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 <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/binfmts.h

@@ -34,7 +34,8 @@ struct linux_binprm{
 #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;