Re: [RFC][PATCH] Prevent overriding of Symbols in the Kernel, avoiding Undefined behaviour

[Ashutosh Naik <ashutosh.naik@gmail.com>]

On 12/14/05, Rusty Russell <rusty@rustcorp.com.au> wrote:

>         Patch looks good!  A few nits still:

Have resolved all the nits ( hopefully :) )

> We already do this to resolve (more) symbols, so I don't see it as a
> problem.  However, I believe that lock is redundant here: we need both
> locks to write the list, but either is sufficient for reading, and we
> already hold the sem.

Ya, the lock is redundant here, as we are already inside a semaphore.

Signed-off-by: Ashutosh Naik <ashutosh.naik@gmail.com>
Signed-off-by: Anand Krishnan <anandhkrishnan@yahoo.co.in>


--- linux-2.6.15-rc5/kernel/module.c.orig       2005-12-14
09:27:53.000000000 +0530
+++ linux-2.6.15-rc5/kernel/module.c    2005-12-14 09:18:31.000000000 +0530
@@ -1204,6 +1204,39 @@ void *__symbol_get(const char *symbol)
 }
 EXPORT_SYMBOL_GPL(__symbol_get);

+/*
+ * Ensure that an exported symbol [global namespace] does not already exist
+ * in the Kernel or in some other modules exported symbol table.
+ */
+static int verify_export_symbols(struct module *mod)
+{
+       const char *name = NULL;
+       unsigned long i, ret = 0;
+       struct module *owner;
+       const unsigned long *crc;
+
+       for (i = 0; i < mod->num_syms; i++)
+               if (!__find_symbol(mod->syms[i].name, &owner, &crc, 1)) {
+                       name = mod->syms[i].name;
+                       ret = -ENOEXEC;
+                       goto dup;
+               }
+
+       for (i = 0; i < mod->num_gpl_syms; i++)
+               if (!__find_symbol(mod->gpl_syms[i].name, &owner, &crc, 1)) {
+                       name = mod->gpl_syms[i].name;
+                       ret = -ENOEXEC;
+                       goto dup;
+               }
+
+dup:
+       if (ret)
+               printk(KERN_ERR "%s: exports duplicate symbol %s
(owned by %s)\n",
+                       mod->name, name, module_name(owner));
+
+       return ret;
+}
+
 /* Change all symbols so that sh_value encodes the pointer directly. */
 static int simplify_symbols(Elf_Shdr *sechdrs,
                            unsigned int symindex,
@@ -1767,6 +1800,12 @@ static struct module *load_module(void _
                        goto cleanup;
        }

+        /* Find duplicate symbols */
+       err = verify_export_symbols(mod);
+
+       if (err < 0)
+               goto cleanup;
+
        /* Set up and sort exception table */
        mod->num_exentries = sechdrs[exindex].sh_size / sizeof(*mod->extable);
        mod->extable = extable = (void *)sechdrs[exindex].sh_addr;