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

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Date: Tue, 9 Apr 2024 17:23:35 -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

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;
---



>
> 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