elf: Extend glibc.rtld.execstack tunable to force executable stack (BZ 32653)

From the bug report [1], multiple programs still require to dlopen
shared libraries with either missing PT_GNU_STACK or with the executable
bit set.  Although, in some cases, it seems to be a hard-craft assembly
source without the required .note.GNU-stack marking (so the static linker
is forced to set the stack executable if the ABI requires it), other
cases seem that the library uses trampolines [2].

Unfortunately, READ_IMPLIES_EXEC is not an option since on some ABIs
(x86_64), the kernel clears the bit, making it unsupported.  To avoid
reinstating the broken code that changes stack permission on dlopen
(0ca8785a28), this patch extends the glibc.rtld.execstack tunable to
allow an option to force an executable stack at the program startup.

The tunable is a security issue because it defeats the PT_GNU_STACK
hardening.  It has the slight advantage of making it explicit by the
caller, and, as for other tunables, this is disabled for setuid binaries.
A tunable also allows us to eventually remove it, but from previous
experiences, it would require some time.

Checked on aarch64-linux-gnu, x86_64-linux-gnu, and i686-linux-gnu.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=32653
[2] https://github.com/conda-forge/ctng-compiler-activation-feedstock/issues/143
Reviewed-by: Sam James <sam@gentoo.org>

diff --git a/NEWS b/NEWS
index c8dc31925e579b7bf8bc9f9c9a4b80081e70d70b..740225ad1294c88f9aac8ecfd95d9dc003ed3326 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -23,7 +23,9 @@ Major new features:
 
 Deprecated and removed features, and other changes affecting compatibility:
 
-  [Add deprecations, removals and changes affecting compatibility here]
+* The glibc.rtld.execstack now supports a compatibility mode to allow
+  programs that require an executable stack through dynamic loaded
+  shared libraries.
 
 Changes to build and runtime requirements:
 

diff --git a/elf/Makefile b/elf/Makefile
index 250348c2317235f21736e32f1aa16eb469da893b..c3864ca35082f1138e0c4a8bbef99ed2009bbf82 100644 (file)
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -60,6 +60,7 @@ dl-routines = \
   dl-deps \
   dl-exception \
   dl-execstack \
+  dl-execstack-tunable \
   dl-find_object \
   dl-fini \
   dl-init \
@@ -572,9 +573,11 @@ tests-execstack-yes = \
   tst-execstack \
   tst-execstack-needed \
   tst-execstack-prog \
+  tst-execstack-tunable \
   # tests-execstack-yes
 tests-execstack-static-yes = \
-  tst-execstack-prog-static
+  tst-execstack-prog-static \
+  tst-execstack-prog-static-tunable \
   # tests-execstack-static-yes
 ifeq (yes,$(run-built-tests))
 tests-execstack-special-yes = \
@@ -2023,6 +2026,14 @@ LDFLAGS-tst-execstack-prog = -Wl,-z,execstack
 CFLAGS-tst-execstack-prog.c += -Wno-trampolines
 CFLAGS-tst-execstack-mod.c += -Wno-trampolines
 
+# It expects loading a module with executable stack to work.
+CFLAGS-tst-execstack-tunable.c += -DUSE_PTHREADS=0 -DDEFAULT_RWX_STACK=1
+$(objpfx)tst-execstack-tunable.out: $(objpfx)tst-execstack-mod.so
+tst-execstack-tunable-ENV = GLIBC_TUNABLES=glibc.rtld.execstack=2
+
+LDFLAGS-tst-execstack-prog-static-tunable = -Wl,-z,noexecstack
+tst-execstack-prog-static-tunable-ENV = GLIBC_TUNABLES=glibc.rtld.execstack=2
+
 LDFLAGS-tst-execstack-prog-static = -Wl,-z,execstack
 ifeq ($(have-no-error-execstack),yes)
 LDFLAGS-tst-execstack-prog-static += -Wl,--no-error-execstack

diff --git a/elf/dl-execstack-tunable.c b/elf/dl-execstack-tunable.c
new file mode 100644 (file)
index 0000000..6cef1a3
--- /dev/null
+++ b/elf/dl-execstack-tunable.c
@@ -0,0 +1,39 @@
+/* Stack executability handling for GNU dynamic linker.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <ldsodefs.h>
+#include <dl-tunables.h>
+
+void
+_dl_handle_execstack_tunable (void)
+{
+  switch (TUNABLE_GET (glibc, rtld, execstack, int32_t, NULL))
+    {
+    case stack_tunable_mode_disable:
+      if ((__glibc_unlikely (GL(dl_stack_flags)) & PF_X))
+       _dl_fatal_printf (
+"Fatal glibc error: executable stack is not allowed\n");
+      break;
+
+    case stack_tunable_mode_force:
+      if (_dl_make_stack_executable (&__libc_stack_end) != 0)
+       _dl_fatal_printf (
+"Fatal glibc error: cannot enable executable stack as tunable requires");
+      break;
+    }
+}

diff --git a/elf/dl-support.c b/elf/dl-support.c
index c7860f327adaf6f22079b28583e60e935a6a9ebc..7b2a1c35d53786f57bf4b432e3e2bca1e850fe6b 100644 (file)
--- a/elf/dl-support.c
+++ b/elf/dl-support.c
@@ -331,9 +331,7 @@ _dl_non_dynamic_init (void)
        break;
       }
 
