kallsyms: Refactor kallsyms_show_value() to take cred

In order to perform future tests against the cred saved during open(),
switch kallsyms_show_value() to operate on a cred, and have all current
callers pass current_cred(). This makes it very obvious where callers
are checking the wrong credential in their "read" contexts. These will
be fixed in the coming patches.

Additionally switch return value to bool, since it is always used as a
direct permission check, not a 0-on-success, negative-on-error style
function return.

Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/filter.h

@@ -889,7 +889,7 @@ static inline bool bpf_dump_raw_ok(void)
 	/* Reconstruction of call-sites is dependent on kallsyms,
 	 * thus make dump the same restriction.
 	 */
-	return kallsyms_show_value() == 1;
+	return kallsyms_show_value(current_cred());
 }
 
 struct bpf_prog *bpf_patch_insn_single(struct bpf_prog *prog, u32 off,

include/linux/kallsyms.h

@@ -18,6 +18,7 @@
 #define KSYM_SYMBOL_LEN (sizeof("%s+%#lx/%#lx [%s]") + (KSYM_NAME_LEN - 1) + \
 			 2*(BITS_PER_LONG*3/10) + (MODULE_NAME_LEN - 1) + 1)
 
+struct cred;
 struct module;
 
 static inline int is_kernel_inittext(unsigned long addr)
@@ -98,7 +99,7 @@ int lookup_symbol_name(unsigned long addr, char *symname);
 int lookup_symbol_attrs(unsigned long addr, unsigned long *size, unsigned long *offset, char *modname, char *name);
 
 /* How and when do we show kallsyms values? */
-extern int kallsyms_show_value(void);
+extern bool kallsyms_show_value(const struct cred *cred);
 
 #else /* !CONFIG_KALLSYMS */
 
@@ -158,7 +159,7 @@ static inline int lookup_symbol_attrs(unsigned long addr, unsigned long *size, u
 	return -ERANGE;
 }
 
-static inline int kallsyms_show_value(void)
+static inline bool kallsyms_show_value(const struct cred *cred)
 {
 	return false;
 }

kernel/kallsyms.c

@@ -644,19 +644,20 @@ static inline int kallsyms_for_perf(void)
  * Otherwise, require CAP_SYSLOG (assuming kptr_restrict isn't set to
  * block even that).
  */
-int kallsyms_show_value(void)
+bool kallsyms_show_value(const struct cred *cred)
 {
 	switch (kptr_restrict) {
 	case 0:
 		if (kallsyms_for_perf())
-			return 1;
+			return true;
 	/* fallthrough */
 	case 1:
-		if (has_capability_noaudit(current, CAP_SYSLOG))
-			return 1;
+		if (security_capable(cred, &init_user_ns, CAP_SYSLOG,
+				     CAP_OPT_NOAUDIT) == 0)
+			return true;
 	/* fallthrough */
 	default:
-		return 0;
+		return false;
 	}
 }
 
@@ -673,7 +674,11 @@ static int kallsyms_open(struct inode *inode, struct file *file)
 		return -ENOMEM;
 	reset_iter(iter, 0);
 
-	iter->show_value = kallsyms_show_value();
+	/*
+	 * Instead of checking this on every s_show() call, cache
+	 * the result here at open time.
+	 */
+	iter->show_value = kallsyms_show_value(file->f_cred);
 	return 0;
 }
 

kernel/kprobes.c

@@ -2448,7 +2448,7 @@ static void report_probe(struct seq_file *pi, struct kprobe *p,
 	else
 		kprobe_type = "k";
 
-	if (!kallsyms_show_value())
+	if (!kallsyms_show_value(current_cred()))
 		addr = NULL;
 
 	if (sym)
@@ -2540,7 +2540,7 @@ static int kprobe_blacklist_seq_show(struct seq_file *m, void *v)
 	 * If /proc/kallsyms is not showing kernel address, we won't
 	 * show them here either.
 	 */
-	if (!kallsyms_show_value())
+	if (!kallsyms_show_value(current_cred()))
 		seq_printf(m, "0x%px-0x%px\t%ps\n", NULL, NULL,
 			   (void *)ent->start_addr);
 	else

kernel/module.c

@@ -4377,7 +4377,7 @@ static int modules_open(struct inode *inode, struct file *file)
 
 	if (!err) {
 		struct seq_file *m = file->private_data;
-		m->private = kallsyms_show_value() ? NULL : (void *)8ul;
+		m->private = kallsyms_show_value(current_cred()) ? NULL : (void *)8ul;
 	}
 
 	return err;