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>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 1dd756731c95..b512e48f6d8e 100644
--- a/include/linux/binfmts.h
+++ b/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;