module: fix crash in get_ksymbol() when oopsing in module init

Andrew had the sole pleasure of tickling this bug in linux-next; when we set
up "info->strtab" it's pointing into the temporary copy of the module.  For
most uses that is fine, but kallsyms keeps a pointer around during module
load (inside mod->strtab).

If we oops for some reason inside a module's init function, kallsyms will use
the mod->strtab pointer into the now-freed temporary module copy.

(Later oopses work fine: after init we overwrite mod->strtab to point to a
 compacted core-only strtab).
Reported-by: NAndrew "Grumpy" Morton <akpm@linux-foundation.org>
Signed-off-by: NRusty "Buggy" Russell <rusty@rustcorp.com.au>
Tested-by: NAndrew "Happy" Morton <akpm@linux-foundation.org>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

kernel/module.c

@@ -2045,7 +2045,8 @@ static void add_kallsyms(struct module *mod, struct load_info *info,
 
 	mod->symtab = (void *)symsec->sh_addr;
 	mod->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
-	mod->strtab = info->strtab;
+	/* Make sure we get permanent strtab: don't use info->strtab. */
+	mod->strtab = (void *)info->sechdrs[info->index.str].sh_addr;
 
 	/* Set types up while we still have access to sections. */
 	for (i = 0; i < mod->num_symtab; i++)