Improve DST handling (Bug 23102, Bug 21942, Bug 18018, Bug 23259).

This commit improves DST handling significantly in the following
ways: firstly is_dst () is overhauled to correctly process DST
sequences that would be accepted given the ELF gABI.  This means that
we actually now accept slightly more sequences than before.  Now we
accept $ORIGIN$ORIGIN, but in the past we accepted only $ORIGIN\0 or
$ORIGIN/..., but this kind of behaviour results in unexpected
and uninterpreted DST sequences being used as literal search paths
leading to security defects.  Therefore the first step in correcting
this defect is making is_dst () properly account for all DSTs
and making the function context free in the sense that it counts
DSTs without knowledge of path, or AT_SECURE.  Next, _dl_dst_count ()
is also simplified to count all DSTs regardless of context.
Then in _dl_dst_substitute () we reintroduce context-dependent
processing for such things as AT_SECURE handling.  At the level of
_dl_dst_substitute we can have access to things like the true start
of the string sequence to validate $ORIGIN-based paths rooted in
trusted directories.  Lastly, we tighten up the accepted sequences
in AT_SECURE, and avoid leaving known unexpanded DSTs, this is
noted in the NEWS entry.

Verified with a sequence of 68 tests on x86_64 that cover
non-AT_SECURE and AT_SECURE testing using a sysroot (requires root
to run).  The tests cover cases for bug 23102, bug 21942, bug 18018,
and bug 23259.  These tests are not yet appropriate for the glibc
regression testsuite, but with the upcoming test-in-container testing
framework it should be possible to include these tests upstream soon.

See the mailing list for the tests:
https://www.sourceware.org/ml/libc-alpha/2018-06/msg00251.html

1 files changed, 134 insertions, 79 deletions

diff --git a/elf/dl-load.c b/elf/dl-load.c
index c52d011efd..e81601f36d 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -121,12 +121,6 @@ is_trusted_path_normalize (const char *path, size_t len)
   if (len == 0)
     return false;
 
-  if (*path == ':')
-    {
-      ++path;
-      --len;
-    }
-
   char *npath = (char *) alloca (len + 2);
   char *wnp = npath;
   while (*path != '\0')
@@ -177,114 +171,165 @@ is_trusted_path_normalize (const char *path, size_t len)
   return false;
 }
 
+/* Given a substring starting at INPUT, just after the DST '$' start
+   token, determine if INPUT contains DST token REF, following the
+   ELF gABI rules for DSTs:
+
+   * Longest possible sequence using the rules (greedy).
+
+   * Must start with a $ (enforced by caller).
+
+   * Must follow $ with one underscore or ASCII [A-Za-z] (caller
+     follows these rules for REF) or '{' (start curly quoted name).
+
+   * Must follow first two characters with zero or more [A-Za-z0-9_]
+     (enforced by caller) or '}' (end curly quoted name).
 
+   If the sequence is a DST matching REF then the length of the DST
+   (excluding the $ sign but including curly braces, if any) is
+   returned, otherwise 0.  */
 static size_t
-is_dst (const char *start, const char *name, const char *str, int secure)
+is_dst (const char *input, const char *ref)
 {
-  size_t len;
   bool is_curly = false;
 
-  if (name[0] == '{')
+  /* Is a ${...} input sequence?  */
+  if (input[0] == '{')
     {
       is_curly = true;
-      ++name;
+      ++input;
     }
 
-  len = 0;
-  while (name[len] == str[len] && name[len] != '\0')
-    ++len;
-
-  if (is_curly)
-    {
-      if (name[len] != '}')
-	return 0;
-
-      /* Point again at the beginning of the name.  */
-      --name;
-      /* Skip over closing curly brace and adjust for the --name.  */
-      len += 2;
-    }
-  else if (name[len] != '\0' && name[len] != '/')
-    return 0;
-
-  if (__glibc_unlikely (secure)
-      && ((name[len] != '\0' && name[len] != '/')
-	  || (name != start + 1)))
+  /* Check for matching name, following closing curly brace (if
+     required), or trailing characters which are part of an
+     identifier.  */
+  size_t rlen = strlen (ref);
+  if (strncmp (input, ref, rlen) != 0
+      || (is_curly && input[rlen] != '}')
+      || ((input[rlen] >= 'A' && input[rlen] <= 'Z')
+	  || (input[rlen] >= 'a' && input[rlen] <= 'z')
+	  || (input[rlen] >= '0' && input[rlen] <= '9')
+	  || (input[rlen] == '_')))
     return 0;
 
