Re: [PATCHv3 19/27] timens/fs/proc: Introduce /proc/pid/timens_offsets

[Jann Horn]

On Thu, Apr 25, 2019 at 6:15 PM Dmitry Safonov <dima@xxxxxxxxxx> wrote:
> API to set time namespace offsets for children processes, i.e.:
> echo "clockid off_ses off_nsec" > /proc/self/timens_offsets
[...]
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 6a803a0b75df..76d58e9b5178 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
[...]
> @@ -1521,6 +1522,103 @@ static const struct file_operations proc_pid_sched_autogroup_operations = {
>
>  #endif /* CONFIG_SCHED_AUTOGROUP */
>
> +#ifdef CONFIG_TIME_NS
> +static int timens_offsets_show(struct seq_file *m, void *v)
> +{
> +       struct inode *inode = m->private;
> +       struct task_struct *p;
> +
> +       p = get_proc_task(inode);

(FYI, this could also be "p = get_proc_task(file_inode(m->file));".
But this works, too.)

> +       if (!p)
> +               return -ESRCH;
> +       proc_timens_show_offsets(p, m);
> +
> +       put_task_struct(p);
> +
> +       return 0;
> +}
> +
> +static ssize_t
> +timens_offsets_write(struct file *file, const char __user *buf,
> +           size_t count, loff_t *ppos)
> +{
> +       struct inode *inode = file_inode(file);
> +       struct proc_timens_offset offsets[2];
> +       char *kbuf = NULL, *pos, *next_line;
> +       struct task_struct *p;
> +       int ret, noffsets;
> +
> +       /* Only allow < page size writes at the beginning of the file */
> +       if ((*ppos != 0) || (count >= PAGE_SIZE))
> +               return -EINVAL;
> +
> +       /* Slurp in the user data */
> +       kbuf = memdup_user_nul(buf, count);
> +       if (IS_ERR(kbuf))
> +               return PTR_ERR(kbuf);
> +
> +       /* Parse the user data */
> +       ret = -EINVAL;
> +       noffsets = 0;
> +       pos = kbuf;
> +       for (; pos; pos = next_line) {
> +               struct proc_timens_offset *off = &offsets[noffsets];
> +               int err;
> +
> +               /* Find the end of line and ensure I don't look past it */
> +               next_line = strchr(pos, '\n');
> +               if (next_line) {
> +                       *next_line = '\0';
> +                       next_line++;
> +                       if (*next_line == '\0')
> +                               next_line = NULL;
> +               }
> +
> +               err = sscanf(pos, "%u %lld %lu", &off->clockid,
> +                               &off->val.tv_sec, &off->val.tv_nsec);
> +               if (err != 3 || off->val.tv_nsec >= NSEC_PER_SEC)
> +                       goto out;
> +               if (noffsets++ == ARRAY_SIZE(offsets))
> +                       break;

This is equivalent to:

if (noffsets == ARRAY_SIZE(offsets))
        break;
noffsets++;

So we can reach the start of the loop with
noffsets==ARRAY_SIZE(offsets), right? Which means that an
out-of-bounds write can happen?

I think that for code like this, it makes sense to write the increment
and the check out separately so that it's easier to spot problems;
e.g. like this:

noffsets++;
if (noffsets == ARRAY_SIZE(offsets))
        break;

> +       }
> +
> +       ret = -ESRCH;
> +       p = get_proc_task(inode);
> +       if (!p)
> +               goto out;
> +       ret = proc_timens_set_offset(p, offsets, noffsets);
> +       put_task_struct(p);
> +       if (ret)
> +               goto out;
> +
> +       ret = count;
> +out:
> +       kfree(kbuf);
> +       return ret;
> +}
> +
> +static int timens_offsets_open(struct inode *inode, struct file *filp)
> +{
> +       int ret;
> +
> +       ret = single_open(filp, timens_offsets_show, NULL);
> +       if (!ret) {
> +               struct seq_file *m = filp->private_data;
> +
> +               m->private = inode;
> +       }
> +       return ret;
> +}

Why did you write it like this? Wouldn't the following be equivalent?

static int timens_offsets_open(struct inode *inode, struct file *file)
{
        return single_open(file, timens_offsets_show, inode);
}

(But also, you can reach the inode of a seq_file as file_inode(m->file).)

[...]
> diff --git a/kernel/time_namespace.c b/kernel/time_namespace.c
> index e806accc4eaf..9ad4b63c4ed2 100644
> --- a/kernel/time_namespace.c
> +++ b/kernel/time_namespace.c
[...]
> +
> +int proc_timens_set_offset(struct task_struct *p,
> +                          struct proc_timens_offset *offsets, int noffsets)
> +{
> +       struct ns_common *ns;
> +       struct time_namespace *time_ns;
> +       struct timens_offsets *ns_offsets;
> +       struct timespec64 *offset;
> +       struct timespec64 tp;
> +       int i, err;
> +
> +       ns = timens_for_children_get(p);
> +       if (!ns)
> +               return -ESRCH;
> +       time_ns = to_time_ns(ns);
> +
> +       if (!time_ns->offsets || time_ns->initialized ||
> +           !ns_capable(time_ns->user_ns, CAP_SYS_TIME)) {

Capability checks in VFS read/write handlers are bad. Please pass
through the file pointer to this function and replace the call with
"file_ns_capable(file, time_ns->user_ns, CAP_SYS_TIME)".

> +               put_time_ns(time_ns);
> +               return -EPERM;
> +       }
[...]
> +}