[PATCH AUTOSEL 3.18 28/29] module: exclude SHN_UNDEF symbols from kallsyms api

[Sasha Levin <Alexander.Levin@microsoft.com>]

From: Jessica Yu <jeyu@kernel.org>

[ Upstream commit 9f2d1e68cf4d641def734adaccfc3823d3575e6c ]

Livepatch modules are special in that we preserve their entire symbol
tables in order to be able to apply relocations after module load. The
unwanted side effect of this is that undefined (SHN_UNDEF) symbols of
livepatch modules are accessible via the kallsyms api and this can
confuse symbol resolution in livepatch (klp_find_object_symbol()) and
cause subtle bugs in livepatch.

Have the module kallsyms api skip over SHN_UNDEF symbols. These symbols
are usually not available for normal modules anyway as we cut down their
symbol tables to just the core (non-undefined) symbols, so this should
really just affect livepatch modules. Note that this patch doesn't
affect the display of undefined symbols in /proc/kallsyms.

Reported-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jessica Yu <jeyu@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 kernel/module.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/module.c b/kernel/module.c
index 737d53099360..bc417377930b 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3592,7 +3592,7 @@ static unsigned long mod_find_symname(struct module *mod, const char *name)
 
 	for (i = 0; i < kallsyms->num_symtab; i++)
 		if (strcmp(name, symname(kallsyms, i)) == 0 &&
-		    kallsyms->symtab[i].st_info != 'U')
+		    kallsyms->symtab[i].st_shndx != SHN_UNDEF)
 			return kallsyms->symtab[i].st_value;
 	return 0;
 }
@@ -3636,6 +3636,10 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *,
 		if (mod->state == MODULE_STATE_UNFORMED)
 			continue;
 		for (i = 0; i < kallsyms->num_symtab; i++) {
+
+			if (kallsyms->symtab[i].st_shndx == SHN_UNDEF)
+				continue;
+
 			ret = fn(data, symname(kallsyms, i),
 				 mod, kallsyms->symtab[i].st_value);
 			if (ret != 0)
-- 
2.17.1