-  return len;
+  if (is_curly)
+    /* Count the two curly braces.  */
+    return rlen + 2;
+  else
+    return rlen;
 }
 
-
+/* INPUT is the start of a DST sequence at the first '$' occurrence.
+   If there is a DST we call into _dl_dst_count to count the number of
+   DSTs.  We count all known DSTs regardless of __libc_enable_secure;
+   the caller is responsible for enforcing the security of the
+   substitution rules (usually _dl_dst_substitute).  */
 size_t
-_dl_dst_count (const char *name)
+_dl_dst_count (const char *input)
 {
-  const char *const start = name;
   size_t cnt = 0;
 
+  input = strchr (input, '$');
+
+  /* Most likely there is no DST.  */
+  if (__glibc_likely (input == NULL))
+    return 0;
+
   do
     {
       size_t len;
 
-      /* $ORIGIN is not expanded for SUID/GUID programs (except if it
-	 is $ORIGIN alone) and it must always appear first in path.  */
-      ++name;
-      if ((len = is_dst (start, name, "ORIGIN", __libc_enable_secure)) != 0
-	  || (len = is_dst (start, name, "PLATFORM", 0)) != 0
-	  || (len = is_dst (start, name, "LIB", 0)) != 0)
+      ++input;
+      /* All DSTs must follow ELF gABI rules, see is_dst ().  */
+      if ((len = is_dst (input, "ORIGIN")) != 0
+	  || (len = is_dst (input, "PLATFORM")) != 0
+	  || (len = is_dst (input, "LIB")) != 0)
 	++cnt;
 
-      name = strchr (name + len, '$');
+      /* There may be more than one DST in the input.  */
+      input = strchr (input + len, '$');
     }
-  while (name != NULL);
+  while (input != NULL);
 
   return cnt;
 }
 
-
+/* Process INPUT for DSTs and store in RESULT using the information
+   from link map L to resolve the DSTs. This function only handles one
+   path at a time and does not handle colon-separated path lists (see
+   fillin_rpath ()).  Lastly the size of result in bytes should be at
+   least equal to the value returned by DL_DST_REQUIRED.  Note that it
+   is possible for a DT_NEEDED, DT_AUXILIARY, and DT_FILTER entries to
+   have colons, but we treat those as literal colons here, not as path
+   list delimeters.  */
 char *
-_dl_dst_substitute (struct link_map *l, const char *name, char *result)
+_dl_dst_substitute (struct link_map *l, const char *input, char *result)
 {
-  const char *const start = name;
-
-  /* Now fill the result path.  While copying over the string we keep
-     track of the start of the last path element.  When we come across
-     a DST we copy over the value or (if the value is not available)
-     leave the entire path element out.  */
+  /* Copy character-by-character from input into the working pointer
+     looking for any DSTs.  We track the start of input and if we are
+     going to check for trusted paths, all of which are part of $ORIGIN
+     handling in SUID/SGID cases (see below).  In some cases, like when
+     a DST cannot be replaced, we may set result to an empty string and
+     return.  */
   char *wp = result;
-  char *last_elem = result;
+  const char *start = input;
   bool check_for_trusted = false;
 
   do
     {
-      if (__glibc_unlikely (*name == '$'))
+      if (__glibc_unlikely (*input == '$'))
 	{
 	  const char *repl = NULL;
 	  size_t len;
 
-	  ++name;
-	  if ((len = is_dst (start, name, "ORIGIN", __libc_enable_secure)) != 0)
+	  ++input;
+	  if ((len = is_dst (input, "ORIGIN")) != 0)
 	    {
-	      repl = l->l_origin;
+	      /* For SUID/GUID programs we normally ignore the path with
+		 $ORIGIN in DT_RUNPATH, or DT_RPATH.  However, there is
+		 one exception to this rule, and it is:
+
+		   * $ORIGIN appears as the first path element, and is
+		     the only string in the path or is immediately
+		     followed by a path separator and the rest of the
+		     path.
+
+		   * The path is rooted in a trusted directory.
+
+		 This exception allows such programs to reference
+		 shared libraries in subdirectories of trusted
+		 directories.  The use case is one of general
+		 organization and deployment flexibility.
+		 Trusted directories are usually such paths as "/lib64"
+		 or "/usr/lib64", and the usual RPATHs take the form of
+		 [$ORIGIN/../$LIB/somedir].  */
+	      if (__glibc_unlikely (__libc_enable_secure)
+		  && !(input == start + 1
+		       && (input[len] == '\0' || input[len] == '/')))
+		repl = (const char *) -1;
+	      else
+	        repl = l->l_origin;
+
 	      check_for_trusted = (__libc_enable_secure
 				   && l->l_type == lt_executable);
 	    }
-	  else if ((len = is_dst (start, name, "PLATFORM", 0)) != 0)
+	  else if ((len = is_dst (input, "PLATFORM")) != 0)
 	    repl = GLRO(dl_platform);
-	  else if ((len = is_dst (start, name, "LIB", 0)) != 0)
+	  else if ((len = is_dst (input, "LIB")) != 0)
 	    repl = DL_DST_LIB;
 
