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Re: [PATCH v2] elf: Check for empty tokens before dynamic string token expansion [BZ #22625]


On 2017-12-19 00:16, Dmitry V. Levin wrote:
> On Mon, Dec 18, 2017 at 09:42:13PM +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).
> > 
> > To fix that, move the check for empty tokens before the dynamic string
> > token expansion. Change expand_dynamic_string_token to return NULL
> > instead of an empty string, and check for NULL pointer returned 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 (expand_dynamic_string_token): Return NULL instead
> > 	of an empty string.
> > 	(fillin_rpath): Check for empty tokens before dynamic string token
> > 	expansion. Check for NULL pointer possibly returned by
> > 	expand_dynamic_string_token.
> > 	(decompose_rpath): Check for empty path after dynamic string
> > 	token expansion.
> > ---
> >  ChangeLog     | 12 ++++++++++++
> >  NEWS          |  4 ++++
> >  elf/dl-load.c | 43 ++++++++++++++++++++++++++++++++++++-------
> >  3 files changed, 52 insertions(+), 7 deletions(-)
> > 
> > 
> > This new version includes the suggestions done by Dmitry. It would be
> > nice if it could be reviewed by at least one more person, especially 
> > the part modifying expand_dynamic_string_token.
> > 
> > diff --git a/ChangeLog b/ChangeLog
> > index 4a7164354f..24a5223485 100644
> > --- a/ChangeLog
> > +++ b/ChangeLog
> > @@ -1,3 +1,15 @@
> > +2017-12-17  Aurelien Jarno  <aurelien@aurel32.net>
> > +	    Dmitry V. Levin  <ldv@altlinux.org>
> > +
> > +	[BZ #22625]
> > +	* elf/dl-load.c (expand_dynamic_string_token): Return NULL instead
> > +	of an empty string.
> > +	(fillin_rpath): Check for empty tokens before dynamic string token
> > +	expansion. Check for NULL pointer possibly returned by
> > +	expand_dynamic_string_token.
> > +	(decompose_rpath): Check for empty path after dynamic string
> > +	token expansion.
> > +
> >  2017-12-18  Sergei Trofimovich  <slyfox@gentoo.org>
> >  
> >  	[BZ #22624]
> > diff --git a/NEWS b/NEWS
> > index a43ff26e83..0d43e93a17 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 e7d97dcc56..827ec1c491 100644
> > --- a/elf/dl-load.c
> > +++ b/elf/dl-load.c
> > @@ -384,7 +384,17 @@ expand_dynamic_string_token (struct link_map *l, const char *s, int is_path)
> >    if (result == NULL)
> >      return NULL;
> >  
> > -  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;
> >  }
> >  
> >  
> > @@ -433,22 +443,33 @@ fillin_rpath (char *rpath, struct r_search_path_elem **result, const char *sep,
> >  {
> >    char *cp;
> >    size_t nelems = 0;
> > -  char *to_free;
> >  
> >    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);
> > +      char *to_free = NULL;
> > +      size_t len;
> >  
> >        /* `strsep' can pass an empty string.  This has to be
> >  	 interpreted as `use the current directory'. */
> > -      if (len == 0)
> > +      if (*cp == '\0')
> >  	{
> >  	  static const char curwd[] = "./";
> >  	  cp = (char *) curwd;
> > +	  len = 0;
> > +	}
> > +      else
> > +	{
> > +	  to_free = cp = expand_dynamic_string_token (l, cp, 1);
> > +
> > +	  /* expand_dynamic_string_token can return NULL in case of empty
> > +	     path or memory allocation failure.  */
> > +	  if (cp == NULL)
> > +	    continue;
> > +
> > +	  /* Recompute the length after dynamic string token expansion
> > +	     and ignore empty paths.  */
> 
> Just /* Compute the length after dynamic string token expansion.  */

Good catch, I have fixed that locally.
 
> 
> > +	  len = strlen (cp);
> >  	}
> >  
> >        /* Remove trailing slashes (except for "/").  */
> > @@ -620,6 +641,14 @@ 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;
> 
> I think the patch is fine.
> 

Thanks for the review, let's see if someone else has comments on it.

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net

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