[PATCH 3/5] module: Do not expose section addresses to non-CAP_SYSLOG

[Kees Cook]

The printing of section addresses in /sys/module/*/sections/* was not
using the correct credentials to evaluate visibility.

Before:

 # cat /sys/module/*/sections/.*text
 0xffffffffc0458000
 ...
 # capsh --drop=CAP_SYSLOG -- -c "cat /sys/module/*/sections/.*text"
 0xffffffffc0458000
 ...

After:

 # cat /sys/module/*/sections/*.text
 0xffffffffc0458000
 ...
 # capsh --drop=CAP_SYSLOG -- -c "cat /sys/module/*/sections/.*text"
 0x0000000000000000
 ...

Additionally replaces the existing (safe) /proc/modules check with
file->f_cred for consistency.

Cc: sta...@vger.kernel.org
Reported-by: Dominik Czarnota <dominik.czarn...@trailofbits.com>
Fixes: be71eda5383f ("module: Fix display of wrong module .text address")
Signed-off-by: Kees Cook <keesc...@chromium.org>
---
 kernel/module.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 9e2954519259..e6c7571092cb 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1530,8 +1530,8 @@ static ssize_t module_sect_read(struct file *file, struct 
kobject *kobj,
        if (pos != 0)
                return -EINVAL;
 
-       return sprintf(buf, "0x%px\n", kptr_restrict < 2 ?
-                      (void *)sattr->address : NULL);
+       return sprintf(buf, "0x%px\n",
+                      kallsyms_show_value(file->f_cred) ? (void 
*)sattr->address : NULL);
 }
 
 static void free_sect_attrs(struct module_sect_attrs *sect_attrs)
@@ -4380,7 +4380,7 @@ static int modules_open(struct inode *inode, struct file 
*file)
 
        if (!err) {
                struct seq_file *m = file->private_data;
-               m->private = kallsyms_show_value(current_cred()) ? NULL : (void 
*)8ul;
+               m->private = kallsyms_show_value(file->f_cred) ? NULL : (void 
*)8ul;
        }
 
        return err;
-- 
2.25.1