-  if ((__glibc_unlikely (GL(dl_stack_flags)) & PF_X)
-      && TUNABLE_GET (glibc, rtld, execstack, int32_t, NULL) == 0)
-    _dl_fatal_printf ("Fatal glibc error: executable stack is not allowed\n");
+  _dl_handle_execstack_tunable ();
 
   call_function_static_weak (_dl_find_object_init);
 

diff --git a/elf/dl-tunables.list b/elf/dl-tunables.list
index 0b6721bc51a0a053b12d8970a224151628bf10b9..c03c9967f09a41ac3f3c05fc3851a7f360872ed1 100644 (file)
--- a/elf/dl-tunables.list
+++ b/elf/dl-tunables.list
@@ -138,7 +138,7 @@ glibc {
     execstack {
       type: INT_32
       minval: 0
-      maxval: 1
+      maxval: 2
       default: 1
     }
   }

diff --git a/elf/rtld.c b/elf/rtld.c
index 099c447e800128537a92f2cf4682d653d2d65931..3af8ee63c39202f047ae67edf3a52a6d1c8ac872 100644 (file)
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -1622,9 +1622,9 @@ dl_main (const ElfW(Phdr) *phdr,
 
   bool has_interp = rtld_setup_main_map (main_map);
 
-  if ((__glibc_unlikely (GL(dl_stack_flags)) & PF_X)
-      && TUNABLE_GET (glibc, rtld, execstack, int32_t, NULL) == 0)
-    _dl_fatal_printf ("Fatal glibc error: executable stack is not allowed\n");
+  /* Handle this after PT_GNU_STACK parse, because it updates dl_stack_flags
+     if required.  */
+  _dl_handle_execstack_tunable ();
 
   /* If the current libname is different from the SONAME, add the
      latter as well.  */

diff --git a/elf/tst-execstack-prog-static-tunable.c b/elf/tst-execstack-prog-static-tunable.c
new file mode 100644 (file)
index 0000000..88b0ca1
--- /dev/null
+++ b/elf/tst-execstack-prog-static-tunable.c
@@ -0,0 +1 @@
+#include <tst-execstack-prog-static.c>

diff --git a/elf/tst-execstack-tunable.c b/elf/tst-execstack-tunable.c
new file mode 100644 (file)
index 0000000..9f03b0f
--- /dev/null
+++ b/elf/tst-execstack-tunable.c
@@ -0,0 +1 @@
+#include <tst-execstack.c>

diff --git a/elf/tst-rtld-list-tunables.exp b/elf/tst-rtld-list-tunables.exp
index 9f5990f3404d93e0c4271ad2439318df076498dc..8df6f5906e6ccc1cc4094947e7084fbb8605dae3 100644 (file)
--- a/elf/tst-rtld-list-tunables.exp
+++ b/elf/tst-rtld-list-tunables.exp
@@ -13,6 +13,6 @@ glibc.malloc.top_pad: 0x20000 (min: 0x0, max: 0x[f]+)
 glibc.malloc.trim_threshold: 0x0 (min: 0x0, max: 0x[f]+)
 glibc.rtld.dynamic_sort: 2 (min: 1, max: 2)
 glibc.rtld.enable_secure: 0 (min: 0, max: 1)
-glibc.rtld.execstack: 1 (min: 0, max: 1)
+glibc.rtld.execstack: 1 (min: 0, max: 2)
 glibc.rtld.nns: 0x4 (min: 0x1, max: 0x10)
 glibc.rtld.optional_static_tls: 0x200 (min: 0x0, max: 0x[f]+)

diff --git a/manual/tunables.texi b/manual/tunables.texi
index 7f0246c7895962aac6b67160ba8c21ee93c8b9fb..67064f595ee6d9f0ded4d3cef5057fc646e14f27 100644 (file)
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -365,8 +365,11 @@ change the main stack permission if kernel starts with a non-executable stack.
 The @code{glibc.rtld.execstack} can be used to control whether an executable
 stack is allowed from the main program.  Setting the value to @code{0} disables
 the ABI auto-negotiation (meaning no executable stacks even if the ABI or ELF
-header requires it), while @code{1} enables auto-negotiation (although the
-program might not need an executable stack).
+header requires it), @code{1} enables auto-negotiation (although the program
+might not need an executable stack), while @code{2} forces an executable
+stack at process start.  Tthis is provided for compatibility reasons, when
+the program dynamically loads modules with @code{dlopen} which require
+an executable stack.
 
 When executable stacks are not allowed, and if the main program requires it,
 the loader will fail with an error message.
@@ -380,7 +383,8 @@ of hardware capabilities and kernel configuration.
 @strong{NB:} Trying to load a dynamic shared library with @code{dlopen} or
 @code{dlmopen} that requires an executable stack will always fail if the
 main program does not require an executable stack at loading time.  This
-is enforced regardless of the tunable value.
+can be worked around by setting the tunable to @code{2}, where the stack is
+always executable.
 @end deftp
 
 @node Elision Tunables

diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 5b12a41a183d8260658573ada5e42b77b5ca24ec..b5d5b3106cfd853367216e193527b93126de3849 100644 (file)
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -717,6 +717,19 @@ extern const ElfW(Phdr) *_dl_phdr;
 extern size_t _dl_phnum;
 #endif
 
+/* Possible values for the glibc.rtld.execstack tunable.  */
+enum stack_tunable_mode
+  {
+    /* Do not allow executable stacks, even if program requires it.  */
+    stack_tunable_mode_disable = 0,
+    /* Follows either ABI requirement, or the PT_GNU_STACK value.  */
+    stack_tunable_mode_enable = 1,
+    /* Always enable an executable stack.  */
+    stack_tunable_mode_force = 2
+  };
+
+void _dl_handle_execstack_tunable (void) attribute_hidden;
+
 /* This function changes the permission of the memory region pointed
    by STACK_ENDP to executable and update the internal memory protection
    flags for future thread stack creation.  */