elf: Remove ad-hoc restrictions on dlopen callers [BZ #22787]

This looks like a post-exploitation hardening measure: If an attacker is
able to redirect execution flow, they could use that to load a DSO which
contains additional code (or perhaps make the stack executable).

However, the checks are not in the correct place to be effective: If
they are performed before the critical operation, an attacker with
sufficient control over execution flow could simply jump directly to
the code which performs the operation, bypassing the check.  The check
would have to be executed unconditionally after the operation and
terminate the process in case a caller violation was detected.

Furthermore, in _dl_check_caller, there was a fallback reading global
writable data (GL(dl_rtld_map).l_map_start and
GL(dl_rtld_map).l_text_end), which could conceivably be targeted by an
attacker to disable the check, too.

Other critical functions (such as system) remain completely
unprotected, so the value of these additional checks does not appear
that large.  Therefore this commit removes this functionality.

1 files changed, 0 insertions, 9 deletions

diff --git a/elf/dl-open.c b/elf/dl-open.c
index 0e37dd7..9dde4ac 100644
--- a/elf/dl-open.c
+++ b/elf/dl-open.c
@@ -28,7 +28,6 @@
 #include <sys/param.h>
 #include <libc-lock.h>
 #include <ldsodefs.h>
-#include <caller.h>
 #include <sysdep-cancel.h>
 #include <tls.h>
 #include <stap-probe.h>
@@ -47,8 +46,6 @@ struct dl_open_args
   int mode;
   /* This is the caller of the dlopen() function.  */
   const void *caller_dlopen;
-  /* This is the caller of _dl_open().  */
-  const void *caller_dl_open;
   struct link_map *map;
   /* Namespace ID.  */
   Lmid_t nsid;
@@ -187,11 +184,6 @@ dl_open_worker (void *a)
   int mode = args->mode;
   struct link_map *call_map = NULL;
 
-  /* Check whether _dl_open() has been called from a valid DSO.  */
-  if (__check_caller (args->caller_dl_open,
-		      allow_libc|allow_libdl|allow_ldso) != 0)
-    _dl_signal_error (0, "dlopen", NULL, N_("invalid caller"));
-
   /* Determine the caller's map if necessary.  This is needed in case
      we have a DST, when we don't know the namespace ID we have to put
      the new object in, or when the file name has no path in which
@@ -583,7 +575,6 @@ no more namespaces available for dlmopen()"));
   args.file = file;
   args.mode = mode;
   args.caller_dlopen = caller_dlopen;
-  args.caller_dl_open = RETURN_ADDRESS (0);
   args.map = NULL;
   args.nsid = nsid;
   args.argc = argc;