binfmt_misc.c: avoid potential kernel stack overflow

This can be triggered with root help only, but...

Register the ":text:E::txt::/root/cat.txt:' rule in binfmt_misc (by root) and
try launching the cat.txt file (by anyone) :) The result is - the endless
recursion in the load_misc_binary -> open_exec -> load_misc_binary chain and
stack overflow.

There's a similar problem with binfmt_script, and there's a sh_bang memner on
linux_binprm structure to handle this, but simply raising this in binfmt_misc
may break some setups when the interpreter of some misc binaries is a script.

So the proposal is to turn sh_bang into a bit, add a new one (the misc_bang)
and raise it in load_misc_binary. After this, even if we set up the misc ->
script -> misc loop for binfmts one of them will step on its own bang and
exit.

Signed-off-by: Pavel Emelyanov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/binfmt_em86.c

···
 			return -ENOEXEC;
 	}
 
-	bprm->sh_bang++;	/* Well, the bang-shell is implicit... */
+	bprm->sh_bang = 1;	/* Well, the bang-shell is implicit... */
 	allow_write_access(bprm->file);
 	fput(bprm->file);
 	bprm->file = NULL;

fs/binfmt_misc.c

···
 	if (!enabled)
 		goto _ret;
 
+	retval = -ENOEXEC;
+	if (bprm->misc_bang)
+		goto _ret;
+
+	bprm->misc_bang = 1;
+
 	/* to keep locking time low, we copy the interpreter string */
 	read_lock(&entries_lock);
 	fmt = check_file(bprm);

fs/binfmt_script.c

···
 	 * Sorta complicated, but hopefully it will work.  -TYT
 	 */
 
-	bprm->sh_bang++;
+	bprm->sh_bang = 1;
 	allow_write_access(bprm->file);
 	fput(bprm->file);
 	bprm->file = NULL;

include/linux/binfmts.h

···
 #endif
 	struct mm_struct *mm;
 	unsigned long p; /* current top of mem */
-	int sh_bang;
+	unsigned int sh_bang:1,
+		     misc_bang:1;
 	struct file * file;
 	int e_uid, e_gid;
 	kernel_cap_t cap_inheritable, cap_permitted;