2020-07-02 23:27:27

by Kees Cook

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

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: [email protected]
Reported-by: Dominik Czarnota <[email protected]>
Fixes: be71eda5383f ("module: Fix display of wrong module .text address")
Signed-off-by: Kees Cook <[email protected]>
---
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

2020-07-08 16:13:37

by Jessica Yu

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

+++ Kees Cook [02/07/20 16:26 -0700]:
>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: [email protected]
>Reported-by: Dominik Czarnota <[email protected]>
>Fixes: be71eda5383f ("module: Fix display of wrong module .text address")
>Signed-off-by: Kees Cook <[email protected]>

Tested-by: Jessica Yu <[email protected]>
Acked-by: Jessica Yu <[email protected]>