linux-kernel - Re: [linux-next PATCH] rapidio: Fix error handling path

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Date:   Wed, 16 Sep 2020 13:02:32 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     mporter@...nel.crashing.org, alex.bou9@...il.com,
        akpm@...ux-foundation.org, gustavoars@...nel.org,
        jhubbard@...dia.com, madhuparnabhowmik10@...il.com,
        linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [linux-next PATCH] rapidio: Fix error handling path

On Wed, Sep 16, 2020 at 09:12:17AM +0530, Souptick Joarder wrote:
> There is an error when pin_user_pages_fast() returns -ERRNO and
> inside error handling path driver end up calling unpin_user_pages()
> with -ERRNO which is not correct.
> 
> This patch will fix the problem.

There are a few ways we could prevent bug in the future.

1) This could have been caught with existing static analysis tools
   which warn about when a value is set but not used.

2) I've created a Smatch check which warngs about:

	drivers/rapidio/devices/rio_mport_cdev.c:955 rio_dma_transfer() warn: unpinning negative pages 'nr_pages'

   I'll test it out tonight and see how well it works.  I don't
   immediately see any other bugs allthough Smatch doesn't like the code
   in siw_umem_release().  It uses "min_t(int" which suggests that
   negative pages are okay.

	   int to_free = min_t(int, PAGES_PER_CHUNK, num_pages);

3) We could add a check in unpin_user_pages().

	if (WARN_ON(IS_ERR_VALUE(npages)))
		return;

It's not possible to pin more than "ULONG_MAX - 4095" because otherwise
returning error pointers wouldn't work.  So this check can't break
anything and it could prevent a crash.

regards,
dan carpenter