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>

fs/binfmt_em86.c

diff --git a/fs/binfmt_em86.c b/fs/binfmt_em86.c
index f95ae97..f9c88d0 100644
--- a/fs/binfmt_em86.c
+++ b/fs/binfmt_em86.c
@@ -43,7 +43,7 @@
 			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

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index dbf0ac0..7191306 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -115,6 +115,12 @@
 	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

diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
index ab33939..9e3963f 100644
--- a/fs/binfmt_script.c
+++ b/fs/binfmt_script.c
@@ -29,7 +29,7 @@
 	 * 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

diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 1dd7567..b512e48 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -34,7 +34,8 @@
 #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;