linux-hardening - Re: [PATCH] libfs: fix accidental overflow in offset calculation

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Date: Fri, 10 May 2024 03:26:08 +0000
From: Justin Stitt <justinstitt@...gle.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Nathan Chancellor <nathan@...nel.org>, Bill Wendling <morbo@...gle.com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] libfs: fix accidental overflow in offset calculation

Hi,

On Fri, May 10, 2024 at 02:04:51AM +0100, Al Viro wrote:
> On Fri, May 10, 2024 at 01:49:06AM +0100, Al Viro wrote:
> > On Fri, May 10, 2024 at 12:35:51AM +0000, Justin Stitt wrote:
> > > @@ -147,7 +147,9 @@ loff_t dcache_dir_lseek(struct file *file, loff_t offset, int whence)
> > >  	struct dentry *dentry = file->f_path.dentry;
> > >  	switch (whence) {
> > >  		case 1:
> > > -			offset += file->f_pos;
> > > +			/* cannot represent offset with loff_t */
> > > +			if (check_add_overflow(offset, file->f_pos, &offset))
> > > +				return -EOVERFLOW;
> > 
> > Instead of -EINVAL it correctly returns in such cases?  Why?
> 
> We have file->f_pos in range 0..LLONG_MAX.  We are adding a value in
> range LLONG_MIN..LLONG_MAX.  The sum (in $\Bbb Z$) cannot be below
> LLONG_MIN or above 2 * LLONG_MAX, so if it can't be represented by loff_t,
> it must have been in range LLONG_MAX + 1 .. 2 * LLONG_MAX.  Result of
> wraparound would be equal to that sum - 2 * LLONG_MAX - 2, which is going
> to be in no greater than -2.  We will run
> 			fallthrough;
> 		case 0:
> 			if (offset >= 0)
> 				break;
> 			fallthrough;
> 		default:
> 			return -EINVAL;
> and fail with -EINVAL.

This feels like a case of accidental correctness. You demonstrated that
even with overflow we end up going down a control path that returns an
error code so all is good. However, I think finding the solution
shouldn't require as much mental gymnastics. We clearly don't want our
file offsets to wraparound and a plain-and-simple check for that lets
readers of the code understand this.

> 
> Could you explain why would -EOVERFLOW be preferable and why should we
> engage in that bit of cargo cult?

I noticed some patterns in fs/ regarding -EOVERFLOW and thought this was
a good application of this particular error code.

Some examples:

read_write.c::do_sendfile()
  ...
	if (unlikely(pos + count > max)) {
		retval = -EOVERFLOW;
		if (pos >= max)
			goto fput_out;
		count = max - pos;
	}

read_write.c::generic_copy_file_checks()
  ...
	/* Ensure offsets don't wrap. */
	if (pos_in + count < pos_in || pos_out + count < pos_out)
		return -EOVERFLOW;

... amongst others.

So to answer your question, I don't have strong feelings about what the
error code should be; I was just attempting to match patterns I had seen
within this section of the codebase when handling overflow/wraparound.

If -EOVERFLOW is technically incorrect or if it is just bad style, I'm
happy to send a new version swapping it over to -EINVAL

Thanks
Justin