[PATCH] exec: fix set_binfmt() vs sys_delete_module() race

[Oleg Nesterov <oleg@redhat.com>]

sys_delete_module() can set MODULE_STATE_GOING after search_binary_handler()
does try_module_get(). In this case set_binfmt()->try_module_get() fails but
since none of the callers check the returned error, the task will run with
the wrong old ->binfmt.

The proper fix should change all ->load_binary() methods, but we can rely
on fact that the caller must hold a reference to binfmt->module and use
__module_get() which never fails.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

--- WAIT/include/linux/binfmts.h~SET_BINFMT	2009-05-03 17:15:27.000000000 +0200
+++ WAIT/include/linux/binfmts.h	2009-07-24 19:02:19.000000000 +0200
@@ -119,7 +119,7 @@ extern int bprm_mm_init(struct linux_bin
 extern int copy_strings_kernel(int argc,char ** argv,struct linux_binprm *bprm);
 extern void install_exec_creds(struct linux_binprm *bprm);
 extern void do_coredump(long signr, int exit_code, struct pt_regs *regs);
-extern int set_binfmt(struct linux_binfmt *new);
+extern void set_binfmt(struct linux_binfmt *new);
 extern void free_bprm(struct linux_binprm *);
 
 #endif /* __KERNEL__ */
--- WAIT/fs/exec.c~SET_BINFMT	2009-07-02 19:27:36.000000000 +0200
+++ WAIT/fs/exec.c	2009-07-24 19:00:40.000000000 +0200
@@ -1377,18 +1377,14 @@ out_ret:
 	return retval;
 }
 
-int set_binfmt(struct linux_binfmt *new)
+void set_binfmt(struct linux_binfmt *new)
 {
-	struct linux_binfmt *old = current->binfmt;
+	if (current->binfmt)
+		module_put(current->binfmt->module);
 
-	if (new) {
-		if (!try_module_get(new->module))
-			return -1;
-	}
 	current->binfmt = new;
-	if (old)
-		module_put(old->module);
-	return 0;
+	if (new)
+		__module_get(new->module);
 }
 
 EXPORT_SYMBOL(set_binfmt);