netdev - Re: [PATCH bpf-next v10 1/2] bpf: add new helper get_file_path for mapping a file descriptor to a pathname

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Date:   Sat, 23 Nov 2019 04:51:51 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Wenbo Zhang <ethercflow@...il.com>, bpf@...r.kernel.org,
        ast@...nel.org.com, daniel@...earbox.net, yhs@...com,
        andrii.nakryiko@...il.com, netdev@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH bpf-next v10 1/2] bpf: add new helper get_file_path for
 mapping a file descriptor to a pathname

On Fri, Nov 22, 2019 at 07:18:28PM -0800, Alexei Starovoitov wrote:
> > +	f = fget_raw(fd);
> > +	if (!f)
> > +		goto error;
> > +
> > +	/* For unmountable pseudo filesystem, it seems to have no meaning
> > +	 * to get their fake paths as they don't have path, and to be no
> > +	 * way to validate this function pointer can be always safe to call
> > +	 * in the current context.
> > +	 */
> > +	if (f->f_path.dentry->d_op && f->f_path.dentry->d_op->d_dname)
> > +		return -EINVAL;

An obvious leak here, BTW.

Anyway, what could that be used for?  I mean, if you want to check
something about syscall arguments, that's an unfixably racy way to go.
Descriptor table can be a shared data structure, and two consequent
fdget() on the same number can bloody well yield completely unrelated
struct file references.

IOW, anything that does descriptor -> struct file * translation more than
once is an instant TOCTOU suspect.  In this particular case, the function
will produce a pathname of something that was once reachable via descriptor
with this number; quite possibly never before that function had been called
_and_ not once after it has returned.