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>

4 files changed, 19 insertions, 28 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 */
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 1e295ba12c0d..adbc6603abba 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -50,7 +50,7 @@ LSM_HOOK(int, 0, settime, const struct timespec64 *ts,
 	 const struct timezone *tz)
 LSM_HOOK(int, 0, vm_enough_memory, struct mm_struct *mm, long pages)
 LSM_HOOK(int, 0, bprm_creds_for_exec, struct linux_binprm *bprm)
-LSM_HOOK(int, 0, bprm_repopulate_creds, struct linux_binprm *bprm)
+LSM_HOOK(int, 0, bprm_creds_from_file, struct linux_binprm *bprm, struct file *file)
 LSM_HOOK(int, 0, bprm_check_security, struct linux_binprm *bprm)
 LSM_HOOK(void, LSM_RET_VOID, bprm_committing_creds, struct linux_binprm *bprm)
 LSM_HOOK(void, LSM_RET_VOID, bprm_committed_creds, struct linux_binprm *bprm)
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index f68076d440f3..c523c18efa0e 100644
--- a/include/linux/lsm_hooks.h
+++ b/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.
diff --git a/include/linux/security.h b/include/linux/security.h
index 6dcec9375e8f..8444fae7c5b9 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -140,7 +140,7 @@ extern int cap_capset(struct cred *new, const struct cred *old,
 		      const kernel_cap_t *effective,
 		      const kernel_cap_t *inheritable,
 		      const kernel_cap_t *permitted);
-extern int cap_bprm_repopulate_creds(struct linux_binprm *bprm);
+extern int cap_bprm_creds_from_file(struct linux_binprm *bprm, struct file *file);
 extern int cap_inode_setxattr(struct dentry *dentry, const char *name,
 			      const void *value, size_t size, int flags);
 extern int cap_inode_removexattr(struct dentry *dentry, const char *name);
@@ -277,7 +277,7 @@ int security_syslog(int type);
 int security_settime64(const struct timespec64 *ts, const struct timezone *tz);
 int security_vm_enough_memory_mm(struct mm_struct *mm, long pages);
 int security_bprm_creds_for_exec(struct linux_binprm *bprm);
-int security_bprm_repopulate_creds(struct linux_binprm *bprm);
+int security_bprm_creds_from_file(struct linux_binprm *bprm, struct file *file);
 int security_bprm_check(struct linux_binprm *bprm);
 void security_bprm_committing_creds(struct linux_binprm *bprm);
 void security_bprm_committed_creds(struct linux_binprm *bprm);
@@ -575,9 +575,10 @@ static inline int security_bprm_creds_for_exec(struct linux_binprm *bprm)
 	return 0;
 }
 
-static inline int security_bprm_repopulate_creds(struct linux_binprm *bprm)
+static inline int security_bprm_creds_from_file(struct linux_binprm *bprm,
+						struct file *file)
 {
-	return cap_bprm_repopulate_creds(bprm);
+	return cap_bprm_creds_from_file(bprm, file);
 }
 
 static inline int security_bprm_check(struct linux_binprm *bprm)