linux-kernel - Re: [PATCH] pstore: honor dmesg_restrict sysctl on dmesg dumps

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Date:	Mon, 20 Oct 2014 11:15:57 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Sebastian Schmidt <yath@...h.de>
Cc:	Anton Vorontsov <anton@...msg.org>,
	Colin Cross <ccross@...roid.com>,
	Tony Luck <tony.luck@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pstore: honor dmesg_restrict sysctl on dmesg dumps

On Sun, Oct 19, 2014 at 11:05 AM, Sebastian Schmidt <yath@...h.de> wrote:
> When the kernel.dmesg_restrict restriction is in place, only users with
> CAP_SYSLOG should be able to access crash dumps (like: attacker is
> trying to exploit a bug, watchdog reboots, attacker can happily read
> crash dumps and logs).
>
> This puts the restriction on console-* types as well as sensitive
> information could have been leaked there.
>
> Other log types are unaffected.
>
> Signed-off-by: Sebastian Schmidt <yath@...h.de>

This looks sensible to me; thanks!

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  fs/pstore/inode.c      | 22 ++++++++++++++++++++++
>  include/linux/syslog.h |  1 +
>  kernel/printk/printk.c |  2 +-
>  3 files changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/fs/pstore/inode.c b/fs/pstore/inode.c
> index fafb7a0..5041660 100644
> --- a/fs/pstore/inode.c
> +++ b/fs/pstore/inode.c
> @@ -36,6 +36,7 @@
>  #include <linux/slab.h>
>  #include <linux/spinlock.h>
>  #include <linux/uaccess.h>
> +#include <linux/syslog.h>
>
>  #include "internal.h"
>
> @@ -120,6 +121,18 @@ static const struct seq_operations pstore_ftrace_seq_ops = {
>         .show   = pstore_ftrace_seq_show,
>  };
>
> +static int pstore_check_syslog_permissions(struct pstore_private *ps)
> +{
> +       switch (ps->type) {
> +       case PSTORE_TYPE_DMESG:
> +       case PSTORE_TYPE_CONSOLE:
> +               return check_syslog_permissions(SYSLOG_ACTION_READ_ALL,
> +                       SYSLOG_FROM_READER);
> +       default:
> +               return 0;
> +       }
> +}
> +
>  static ssize_t pstore_file_read(struct file *file, char __user *userbuf,
>                                                 size_t count, loff_t *ppos)
>  {
> @@ -138,6 +151,10 @@ static int pstore_file_open(struct inode *inode, struct file *file)
>         int err;
>         const struct seq_operations *sops = NULL;
>
> +       err = pstore_check_syslog_permissions(ps);
> +       if (err)
> +               return err;
> +
>         if (ps->type == PSTORE_TYPE_FTRACE)
>                 sops = &pstore_ftrace_seq_ops;
>
> @@ -174,6 +191,11 @@ static const struct file_operations pstore_file_operations = {
>  static int pstore_unlink(struct inode *dir, struct dentry *dentry)
>  {
>         struct pstore_private *p = dentry->d_inode->i_private;
> +       int err;
> +
> +       err = pstore_check_syslog_permissions(p);
> +       if (err)
> +               return err;
>
>         if (p->psi->erase)
>                 p->psi->erase(p->type, p->id, p->count,
> diff --git a/include/linux/syslog.h b/include/linux/syslog.h
> index 98a3153..9def529 100644
> --- a/include/linux/syslog.h
> +++ b/include/linux/syslog.h
> @@ -48,5 +48,6 @@
>  #define SYSLOG_FROM_PROC             1
>
>  int do_syslog(int type, char __user *buf, int count, bool from_file);
> +int check_syslog_permissions(int type, bool from_file);
>
>  #endif /* _LINUX_SYSLOG_H */
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index ced2b84..c8755e7 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -480,7 +480,7 @@ static int syslog_action_restricted(int type)
>                type != SYSLOG_ACTION_SIZE_BUFFER;
>  }
>
> -static int check_syslog_permissions(int type, bool from_file)
> +int check_syslog_permissions(int type, bool from_file)
>  {
>         /*
>          * If this is from /proc/kmsg and we've already opened it, then we've
> --
> 2.1.1
>



-- 
Kees Cook
Chrome OS Security
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