1 files changed, 37 insertions, 1 deletions

diff --git a/fs/open.c b/fs/open.c
index 8038cf652583..4401a73d4032 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -368,7 +368,37 @@ COMPAT_SYSCALL_DEFINE6(fallocate, int, fd, int, mode, compat_arg_u64_dual(offset
  * access() needs to use the real uid/gid, not the effective uid/gid.
  * We do this by temporarily clearing all FS-related capabilities and
  * switching the fsuid/fsgid around to the real ones.
+ *
+ * Creating new credentials is expensive, so we try to skip doing it,
+ * which we can if the result would match what we already got.
  */
+static bool access_need_override_creds(int flags)
+{
+	const struct cred *cred;
+
+	if (flags & AT_EACCESS)
+		return false;
+
+	cred = current_cred();
+	if (!uid_eq(cred->fsuid, cred->uid) ||
+	    !gid_eq(cred->fsgid, cred->gid))
+		return true;
+
+	if (!issecure(SECURE_NO_SETUID_FIXUP)) {
+		kuid_t root_uid = make_kuid(cred->user_ns, 0);
+		if (!uid_eq(cred->uid, root_uid)) {
+			if (!cap_isclear(cred->cap_effective))
+				return true;
+		} else {
+			if (!cap_isidentical(cred->cap_effective,
+			    cred->cap_permitted))
+				return true;
+		}
+	}
+
+	return false;
+}
+
 static const struct cred *access_override_creds(void)
 {
 	const struct cred *old_cred;
@@ -378,6 +408,12 @@ static const struct cred *access_override_creds(void)
 	if (!override_cred)
 		return NULL;
 
+	/*
+	 * XXX access_need_override_creds performs checks in hopes of skipping
+	 * this work. Make sure it stays in sync if making any changes in this
+	 * routine.
+	 */
+
 	override_cred->fsuid = override_cred->uid;
 	override_cred->fsgid = override_cred->gid;
 
@@ -437,7 +473,7 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
 	if (flags & AT_EMPTY_PATH)
 		lookup_flags |= LOOKUP_EMPTY;
 
-	if (!(flags & AT_EACCESS)) {
+	if (access_need_override_creds(flags)) {
 		old_cred = access_override_creds();
 		if (!old_cred)
 			return -ENOMEM;