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>

2 files changed, 15 insertions, 13 deletions

diff --git a/security/commoncap.c b/security/commoncap.c
index 6de72d22dc6c..59bf3c1674c8 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -647,7 +647,8 @@ int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data
  * its xattrs and, if present, apply them to the proposed credentials being
  * constructed by execve().
  */
-static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_fcap)
+static int get_file_caps(struct linux_binprm *bprm, struct file *file,
+			 bool *effective, bool *has_fcap)
 {
 	int rc = 0;
 	struct cpu_vfs_cap_data vcaps;
@@ -657,7 +658,7 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_f
 	if (!file_caps_enabled)
 		return 0;
 
-	if (!mnt_may_suid(bprm->file->f_path.mnt))
+	if (!mnt_may_suid(file->f_path.mnt))
 		return 0;
 
 	/*
@@ -665,10 +666,10 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_f
 	 * explicit that capability bits are limited to s_user_ns and its
 	 * descendants.
 	 */
-	if (!current_in_userns(bprm->file->f_path.mnt->mnt_sb->s_user_ns))
+	if (!current_in_userns(file->f_path.mnt->mnt_sb->s_user_ns))
 		return 0;
 
-	rc = get_vfs_caps_from_disk(bprm->file->f_path.dentry, &vcaps);
+	rc = get_vfs_caps_from_disk(file->f_path.dentry, &vcaps);
 	if (rc < 0) {
 		if (rc == -EINVAL)
 			printk(KERN_NOTICE "Invalid argument reading file caps for %s\n",
@@ -797,26 +798,27 @@ static inline bool nonroot_raised_pE(struct cred *new, const struct cred *old,
 }
 
 /**
- * cap_bprm_repopulate_creds - Set up the proposed credentials for execve().
+ * cap_bprm_creds_from_file - Set up the proposed credentials for execve().
  * @bprm: The execution parameters, including the proposed creds
+ * @file: The file to pull the credentials from
  *
  * 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_repopulate_creds(struct linux_binprm *bprm)
+int cap_bprm_creds_from_file(struct linux_binprm *bprm, struct file *file)
 {
+	/* Process setpcap binaries and capabilities for uid 0 */
 	const struct cred *old = current_cred();
 	struct cred *new = bprm->cred;
 	bool effective = false, has_fcap = false, is_setid;
 	int ret;
 	kuid_t root_uid;
 
-	new->cap_ambient = old->cap_ambient;
 	if (WARN_ON(!cap_ambient_invariant_ok(old)))
 		return -EPERM;
 
-	ret = get_file_caps(bprm, &effective, &has_fcap);
+	ret = get_file_caps(bprm, file, &effective, &has_fcap);
 	if (ret < 0)
 		return ret;
 
@@ -826,7 +828,7 @@ int cap_bprm_repopulate_creds(struct linux_binprm *bprm)
 
 	/* if we have fs caps, clear dangerous personality flags */
 	if (__cap_gained(permitted, new, old))
-		bprm->pf_per_clear |= PER_CLEAR_ON_SETID;
+		bprm->per_clear |= PER_CLEAR_ON_SETID;
 
 	/* Don't let someone trace a set[ug]id/setpcap binary with the revised
 	 * credentials unless they have the appropriate permit.
@@ -889,7 +891,7 @@ int cap_bprm_repopulate_creds(struct linux_binprm *bprm)
 	    (!__is_real(root_uid, new) &&
 	     (effective ||
 	      __cap_grew(permitted, ambient, new))))
-		bprm->active_secureexec = 1;
+		bprm->secureexec = 1;
 
 	return 0;
 }
@@ -1346,7 +1348,7 @@ 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_repopulate_creds, cap_bprm_repopulate_creds),
+	LSM_HOOK_INIT(bprm_creds_from_file, cap_bprm_creds_from_file),
 	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),
diff --git a/security/security.c b/security/security.c
index b890b7e2a765..259b8e750aa2 100644
--- a/security/security.c
+++ b/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_repopulate_creds(struct linux_binprm *bprm)
+int security_bprm_creds_from_file(struct linux_binprm *bprm, struct file *file)
 {
-	return call_int_hook(bprm_repopulate_creds, 0, bprm);
+	return call_int_hook(bprm_creds_from_file, 0, bprm, file);
 }
 
 int security_bprm_check(struct linux_binprm *bprm)