linux-hardening - Re: [PATCH] xfs: xattr: replace strncpy and check for truncation

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Message-ID: <CAFhGd8qwCwycW=PpUWUGxAdU54jwi9D=k3xrNgC62CvM-Q+ukg@mail.gmail.com>
Date: Tue, 9 Apr 2024 17:27:34 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: Chandan Babu R <chandan.babu@...cle.com>, "Darrick J. Wong" <djwong@...nel.org>, 
	linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] xfs: xattr: replace strncpy and check for truncation

On Tue, Apr 9, 2024 at 5:23 PM Justin Stitt <justinstitt@...gle.com> wrote:
>
> Hi,
>
> On Tue, Apr 9, 2024 at 6:32 AM Christoph Hellwig <hch@...radead.org> wrote:
> >
> > On Fri, Apr 05, 2024 at 07:45:08PM +0000, Justin Stitt wrote:
> > > -     memcpy(offset, prefix, prefix_len);
> > > -     offset += prefix_len;
> > > -     strncpy(offset, (char *)name, namelen);                 /* real name */
> > > -     offset += namelen;
> > > -     *offset = '\0';
> > > +
> > > +     combined_len = prefix_len + namelen;
> > > +
> > > +     /* plus one byte for \0 */
> > > +     actual_len = scnprintf(offset, combined_len + 1, "%s%s", prefix, name);
> > > +
> > > +     if (actual_len < combined_len)
> >
> > Shouldn't this be a != ?
>
> I guess it could be. It's a truncation check so I figured just
> checking if the amount of bytes actually copied was less than the
> total would suffice.
>
> >
> > That being said I think this is actually wrong - the attr names are
> > not NULL-terminated on disk, which is why we have the explicit
> > zero terminataion above.
>
> Gotcha, in which case we could use the "%.*s" format specifier which
> allows for a length argument. Does something like this look better?
>
> diff --git a/fs/xfs/xfs_xattr.c b/fs/xfs/xfs_xattr.c
> index 364104e1b38a..1b7e886e0f29 100644
> --- a/fs/xfs/xfs_xattr.c
> +++ b/fs/xfs/xfs_xattr.c
> @@ -206,6 +206,7 @@ __xfs_xattr_put_listent(
>  {
>   char *offset;
>   int arraytop;
> + size_t combined_len, actual_len;
>
>   if (context->count < 0 || context->seen_enough)
>   return;
> @@ -220,11 +221,16 @@ __xfs_xattr_put_listent(
>   return;
>   }
>   offset = context->buffer + context->count;
> - memcpy(offset, prefix, prefix_len);
> - offset += prefix_len;
> - strncpy(offset, (char *)name, namelen); /* real name */
> - offset += namelen;
> - *offset = '\0';
> +
> + combined_len = prefix_len + namelen;
> +
> + /* plus one byte for \0 */
> + actual_len = scnprintf(offset, combined_len + 1, "%.*s%.*s",
> +        prefix_len, prefix, namelen, name);
> +
> + if (actual_len < combined_len)
> + xfs_warn(context->dp->i_mount,
> + "cannot completely copy context buffer resulting in truncation");
>
>  compute_size:
>   context->count += prefix_len + namelen + 1;
> ---

I copy pasted from vim -> gmail and it completely ate all my tabs.
When I actually send the new patch, if needed, it will be formatted
correctly :)

>
>
>
> >
> > How was this tested?
>
> With https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/about/
>
> but using scripts + image from: https://github.com/tytso/xfstests-bld
>
> here's the output log: https://pastebin.com/V2gFhbNZ wherein I ran the
> 5 default ones (I think?):
>
> |        Ran: generic/475 generic/476 generic/521 generic/522 generic/642
> |        Passed all 5 tests
>
> Thanks
> Justin