exec: Convert security_bprm_set_creds into security_bprm_repopulate_creds

Rename bprm->cap_elevated to bprm->active_secureexec and initialize it
in prepare_binprm instead of in cap_bprm_set_creds.  Initializing
bprm->active_secureexec in prepare_binprm allows multiple
implementations of security_bprm_repopulate_creds to play nicely with
each other.

Rename security_bprm_set_creds to security_bprm_reopulate_creds to
emphasize that this path recomputes part of bprm->cred.  This
recomputation avoids the time of check vs time of use problems that
are inherent in unix #! interpreters.

In short two renames and a move in the location of initializing
bprm->active_secureexec.

Link: https://lkml.kernel.org/r/87o8qkzrxp.fsf_-_@x220.int.ebiederm.org
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/exec.c

@@ -1366,7 +1366,7 @@ int begin_new_exec(struct linux_binprm * bprm)
 	 * the final state of setuid/setgid/fscaps can be merged into the
 	 * secureexec flag.
 	 */
-	bprm->secureexec |= bprm->cap_elevated;
+	bprm->secureexec |= bprm->active_secureexec;
 
 	if (bprm->secureexec) {
 		/* Make sure parent cannot signal privileged process. */
@@ -1634,10 +1634,10 @@ int prepare_binprm(struct linux_binprm *bprm)
 	int retval;
 	loff_t pos = 0;
 
+	/* Recompute parts of bprm->cred based on bprm->file */
+	bprm->active_secureexec = 0;
 	bprm_fill_uid(bprm);
-
+	retval = security_bprm_repopulate_creds(bprm);
-	/* fill in binprm security blob */
-	retval = security_bprm_set_creds(bprm);
 	if (retval)
 		return retval;
 

include/linux/binfmts.h

@@ -27,10 +27,10 @@ struct linux_binprm {
 	unsigned long argmin; /* rlimit marker for copy_strings() */
 	unsigned int
 		/*
-		 * True if most recent call to cap_bprm_set_creds
+		 * True if most recent call to security_bprm_set_creds
 		 * resulted in elevated privileges.
 		 */
-		cap_elevated:1,
+		active_secureexec:1,
 		/*
 		 * Set by bprm_creds_for_exec hook to indicate a
 		 * privilege-gaining exec has happened. Used to set

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_set_creds, struct linux_binprm *bprm)
+LSM_HOOK(int, 0, bprm_repopulate_creds, struct linux_binprm *bprm)
 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)

include/linux/lsm_hooks.h

@@ -44,7 +44,7 @@
  *	request libc enable secure mode.
  *	@bprm contains the linux_binprm structure.
  *	Return 0 if the hook is successful and permission is granted.
- * @bprm_set_creds:
+ * @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
@@ -53,7 +53,7 @@
  *	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->cap_elevated to 1 if AT_SECURE should be set to
+ *	The hook must set @bprm->active_secureexec to 1 if AT_SECURE should be set to
  *	request libc enable secure mode.
  *	@bprm contains the linux_binprm structure.
  *	Return 0 if the hook is successful and permission is granted.

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_set_creds(struct linux_binprm *bprm);
+extern int cap_bprm_repopulate_creds(struct linux_binprm *bprm);
 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_set_creds(struct linux_binprm *bprm);
+int security_bprm_repopulate_creds(struct linux_binprm *bprm);
 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 +575,9 @@ static inline int security_bprm_creds_for_exec(struct linux_binprm *bprm)
 	return 0;
 }
 
-static inline int security_bprm_set_creds(struct linux_binprm *bprm)
+static inline int security_bprm_repopulate_creds(struct linux_binprm *bprm)
 {
-	return cap_bprm_set_creds(bprm);
+	return cap_bprm_repopulate_creds(bprm);
 }
 
 static inline int security_bprm_check(struct linux_binprm *bprm)

security/commoncap.c

@@ -797,14 +797,14 @@ static inline bool nonroot_raised_pE(struct cred *new, const struct cred *old,
 }
 
 /**
- * cap_bprm_set_creds - Set up the proposed credentials for execve().
+ * cap_bprm_repopulate_creds - Set up the proposed credentials for execve().
  * @bprm: The execution parameters, including the proposed creds
  *
  * Set up the proposed credentials for a new execution context being
  * constructed by execve().  The proposed creds in @bprm->cred is altered,
  * which won't take effect immediately.  Returns 0 if successful, -ve on error.
  */
-int cap_bprm_set_creds(struct linux_binprm *bprm)
+int cap_bprm_repopulate_creds(struct linux_binprm *bprm)
 {
 	const struct cred *old = current_cred();
 	struct cred *new = bprm->cred;
@@ -884,12 +884,11 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 		return -EPERM;
 
 	/* Check for privilege-elevated exec. */
-	bprm->cap_elevated = 0;
 	if (is_setid ||
 	    (!__is_real(root_uid, new) &&
 	     (effective ||
 	      __cap_grew(permitted, ambient, new))))
-		bprm->cap_elevated = 1;
+		bprm->active_secureexec = 1;
 
 	return 0;
 }
@@ -1346,7 +1345,7 @@ static struct security_hook_list capability_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(ptrace_traceme, cap_ptrace_traceme),
 	LSM_HOOK_INIT(capget, cap_capget),
 	LSM_HOOK_INIT(capset, cap_capset),
-	LSM_HOOK_INIT(bprm_set_creds, cap_bprm_set_creds),
+	LSM_HOOK_INIT(bprm_repopulate_creds, cap_bprm_repopulate_creds),
 	LSM_HOOK_INIT(inode_need_killpriv, cap_inode_need_killpriv),
 	LSM_HOOK_INIT(inode_killpriv, cap_inode_killpriv),
 	LSM_HOOK_INIT(inode_getsecurity, cap_inode_getsecurity),

security/security.c

@@ -828,9 +828,9 @@ int security_bprm_creds_for_exec(struct linux_binprm *bprm)
 	return call_int_hook(bprm_creds_for_exec, 0, bprm);
 }
 
-int security_bprm_set_creds(struct linux_binprm *bprm)
+int security_bprm_repopulate_creds(struct linux_binprm *bprm)
 {
-	return call_int_hook(bprm_set_creds, 0, bprm);
+	return call_int_hook(bprm_repopulate_creds, 0, bprm);
 }
 
 int security_bprm_check(struct linux_binprm *bprm)