm structure.
  *	Return 0 if the hook is successful and permission is granted.
- * @bprm_repopulate_creds:
- *	Assuming that the relevant bits of @bprm->cred->security have been
- *	previously set, examine @bprm->file and regenerate them.  This is
- *	so that the credentials derived from the interpreter the code is
- *	actually going to run are used rather than credentials derived
- *	from a script.  This done because the interpreter binary needs to
- *	reopen script, and may end up opening something completely different.
- *	This hook may also optionally check permissions (e.g. for
- *	transitions between security domains).
- *	The hook must set @bprm->active_secureexec to 1 if AT_SECURE should be set to
+ * @bprm_creds_from_file:
+ *	If @file is setpcap, suid, sgid or otherwise marked to change
+ *	privilege upon exec, update @bprm->cred to reflect that change.
+ *	This is called after finding the binary that will be executed.
+ *	without an interpreter.  This ensures that the credentials will not
+ *	be derived from a script that the binary will need to reopen, which
+ *	when reopend may end up being a completely different file.  This
+ *	hook may also optionally check permissions (e.g. for transitions
+ *	between security domains).
+ *	The hook must set @bprm->secureexec to 1 if AT_SECURE should be set to
  *	request libc enable secure mode.
- *	The hook must add to @bprm->pf_per_clear any personality flags that
+ *	The hook must add to @bprm->per_clear any personality flags that
  * 	should be cleared from current->personality.
  *	@bprm contains the linux_binprm structure.
  *	Return 0 if the hook is successful and permission is granted.