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

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Date: Tue, 9 Apr 2024 17:40:49 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	Chandan Babu R <chandan.babu@...cle.com>, 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 09, 2024 at 05:27:34PM -0700, Justin Stitt wrote:
> 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 :)

Yeah, the "%.*s" version looks better.

> >
> >
> >
> > >
> > > 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

Would you mind adding "-g attr,label" into the mix so that you're running all
the functional tests for xattr and fs label functionality?

--D

> >
> > Thanks
> > Justin
>