[kernel-hardening] Re: [PATCH] proc: do not allow negative offsets on /proc/<pid>/environ

[Oleg Nesterov <oleg@redhat.com>]

On 07/22, Djalal Harouni wrote:
>
> __mem_open() which is called by both /proc/<pid>/environ and
> /proc/<pid>/mem ->open() handlers will allow the use of negative offsets.
> /proc/<pid>/mem has negative offsets but not /proc/<pid>/environ.

Probablt the patch makes sense, but I can't understand the changelog...

> Allowing negative offsets on /proc/<pid>/environ can turn it to act like
> /proc/<pid>/mem. A negative offset will pass the
> fs/read_write.c:lseek_execute() and the environ_read() checks and will
> point to another VMA.

which VMA?

environ_read() can only read the memory from [env_start, env_end], and
it should check *ppos anyway to ensure it doesn't read something else.

>  static int mem_open(struct inode *inode, struct file *file)
>  {
> -	return __mem_open(inode, file, PTRACE_MODE_ATTACH);
> +	int ret = __mem_open(inode, file, PTRACE_MODE_ATTACH);
> +	if (!ret)
> +		/* OK to pass negative loff_t, we can catch out-of-range */
> +		file->f_mode |= FMODE_UNSIGNED_OFFSET;
> +
> +	return ret;

I guess you can set FMODE_UNSIGNED_OFFSET unconditionally, it doesn't
matter if __mem_open() fails. But I won't insist.

Oleg.