modules: fix longstanding /proc/kallsyms vs module insertion race.

For CONFIG_KALLSYMS, we keep two symbol tables and two string tables.
There's one full copy, marked SHF_ALLOC and laid out at the end of the
module's init section.  There's also a cut-down version that only
contains core symbols and strings, and lives in the module's core
section.

After module init (and before we free the module memory), we switch
the mod->symtab, mod->num_symtab and mod->strtab to point to the core
versions.  We do this under the module_mutex.

However, kallsyms doesn't take the module_mutex: it uses
preempt_disable() and rcu tricks to walk through the modules, because
it's used in the oops path.  It's also used in /proc/kallsyms.
There's nothing atomic about the change of these variables, so we can
get the old (larger!) num_symtab and the new symtab pointer; in fact
this is what I saw when trying to reproduce.

By grouping these variables together, we can use a
carefully-dereferenced pointer to ensure we always get one or the
other (the free of the module init section is already done in an RCU
callback, so that's safe).  We allocate the init one at the end of the
module init section, and keep the core one inside the struct module
itself (it could also have been allocated at the end of the module
core, but that's probably overkill).

CRs-Fixed: 982779
Change-Id: I519f081967785e44a6ea33b16b1da64b14979963
Reported-by: Weilong Chen <chenweilong@huawei.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=111541
Cc: stable@kernel.org
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Git-commit: 8244062ef1e54502ef55f54cced659913f244c3e
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
[salvares@codeaurora.org: resolved context conflicts in module.c]
Signed-off-by: Sanrio Alvares <salvares@codeaurora.org>

1 files changed, 9 insertions, 8 deletions

diff --git a/include/linux/module.h b/include/linux/module.h
index 3a19c79918e0..b229a9961d02 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -302,6 +302,12 @@ struct mod_tree_node {
 	struct latch_tree_node node;
 };
 
+struct mod_kallsyms {
+	Elf_Sym *symtab;
+	unsigned int num_symtab;
+	char *strtab;
+};
+
 struct module {
 	enum module_state state;
 
@@ -411,14 +417,9 @@ struct module {
 #endif
 
 #ifdef CONFIG_KALLSYMS
-	/*
-	 * We keep the symbol and string tables for kallsyms.
-	 * The core_* fields below are temporary, loader-only (they
-	 * could really be discarded after module init).
-	 */
-	Elf_Sym *symtab, *core_symtab;
-	unsigned int num_symtab, core_num_syms;
-	char *strtab, *core_strtab;
+	/* Protected by RCU and/or module_mutex: use rcu_dereference() */
+	struct mod_kallsyms *kallsyms;
+	struct mod_kallsyms core_kallsyms;
 
 	/* Section attributes */
 	struct module_sect_attrs *sect_attrs;