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>

1 files changed, 2 insertions, 12 deletions

diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 50025ead0b72..aece1b340e7d 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -29,13 +29,8 @@ struct linux_binprm {
 		/* Should an execfd be passed to userspace? */
 		have_execfd:1,
 
-		/* It is safe to use the creds of a script (see binfmt_misc) */
-		preserve_creds:1,
-		/*
-		 * True if most recent call to security_bprm_set_creds
-		 * resulted in elevated privileges.
-		 */
-		active_secureexec:1,
+		/* Use the creds of a script (see binfmt_misc) */
+		execfd_creds:1,
 		/*
 		 * Set by bprm_creds_for_exec hook to indicate a
 		 * privilege-gaining exec has happened. Used to set
@@ -55,11 +50,6 @@ struct linux_binprm {
 	struct file * file;
 	struct cred *cred;	/* new credentials */
 	int unsafe;		/* how unsafe this exec is (mask of LSM_UNSAFE_*) */
-	/*
-	 * bits to clear in current->personality
-	 * recalculated for each bprm->file.
-	 */
-	unsigned int pf_per_clear;
 	unsigned int per_clear;	/* bits to clear in current->personality */
 	int argc, envc;
 	const char * filename;	/* Name of binary as seen by procps */