From fad6dd772d15214036aeea34a1c280dc8390fdfb Mon Sep 17 00:00:00 2001
From: Mateusz Guzik <mjg@FreeBSD.org>
Date: Wed, 29 Jul 2020 17:05:31 +0000
Subject: [PATCH] vfs: elide MAC-induced locking on rename if there are no
 relevant hoooks

---
 sys/kern/vfs_syscalls.c          | 58 ++++++++++++++++++++------------
 sys/security/mac/mac_framework.c |  3 ++
 sys/security/mac/mac_framework.h |  4 +++
 3 files changed, 44 insertions(+), 21 deletions(-)

diff --git a/sys/kern/vfs_syscalls.c b/sys/kern/vfs_syscalls.c
index c16da32de6d..9e9c53d3327 100644
--- a/sys/kern/vfs_syscalls.c
+++ b/sys/kern/vfs_syscalls.c
@@ -3541,6 +3541,33 @@ sys_renameat(struct thread *td, struct renameat_args *uap)
 	    UIO_USERSPACE));
 }
 
+#ifdef MAC
+static int
+kern_renameat_mac(struct thread *td, int oldfd, const char *old, int newfd,
+    const char *new, enum uio_seg pathseg, struct nameidata *fromnd)
+{
+	int error;
+
+	NDINIT_ATRIGHTS(fromnd, DELETE, LOCKPARENT | LOCKLEAF | SAVESTART |
+	    AUDITVNODE1, pathseg, old, oldfd, &cap_renameat_source_rights, td);
+	if ((error = namei(fromnd)) != 0)
+		return (error);
+	error = mac_vnode_check_rename_from(td->td_ucred, fromnd->ni_dvp,
+	    fromnd->ni_vp, &fromnd->ni_cnd);
+	VOP_UNLOCK(fromnd->ni_dvp);
+	if (fromnd->ni_dvp != fromnd->ni_vp)
+		VOP_UNLOCK(fromnd->ni_vp);
+	if (error != 0) {
+		NDFREE(fromnd, NDF_ONLY_PNBUF);
+		vrele(fromnd->ni_dvp);
+		vrele(fromnd->ni_vp);
+		if (fromnd->ni_startdir)
+			vrele(fromnd->ni_startdir);
+	}
+	return (error);
+}
+#endif
+
 int
 kern_renameat(struct thread *td, int oldfd, const char *old, int newfd,
     const char *new, enum uio_seg pathseg)
@@ -3553,30 +3580,19 @@ kern_renameat(struct thread *td, int oldfd, const char *old, int newfd,
 again:
 	bwillwrite();
 #ifdef MAC
-	NDINIT_ATRIGHTS(&fromnd, DELETE, LOCKPARENT | LOCKLEAF | SAVESTART |
-	    AUDITVNODE1, pathseg, old, oldfd,
-	    &cap_renameat_source_rights, td);
-	if ((error = namei(&fromnd)) != 0)
-		return (error);
-	error = mac_vnode_check_rename_from(td->td_ucred, fromnd.ni_dvp,
-	    fromnd.ni_vp, &fromnd.ni_cnd);
-	VOP_UNLOCK(fromnd.ni_dvp);
-	if (fromnd.ni_dvp != fromnd.ni_vp)
-		VOP_UNLOCK(fromnd.ni_vp);
-	if (error != 0) {
-		NDFREE(&fromnd, NDF_ONLY_PNBUF);
-		vrele(fromnd.ni_dvp);
-		vrele(fromnd.ni_vp);
-		if (fromnd.ni_startdir)
-			vrele(fromnd.ni_startdir);
-		return (error);
-	}
-#else
+	if (mac_vnode_check_rename_from_enabled()) {
+		error = kern_renameat_mac(td, oldfd, old, newfd, new, pathseg,
+		    &fromnd);
+		if (error != 0)
+			return (error);
+	} else {
+#endif
 	NDINIT_ATRIGHTS(&fromnd, DELETE, WANTPARENT | SAVESTART | AUDITVNODE1,
-	    pathseg, old, oldfd,
-	    &cap_renameat_source_rights, td);
+	    pathseg, old, oldfd, &cap_renameat_source_rights, td);
 	if ((error = namei(&fromnd)) != 0)
 		return (error);
+#ifdef MAC
+	}
 #endif
 	fvp = fromnd.ni_vp;
 	NDINIT_ATRIGHTS(&tond, RENAME, LOCKPARENT | LOCKLEAF | NOCACHE |
diff --git a/sys/security/mac/mac_framework.c b/sys/security/mac/mac_framework.c
index aea3789d572..41c0779fa78 100644
--- a/sys/security/mac/mac_framework.c
+++ b/sys/security/mac/mac_framework.c
@@ -139,6 +139,7 @@ FPFLAG(vnode_check_read);
 FPFLAG(vnode_check_write);
 FPFLAG(vnode_check_mmap);
 FPFLAG_RARE(vnode_check_poll);
+FPFLAG_RARE(vnode_check_rename_from);
 
 #undef FPFLAG
 #undef FPFLAG_RARE
@@ -427,6 +428,8 @@ struct mac_policy_fastpath_elem mac_policy_fastpath_array[] = {
 		.flag = &mac_vnode_check_mmap_fp_flag },
 	{ .offset = FPO(vnode_check_poll),
 		.flag = &mac_vnode_check_poll_fp_flag },
+	{ .offset = FPO(vnode_check_rename_from),
+		.flag = &mac_vnode_check_rename_from_fp_flag },
 };
 
 static void
diff --git a/sys/security/mac/mac_framework.h b/sys/security/mac/mac_framework.h
index e917eeb3c89..7ef13dcce75 100644
--- a/sys/security/mac/mac_framework.h
+++ b/sys/security/mac/mac_framework.h
@@ -482,6 +482,10 @@ mac_vnode_check_poll(struct ucred *active_cred, struct ucred *file_cred,
 #endif
 int	mac_vnode_check_readdir(struct ucred *cred, struct vnode *vp);
 int	mac_vnode_check_readlink(struct ucred *cred, struct vnode *vp);
+#define mac_vnode_check_rename_from_enabled() __predict_false(mac_vnode_check_rename_from_fp_flag)
+#ifdef MAC
+extern bool mac_vnode_check_rename_from_fp_flag;
+#endif
 int	mac_vnode_check_rename_from(struct ucred *cred, struct vnode *dvp,
 	    struct vnode *vp, struct componentname *cnp);
 int	mac_vnode_check_rename_to(struct ucred *cred, struct vnode *dvp,
-- 
2.45.0