imgact_elf.c indicate that interpreter is an interpreter

Use a value that do_execve will ignore since there's nothing there to do.
With this we can set the 'indirect' flag on rtld, to prevent direct execution
being used to bypass the 'indirect' flag on python3:

root@vuefi:~ # /libexec/ld-elf32.so.1 /usr/bin/python3 -c 'print("pwned")'
pwned
root@vuefi:~ # veriexec -z enforce
root@vuefi:~ # /libexec/ld-elf32.so.1 /usr/bin/python3 -c 'print("pwned")'
/libexec/ld-elf32.so.1: Operation not permitted.
root@vuefi:~ #

load_file seems missnamed since it is only used by load_interp
so rename it to load_interp_file.

Reviewed by:	imp
Sponsored by:	Hewlett Packard Enterprise Development LP
Differential Revision:	https://reviews.freebsd.org/D57649

sys/kern/imgact_elf.c

@@ -94,7 +94,7 @@
 static int __elfN(check_header)(const Elf_Ehdr *hdr);
 static const Elf_Brandinfo *__elfN(get_brandinfo)(struct image_params *imgp,
     const Elf_Phdr *phdr, const char *interp, int32_t *osrel, uint32_t *fctl0);
-static int __elfN(load_file)(struct thread *td, const char *file, u_long *addr,
+static int __elfN(load_interp_file)(struct thread *td, const char *file, u_long *addr,
     u_long *entry);
 static int __elfN(load_section)(const struct image_params *imgp,
     vm_ooffset_t offset, caddr_t vmaddr, size_t memsz, size_t filsz,
@@ -785,7 +785,7 @@ __elfN(load_sections)(const struct image_params *imgp, const Elf_Ehdr *hdr,
  * the entry point for the loaded file.
  */
 static int
-__elfN(load_file)(struct thread *td, const char *file, u_long *addr,
+__elfN(load_interp_file)(struct thread *td, const char *file, u_long *addr,
     u_long *entry)
 {
 	struct {
@@ -823,7 +823,8 @@ __elfN(load_file)(struct thread *td, const char *file, u_long *addr,
 	imgp->td = td;
 	imgp->proc = td->td_proc;
 	imgp->attr = attr;
-
+	imgp->interpreted = IMGACT_INTERP_IGNORE; /* ignored by do_execve */
+	
 	NDINIT(nd, LOOKUP, ISOPEN | FOLLOW | LOCKSHARED | LOCKLEAF,
 	    UIO_SYSSPACE, file);
 	if ((error = namei(nd)) != 0) {
@@ -1092,13 +1093,13 @@ __elfN(load_interp)(struct image_params *imgp, const Elf_Brandinfo *brand_info,
 	if (brand_info->interp_newpath != NULL &&
 	    (brand_info->interp_path == NULL ||
 	    strcmp(interp, brand_info->interp_path) == 0)) {
-		error = __elfN(load_file)(imgp->td,
+		error = __elfN(load_interp_file)(imgp->td,
 		    brand_info->interp_newpath, addr, entry);
 		if (error == 0)
 			return (0);
 	}
 
-	error = __elfN(load_file)(imgp->td, interp, addr, entry);
+	error = __elfN(load_interp_file)(imgp->td, interp, addr, entry);
 	if (error == 0)
 		return (0);
 

sys/kern/kern_exec.c

@@ -678,7 +678,8 @@ interpret:
 	 * Special interpreter operation, cleanup and loop up to try to
 	 * activate the interpreter.
 	 */
-	if (imgp->interpreted) {
+	if (imgp->interpreted != 0 &&
+	    imgp->interpreted != IMGACT_INTERP_IGNORE) {
 		exec_unmap_first_page(imgp);
 		/*
 		 * The text reference needs to be removed for scripts.

sys/sys/imgact.h

@@ -86,6 +86,7 @@ struct image_params {
 	struct ucred *newcred;		/* new credentials if changing */
 #define IMGACT_SHELL	0x1
 #define IMGACT_BINMISC	0x2
+#define IMGACT_INTERP_IGNORE 0x3
 	unsigned char interpreted;	/* mask of interpreters that have run */
 	bool credential_setid;		/* true if becoming setid */
 	bool vmspace_destroyed;		/* we've blown away original vm space */