linux-kernel - Re: [PATCH v5 11/12] S.A.R.A.: /proc/*/mem write limitation

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Date:   Sat, 6 Jul 2019 20:20:35 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Salvatore Mesoraca <s.mesoraca16@...il.com>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux-MM <linux-mm@...ck.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Brad Spengler <spender@...ecurity.net>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Christoph Hellwig <hch@...radead.org>,
        James Morris <james.l.morris@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        PaX Team <pageexec@...email.hu>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v5 11/12] S.A.R.A.: /proc/*/mem write limitation

On Sat, Jul 6, 2019 at 12:55 PM Salvatore Mesoraca
<s.mesoraca16@...il.com> wrote:
> Prevent a task from opening, in "write" mode, any /proc/*/mem
> file that operates on the task's mm.
> A process could use it to overwrite read-only memory, bypassing
> S.A.R.A. restrictions.
[...]
> +static void sara_task_to_inode(struct task_struct *t, struct inode *i)
> +{
> +       get_sara_inode_task(i) = t;

This looks bogus. Nothing is actually holding a reference to `t` here, right?

> +}
> +
>  static struct security_hook_list data_hooks[] __lsm_ro_after_init = {
>         LSM_HOOK_INIT(cred_prepare, sara_cred_prepare),
>         LSM_HOOK_INIT(cred_transfer, sara_cred_transfer),
>         LSM_HOOK_INIT(shm_alloc_security, sara_shm_alloc_security),
> +       LSM_HOOK_INIT(task_to_inode, sara_task_to_inode),
>  };
[...]
> +static int sara_file_open(struct file *file)
> +{
> +       struct task_struct *t;
> +       struct mm_struct *mm;
> +       u16 sara_wxp_flags = get_current_sara_wxp_flags();
> +
> +       /*
> +        * Prevent write access to /proc/.../mem
> +        * if it operates on the mm_struct of the
> +        * current process: it could be used to
> +        * bypass W^X.
> +        */
> +
> +       if (!sara_enabled ||
> +           !wxprot_enabled ||
> +           !(sara_wxp_flags & SARA_WXP_WXORX) ||
> +           !(file->f_mode & FMODE_WRITE))
> +               return 0;
> +
> +       t = get_sara_inode_task(file_inode(file));
> +       if (unlikely(t != NULL &&
> +                    strcmp(file->f_path.dentry->d_name.name,
> +                           "mem") == 0)) {

This should probably at least have a READ_ONCE() somewhere in case the
file concurrently gets renamed?

> +               get_task_struct(t);
> +               mm = get_task_mm(t);
> +               put_task_struct(t);

Getting and dropping a reference to the task_struct here is completely
useless. Either you have a reference, in which case you don't need to
take another one, or you don't have a reference, in which case you
also can't take one.

> +               if (unlikely(mm == current->mm))
> +                       sara_warn_or_goto(error,
> +                                         "write access to /proc/*/mem");

Why is the current process so special that it must be protected more
than other processes? Is the idea here to rely on other protections to
protect all other tasks? This should probably come with a comment that
explains this choice.

> +               mmput(mm);
> +       }
> +       return 0;
> +error:
> +       mmput(mm);
> +       return -EACCES;
> +}