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

[Aurelien Jarno <aurelien at aurel32 dot net>]

On 2017-12-17 23:07, Dmitry V. Levin wrote:
> 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.

Indeed. I was even wondering if we can just use cp = strdup("./") below
to get rid of to_free and limit the amount of different code paths.

> >    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.

Good idea indeed.

> >  	}
> > +      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.

I thought about that, but refrained as much as possible to change any
interface, as it might trigger more bugs, even if it looks ok.

The current bug actually show that a simple change might have big
consequences. Overall it seems the RPATH handling code became very
complex over the time, and would clearly benefit from a rewrite. I
would not be really surprised if other similar bugs are found.

All that said, if a few more persons review that code path, that should
be fine.

> >        /* 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.

Yes, however there is at least one more "continue" in fillin_rpath
which can lead to an empty path after expansion. I am surprised it
didn't trigger before today.

Thanks for the review!

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

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