svn commit: r355080 - in stable: 11/lib/libc/secure 12/lib/libc/secure

[Kyle Evans]

Author: kevans
Date: Mon Nov 25 03:49:38 2019
New Revision: 355080
URL: https://svnweb.freebsd.org/changeset/base/355080

Log:
  MFC r354669, r354672, r354689: move __stack_chk_guard constructor
  
  r354669:
  ssp: add a priority to the __stack_chk_guard constructor
  
  First, this commit is a NOP on GCC <= 4.x; this decidedly doesn't work
  cleanly on GCC 4.2, and it will be gone soon anyways so I chose not to dump
  time into figuring out if there's a way to make it work. xtoolchain-gcc,
  clocking in as GCC6, can cope with it just fine and later versions are also
  generally ok with the syntax. I suspect very few users are running GCC4.2
  built worlds and also experiencing potential fallout from the status quo.
  
  For dynamically linked applications, this change also means very little.
  rtld will run libc ctors before most others, so the situation is
  approximately a NOP for these as well.
  
  The real cause for this change is statically linked applications doing
  almost questionable things in their constructors. qemu-user-static, for
  instance, creates a thread in a global constructor for their async rcu
  callbacks. In general, this works in other places-
  
  - On OpenBSD, __stack_chk_guard is stored in an .openbsd.randomdata section
    that's initialized by the kernel in the static case, or ld.so in the
    dynamic case
  - On Linux, __stack_chk_guard is apparently stored in TLS and such a problem
    is circumvented there because the value is presumed stable in the new
    thread.
  
  On FreeBSD, the rcu thread creation ctor and __guard_setup are both unmarked
  priority. qemu-user-static spins up the rcu thread prior to __guard_setup
  which starts making function calls- some of these are sprinkled with the
  canary. In the middle of one of these functions, __guard_setup is invoked in
  the main thread and __stack_chk_guard changes- qemu-user-static is promptly
  terminated for an SSP violation that didn't actually happen.
  
  This is not an all-too-common problem. We circumvent it here by giving the
  __stack_chk_guard constructor a solid priority. 200 was chosen because that
  gives static applications ample range (down to 101) for working around it
  if they really need to. I suspect most applications will "just work" as
  expected- the default/non-prioritized flavor of __constructor__ functions
  run last, and the canary is generally not expected to change as of this
  point at the very least.
  
  This took approximately three weeks of spare time debugging to pin down.
  
  r354672:
  ssp: rework the logic to use priority=200 on clang builds
  
  The preproc logic was added at the last minute to appease GCC 4.2, and
  kevans@ did clearly not go back and double-check that the logic worked out
  for clang builds to use the new variant.
  
  It turns out that clang defines __GNUC__ == 4. Flip it around and check
  __clang__ as well, leaving a note to remove it later.
  
  r354689:
  ssp: further refine the conditional used for constructor priority
  
  __has_attribute(__constructor__) is a better test for clang than
  defined(__clang__). Switch to it instead.
  
  While we're already here and touching it, pfg@ nailed down when GCC actually
  introduced the priority argument -- 4.3. Use that instead of our
  hammer-guess of GCC >= 5 for the sake of correctness.
  
  PR:		241905

Modified:
  stable/12/lib/libc/secure/stack_protector.c
Directory Properties:
  stable/12/   (props changed)

Changes in other areas also in this revision:
Modified:
  stable/11/lib/libc/secure/stack_protector.c
Directory Properties:
  stable/11/   (props changed)

Modified: stable/12/lib/libc/secure/stack_protector.c
==============================================================================
--- stable/12/lib/libc/secure/stack_protector.c	Mon Nov 25 03:39:13 2019	(r355079)
+++ stable/12/lib/libc/secure/stack_protector.c	Mon Nov 25 03:49:38 2019	(r355080)
@@ -40,11 +40,29 @@ __FBSDID("$FreeBSD$");
 #include <unistd.h>
 #include "libc_private.h"
 
+/*
+ * We give __guard_setup a defined priority early on so that statically linked
+ * applications have a defined priority at which __stack_chk_guard will be
+ * getting initialized.  This will not matter to most applications, because
+ * they're either not usually statically linked or they simply don't do things
+ * in constructors that would be adversely affected by their positioning with
+ * respect to this initialization.
+ *
+ * This conditional should be removed when GCC 4.2 is removed.
+ */
+#if __has_attribute(__constructor__) || __GNUC_PREREQ__(4, 3)
+#define	_GUARD_SETUP_CTOR_ATTR	 \
+    __attribute__((__constructor__ (200), __used__));
+#else
+#define	_GUARD_SETUP_CTOR_ATTR	\
+    __attribute__((__constructor__, __used__));
+#endif
+
 extern int __sysctl(const int *name, u_int namelen, void *oldp,
     size_t *oldlenp, void *newp, size_t newlen);
 
 long __stack_chk_guard[8] = {0, 0, 0, 0, 0, 0, 0, 0};
-static void __guard_setup(void) __attribute__((__constructor__, __used__));
+static void __guard_setup(void) _GUARD_SETUP_CTOR_ATTR;
 static void __fail(const char *);
 void __stack_chk_fail(void);
 void __chk_fail(void);