module: make module_address_lookup safe

module_address_lookup releases preemption then returns a pointer into
the module space.  The only user (kallsyms) copies the result, so just
do that under the preempt disable.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

3 files changed, 30 insertions, 25 deletions

diff --git a/include/linux/module.h b/include/linux/module.h
index c97bdb7eb957..aedc06be1de8 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -446,11 +446,14 @@ static inline void __module_get(struct module *module)
         __mod ? __mod->name : "kernel";         \
 })
 
-/* For kallsyms to ask for address resolution.  NULL means not found. */
-const char *module_address_lookup(unsigned long addr,
-                                  unsigned long *symbolsize,
-                                  unsigned long *offset,
-                                  char **modname);
+/* For kallsyms to ask for address resolution.  namebuf should be at
+ * least KSYM_NAME_LEN long: a pointer to namebuf is returned if
+ * found, otherwise NULL. */
+char *module_address_lookup(unsigned long addr,
+                            unsigned long *symbolsize,
+                            unsigned long *offset,
+                            char **modname,
+                            char *namebuf);
 int lookup_module_symbol_name(unsigned long addr, char *symname);
 int lookup_module_symbol_attrs(unsigned long addr, unsigned long *size, unsigned long *offset, char *modname, char *name);
 
@@ -516,10 +519,11 @@ static inline void module_put(struct module *module)
 #define module_name(mod) "kernel"
 
 /* For kallsyms to ask for address resolution.  NULL means not found. */
-static inline const char *module_address_lookup(unsigned long addr,
-                                                unsigned long *symbolsize,
-                                                unsigned long *offset,
-                                                char **modname)
+static inline char *module_address_lookup(unsigned long addr,
+                                          unsigned long *symbolsize,
+                                          unsigned long *offset,
+                                          char **modname,
+                                          char *namebuf)
 {
         return NULL;
 }
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 2fc25810509e..7dadc71ce516 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -233,10 +233,11 @@ static unsigned long get_symbol_pos(unsigned long addr,
 int kallsyms_lookup_size_offset(unsigned long addr, unsigned long *symbolsize,
                                 unsigned long *offset)
 {
+        char namebuf[KSYM_NAME_LEN];
         if (is_ksym_addr(addr))
                 return !!get_symbol_pos(addr, symbolsize, offset);
 
-        return !!module_address_lookup(addr, symbolsize, offset, NULL);
+        return !!module_address_lookup(addr, symbolsize, offset, NULL, namebuf);
 }
 
 /*
@@ -251,8 +252,6 @@ const char *kallsyms_lookup(unsigned long addr,
                             unsigned long *offset,
                             char **modname, char *namebuf)
 {
-        const char *msym;
-
         namebuf[KSYM_NAME_LEN - 1] = 0;
         namebuf[0] = 0;
 
@@ -268,10 +267,8 @@ const char *kallsyms_lookup(unsigned long addr,
         }
 
         /* see if it's in a module */
-        msym = module_address_lookup(addr, symbolsize, offset, modname);
-        if (msym)
-                return strncpy(namebuf, msym, KSYM_NAME_LEN - 1);
-
+        return module_address_lookup(addr, symbolsize, offset, modname,
+                                     namebuf);
         return NULL;
 }
 
diff --git a/kernel/module.c b/kernel/module.c
index 12067ff34d01..e814cd7da634 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2230,14 +2230,13 @@ static const char *get_ksymbol(struct module *mod,
         return mod->strtab + mod->symtab[best].st_name;
 }
 
-/* For kallsyms to ask for address resolution.  NULL means not found.
-   We don't lock, as this is used for oops resolution and races are a
-   lesser concern. */
-/* FIXME: Risky: returns a pointer into a module w/o lock */
-const char *module_address_lookup(unsigned long addr,
-                                  unsigned long *size,
-                                  unsigned long *offset,
-                                  char **modname)
+/* For kallsyms to ask for address resolution.  NULL means not found.  Careful
+ * not to lock to avoid deadlock on oopses, simply disable preemption. */
+char *module_address_lookup(unsigned long addr,
+                            unsigned long *size,
+                            unsigned long *offset,
+                            char **modname,
+                            char *namebuf)
 {
         struct module *mod;
         const char *ret = NULL;
@@ -2252,8 +2251,13 @@ const char *module_address_lookup(unsigned long addr,
                         break;
                 }
         }
+        /* Make a copy in here where it's safe */
+        if (ret) {
+                strncpy(namebuf, ret, KSYM_NAME_LEN - 1);
+                ret = namebuf;
+        }
         preempt_enable();
-        return ret;
+        return (char *)ret;
 }
 
 int lookup_module_symbol_name(unsigned long addr, char *symname)