Improve the MP safeness associated with the creation of symbolic

links and the execution of ELF binaries. Two problems were found:

1) The link path wasn't tagged as being MP safe and thus was not properly
   protected.
2) The ELF interpreter vnode wasnt being locked in namei(9) and thus was
   insufficiently protected.

This commit makes the following changes:

-Sets the MPSAFE flag in NDINIT for symbolic link paths
-Sets the MPSAFE flag in NDINIT and introduce a vfslocked variable which
 will be used to instruct VFS_UNLOCK_GIANT to unlock Giant if it has been
 picked up.
-Drop in an assertion into vfs_lookup which ensures that if the MPSAFE
 flag is NOT set, that we have picked up giant. If not panic (if WITNESS
 compiled into the kernel). This should help us find conditions where vnode
 operations are in-sufficiently protected.

This is a RELENG_6 candidate.

Discussed with:	jeff
MFC after:	4 days

1 files changed, 7 insertions, 3 deletions

diff --git a/sys/kern/imgact_elf.c b/sys/kern/imgact_elf.c
index 6a88500..b469898 100644
--- a/sys/kern/imgact_elf.c
+++ b/sys/kern/imgact_elf.c
@@ -41,6 +41,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/kernel.h>
 #include <sys/lock.h>
 #include <sys/malloc.h>
+#include <sys/mount.h>
 #include <sys/mutex.h>
 #include <sys/mman.h>
 #include <sys/namei.h>
@@ -515,7 +516,7 @@ __elfN(load_file)(struct proc *p, const char *file, u_long *addr,
 	vm_prot_t prot;
 	u_long rbase;
 	u_long base_addr = 0;
-	int error, i, numsegs;
+	int vfslocked, error, i, numsegs;
 
 	if (curthread->td_proc != p)
 		panic("elf_load_file - thread");	/* XXXKSE DIAGNOSTIC */
@@ -536,12 +537,14 @@ __elfN(load_file)(struct proc *p, const char *file, u_long *addr,
 	imgp->execlabel = NULL;
 
 	/* XXXKSE */
-	NDINIT(nd, LOOKUP, LOCKLEAF|FOLLOW, UIO_SYSSPACE, file, curthread);
-
+	NDINIT(nd, LOOKUP, MPSAFE|LOCKLEAF|FOLLOW, UIO_SYSSPACE, file,
+	    curthread);
+	vfslocked = 0;
 	if ((error = namei(nd)) != 0) {
 		nd->ni_vp = NULL;
 		goto fail;
 	}
+	vfslocked = NDHASGIANT(nd);
 	NDFREE(nd, NDF_ONLY_PNBUF);
 	imgp->vp = nd->ni_vp;
 
@@ -629,6 +632,7 @@ fail:
 	if (nd->ni_vp)
 		vrele(nd->ni_vp);
 
+	VFS_UNLOCK_GIANT(vfslocked);
 	free(tempdata, M_TEMP);
 
 	return (error);