Re: [PATCH] elf: Check for empty tokens before dynamic string token expansion [BZ #22625]

["Dmitry V. Levin" <ldv at altlinux dot org>]

On Sun, Dec 17, 2017 at 06:21:49PM +0100, Aurelien Jarno wrote:
> The fillin_rpath function in elf/dl-load.c loops over each RPATH or
> RUNPATH tokens and intepret empty tokens as the current directory
> ("./"). In practice the check for empty token is done *after* the
> dynamic string token expansion. The expansion process can return an
> empty string for the $ORIGIN token if __libc_enable_secure is set
> or if the path of the binary can not be determined (/proc not mounted).
> 
> Fix that by moving the check for empty tokens before the dynamic string
> token expansion. In addition, check for NULL pointer or empty strings
> return by expand_dynamic_string_token.
> 
> The above changed highlighted a bug in decompose_rpath, an empty array
> is represented by the first element being NULL at the fillin_rpath path
> level, but by using a -1 pointer in decompose_rpath and other functions.
> 
> Changelog:
> 	[BZ #22625]
> 	* elf/dl-load.c (fillin_rpath): Check for empty tokens before
> 	dynamic string token expansion. Check for NULL pointer or empty
> 	string possibly returned by expand_dynamic_string_token.
> 	(decompose_rpath): Check for empty path after dynamic string
> 	token expansion.

Accidentally, I was looking at this issue recently and have been testing
a slightly different patch, so I'd just share my thoughts.

> ---
>  ChangeLog     |  9 +++++++++
>  NEWS          |  4 ++++
>  elf/dl-load.c | 30 ++++++++++++++++++++++++++----
>  3 files changed, 39 insertions(+), 4 deletions(-)
> 
> diff --git a/ChangeLog b/ChangeLog
> index 815e73571d..4ef438d08c 100644
> --- a/ChangeLog
> +++ b/ChangeLog
> @@ -1,3 +1,12 @@
> +2017-12-17  Aurelien Jarno  <aurelien@aurel32.net>
> +
> +	[BZ #22625]
> +	* elf/dl-load.c (fillin_rpath): Check for empty tokens before
> +	dynamic string token expansion. Check for NULL pointer or empty
> +	string possibly returned by expand_dynamic_string_token.
> +	(decompose_rpath): Check for empty path after dynamic string
> +	token expansion.
> +
>  2017-12-16  Aurelien Jarno  <aurelien@aurel32.net>
>  
>  	[BZ #22505]
> diff --git a/NEWS b/NEWS
> index 0670585b4c..8099ecc8f1 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -149,6 +149,10 @@ Security related changes:
>    CVE-2017-1000366 has been applied, but it is mentioned here only because
>    of the CVE assignment.)  Reported by Qualys.
>  
> +  CVE-2017-16997: Incorrect handling of RPATH or RUNPATH containing $ORIGIN
> +  for AT_SECURE or SUID binaries could be used to load libraries from the
> +  current directory.
> +
>  The following bugs are resolved with this release:
>  
>    [The release manager will add the list generated by
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index bbd3be9e20..81a1a03a01 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -433,14 +433,11 @@ fillin_rpath (char *rpath, struct r_search_path_elem **result, const char *sep,
>  {
>    char *cp;
>    size_t nelems = 0;
> -  char *to_free;
> +  char *to_free = NULL;

Let's use this opportunity and move to_free inside the loop.

>    while ((cp = __strsep (&rpath, sep)) != NULL)
>      {
>        struct r_search_path_elem *dirp;
> -
> -      to_free = cp = expand_dynamic_string_token (l, cp, 1);
> -
>        size_t len = strlen (cp);
>  
>        /* `strsep' can pass an empty string.  This has to be

I think the code would be easier to read if len == 0 check below was
changed to *cp == '\0'.  This way you wouldn't have to call strlen twice,
and ...

> @@ -450,6 +447,24 @@ fillin_rpath (char *rpath, struct r_search_path_elem **result, const char *sep,
>  	  static const char curwd[] = "./";
>  	  cp = (char *) curwd;

... an explicit len = 0 assignment would highlight the nastiness
of the trick with len later in fillin_rpath.

>  	}
> +      else
> +	{
> +	  to_free = cp = expand_dynamic_string_token (l, cp, 1);
> +
> +	  /* expand_dynamic_string_token can return NULL in case of memory
> +	     allocation failure.  */
> +	  if (cp == NULL)
> +	    continue;
> +
> +	  /* Recompute the length after dynamic string token expansion
> +	     and ignore empty paths.  */
> +	  len = strlen (cp);
> +	  if (len == 0)
> +	    {
> +	      free (to_free);
> +	      continue;
> +	    }
> +	}

The alternative (that I was testing in my patch) to len == 0 check is to
change expand_dynamic_string_token to return NULL if _dl_dst_substitute
returned an empty string, e.g.

-  return _dl_dst_substitute (l, s, result, is_path);
+  char *retval = _dl_dst_substitute (l, s, result, is_path);
+
+  /* If substitution of dynamic string tokens resulted to an empty string,
+     return NULL as in case of insufficient memory.  */
+  if (__glibc_unlikely (*retval == '\0'))
+    {
+      free (result);
+      return NULL;
+    }
+
+  return retval;

Both users of expand_dynamic_string_token, fillin_rpath and _dl_map_object,
would be happy.

>        /* Remove trailing slashes (except for "/").  */
>        while (len > 1 && cp[len - 1] == '/')
> @@ -620,6 +635,13 @@ decompose_rpath (struct r_search_path_struct *sps,
>       necessary.  */
>    free (copy);
>  
> +  /* There is no path after expansion.  */
> +  if (result[0] == NULL) {
> +    free (result);
> +    sps->dirs = (struct r_search_path_elem **) -1;
> +    return false;
> +  }
> +
>    sps->dirs = result;
>    /* The caller will change this value if we haven't used a real malloc.  */
>    sps->malloced = 1;

Yes, the bug was just sitting here and waiting for a fix in fillin_rpath
to explode.


-- 
ldv

Attachment: signature.asc
Description: PGP signature