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

From: Ashutosh Naik
Date: Tue Dec 13 2005 - 09:28:07 EST


On 12/13/05, Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:
>
> How about something like:
>
> const struct kernel_symbol *sym;
> unsigned int i;
> const unsigned long *crc;
> struct module *owner;
>
> spin_lock_irq(&modlist_lock);
> for (i = 0; i < mod->num_syms; i++)
> if (__find_symbol(mod->syms[i].name, &owner, &crc, 1))
> goto dup;
> for (i = 0; i < num->num_gpl_syms; i++)
> if (__find_symbol(mod->gpl_syms[i].name,&owner,&crc,1))
> goto dup;
> spin_unlock_irq(&modlist_lock);
> return 0;
> dup:
> printk("%s: exports duplicate symbol (owned by %s)\n",
> mod->name, module_name(owner));
> return -ENOEXEC;
> }

Have tried that in the attached patch. However, mod->syms[i].name
would be valid only after a long relocation for loop has run through.
While this adds a wee bit extra overhead, that overhead is only in the
case where the module does actually export a Duplicate Symbol.

This its a question, whether we do the search before relocation ( A
little messier ) or after ( More straight forward)

Regards
Ashutosh
--- /usr/src/linux-2.6.15-rc5-vanilla/kernel/module.c 2005-12-07 19:32:23.000000000 +0530
+++ /usr/src/linux-2.6.15-rc5/kernel/module.c 2005-12-13 19:44:43.000000000 +0530
@@ -1204,6 +1204,42 @@ 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=0;
+ unsigned long i, ret = 0;
+ struct module *owner;
+ const unsigned long *crc;
+
+ spin_lock_irq(&modlist_lock);
+ for (i = 0; i < mod->num_syms; i++)
+ if (unlikely(__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 (unlikely(__find_symbol(mod->gpl_syms[i].name, &owner, &crc,1))) {
+ name = mod->gpl_syms[i].name;
+ ret = -ENOEXEC;
+ goto dup;
+ }
+
+dup:
+ spin_unlock_irq(&modlist_lock);
+
+ if (ret)
+ printk("%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 +1804,12 @@ 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;