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

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

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On 12/13/05, Rusty Russell <rusty@rustcorp.com.au> 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

[-- Attachment #2: mod13Decpatch.txt --]
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--- /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;