 	  if (repl != NULL && repl != (const char *) -1)
 	    {
 	      wp = __stpcpy (wp, repl);
-	      name += len;
+	      input += len;
 	    }
-	  else if (len > 1)
+	  else if (len != 0)
 	    {
-	      /* We cannot use this path element, the value of the
-		 replacement is unknown.  */
-	      wp = last_elem;
-	      break;
+	      /* We found a valid DST that we know about, but we could
+	         not find a replacement value for it, therefore we
+		 cannot use this path and discard it.  */
+	      *result = '\0';
+	      return result;
 	    }
 	  else
 	    /* No DST we recognize.  */
@@ -292,16 +337,26 @@ _dl_dst_substitute (struct link_map *l, const char *name, char *result)
 	}
       else
 	{
-	  *wp++ = *name++;
+	  *wp++ = *input++;
 	}
     }
-  while (*name != '\0');
+  while (*input != '\0');
 
   /* In SUID/SGID programs, after $ORIGIN expansion the normalized
-     path must be rooted in one of the trusted directories.  */
+     path must be rooted in one of the trusted directories.  The $LIB
+     and $PLATFORM DST cannot in any way be manipulated by the caller
+     because they are fixed values that are set by the dynamic loader
+     and therefore any paths using just $LIB or $PLATFORM need not be
+     checked for trust, the authors of the binaries themselves are
+     trusted to have designed this correctly.  Only $ORIGIN is tested in
+     this way because it may be manipulated in some ways with hard
+     links.  */
   if (__glibc_unlikely (check_for_trusted)
-      && !is_trusted_path_normalize (last_elem, wp - last_elem))
-    wp = last_elem;
+      && !is_trusted_path_normalize (result, wp - result))
+    {
+      *result = '\0';
+      return result;
+    }
 
   *wp = '\0';
 
@@ -309,13 +364,13 @@ _dl_dst_substitute (struct link_map *l, const char *name, char *result)
 }
 
 
-/* Return copy of argument with all recognized dynamic string tokens
-   ($ORIGIN and $PLATFORM for now) replaced.  On some platforms it
-   might not be possible to determine the path from which the object
-   belonging to the map is loaded.  In this case the path element
-   containing $ORIGIN is left out.  */
+/* Return a malloc allocated copy of INPUT with all recognized DSTs
+   replaced. On some platforms it might not be possible to determine the
+   path from which the object belonging to the map is loaded.  In this
+   case the path containing the DST is left out.  On error NULL
+   is returned.  */
 static char *
-expand_dynamic_string_token (struct link_map *l, const char *s)
+expand_dynamic_string_token (struct link_map *l, const char *input)
 {
   /* We make two runs over the string.  First we determine how large the
      resulting string is and then we copy it over.  Since this is no
@@ -325,22 +380,22 @@ expand_dynamic_string_token (struct link_map *l, const char *s)
   size_t total;
   char *result;
 
-  /* Determine the number of DST elements.  */
-  cnt = DL_DST_COUNT (s);
+  /* Determine the number of DSTs.  */
+  cnt = _dl_dst_count (input);
 
   /* If we do not have to replace anything simply copy the string.  */
   if (__glibc_likely (cnt == 0))
-    return __strdup (s);
+    return __strdup (input);
 
   /* Determine the length of the substituted string.  */
-  total = DL_DST_REQUIRED (l, s, strlen (s), cnt);
+  total = DL_DST_REQUIRED (l, input, strlen (input), cnt);
 
   /* Allocate the necessary memory.  */
   result = (char *) malloc (total + 1);
   if (result == NULL)
     return NULL;
 
-  return _dl_dst_substitute (l, s, result);
+  return _dl_dst_substitute (l, input, result);
 }