[PATCH] xfs: xattr: replace strncpy and check for truncation

[PATCH] xfs: xattr: replace strncpy and check for truncation

[Justin Stitt]

strncpy is deprecated and as such we should prefer less ambiguous and
more robust string interfaces [1].

There's a lot of manual memory management to get a prefix and name into
a string. Let's use an easier to understand and more robust interface in
scnprintf() to accomplish the same task while enabling us to check for
possible truncation, resulting in a soft warning.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Split from https://lore.kernel.org/all/20240401-strncpy-fs-xfs-xfs_ioctl-c-v1-1-02b9feb1989b@google.com/
with feedback from Christoph H.
---
 fs/xfs/xfs_xattr.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/fs/xfs/xfs_xattr.c b/fs/xfs/xfs_xattr.c
index 364104e1b38a..bc7246eaebdd 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, name);
+
+	if (actual_len < combined_len)
+		xfs_warn(context->dp->i_mount,
+	"cannot completely copy %s%s to context buffer resulting in truncation",
+			prefix, name);
 
 compute_size:
 	context->count += prefix_len + namelen + 1;

---
base-commit: c85af715cac0a951eea97393378e84bb49384734
change-id: 20240405-strncpy-xattr-split2-0a3aff0c6a20

Best regards,
--
Justin Stitt <justinstitt@google.com>

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

[Christoph Hellwig]

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 != ?

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.

How was this tested?

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

[Justin Stitt]

Hi,

On Tue, Apr 9, 2024 at 6:32 AM Christoph Hellwig <hch@infradead.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

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

[Justin Stitt]

On Tue, Apr 9, 2024 at 5:23 PM Justin Stitt <justinstitt@google.com> wrote:
>
> Hi,
>
> On Tue, Apr 9, 2024 at 6:32 AM Christoph Hellwig <hch@infradead.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

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

[Darrick J. Wong]

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@google.com> wrote:
> >
> > Hi,
> >
> > On Tue, Apr 9, 2024 at 6:32 AM Christoph Hellwig <hch@infradead.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
>