Hi James,

quoted

We do have PT_GNU_STACK flags set correctly, this feature is required to
workaround CPU revisions which do not have RIXI support.

RIXI support can be discovered programatically from CP0_Config3.RXI
(cpu_has_rixi in asm/cpu-features.h), so I don't follow why CPUs without
RIXI would require a kernel parameter.

The following patch introduced change in behavior with regards to
stack & heap execute-ability :
commit 1a770b85c1f1c1ee37afd7cef5237ffc4c970f04
Author: Paul Burton [off-list ref]
Date:   Fri Jul 8 11:06:20 2016 +0100

    MIPS: non-exec stack & heap when non-exec PT_GNU_STACK is present
    
    The stack and heap have both been executable by default on MIPS until
    now. This patch changes the default to be non-executable, but only for
    ELF binaries with a non-executable PT_GNU_STACK header present. This
    does apply to both the heap & the stack, despite the name PT_GNU_STACK,
    and this matches the behaviour of other architectures like ARM & x86.
    
    Current MIPS toolchains do not produce the PT_GNU_STACK header, which
    means that we can rely upon this patch not changing the behaviour of
    existing binaries. The new default will only take effect for newly
    compiled binaries once toolchains are updated to support PT_GNU_STACK,
    and since those binaries are newly compiled they can be compiled
    expecting the change in default behaviour. Again this matches the way in
    which the ARM & x86 architectures handled their implementations of
    non-executable memory.
    
    Signed-off-by: Paul Burton [off-list ref]
    Cc: Leonid Yegoshin [off-list ref]
    Cc: Maciej Rozycki [off-list ref]
    Cc: Faraz Shahbazker [off-list ref]
    Cc: Raghu Gandham [off-list ref]
    Cc: Matthew Fortune [off-list ref]
    Cc: linux-mips@linux-mips.org
    Patchwork: https://patchwork.linux-mips.org/patch/13765/
    Signed-off-by: Ralf Baechle [off-list ref]

....

When kernel is detecting the type of mapping it should apply :

fs/binfmt_elf.c:
...
	if (elf_read_implies_exec(loc->elf_ex, executable_stack))
		current->personality |= READ_IMPLIES_EXEC;
...

this effectively calls mips_elf_read_implies_exec() which performs a check:
...
	if (!cpu_has_rixi) {
		/* The CPU doesn't support non-executable memory */
		return 1;
	}

	return 0;
}

This will in turn make stack & heap executable on processors without RIXI, which are practically all processors with MIPS ISA R < 6.

We would like to have an option to override this and force non-executable mappings for such systems.

Kind regards,
Miodrag
________________________________________
From: James Hogan
Sent: Thursday, November 30, 2017 11:09 AM
To: Miodrag Dinic
Cc: David Daney; Aleksandar Markovic; linux-mips@linux-mips.org; Aleksandar Markovic; Andrew Morton; DengCheng Zhu; Ding Tianhong; Douglas Leung; Frederic Weisbecker; Goran Ferenc; Ingo Molnar; James Cowgill; Jonathan Corbet; linux-doc@vger.kernel.org; linux-kernel@vger.kernel.org; Marc Zyngier; Matt Redfearn; Mimi Zohar; Paul Burton; Paul E. McKenney; Petar Jovanovic; Raghu Gandham; Ralf Baechle; Thomas Gleixner; Tom Saeger
Subject: Re: [PATCH v2] MIPS: Add nonxstack=on|off kernel parameter

On Thu, Nov 30, 2017 at 09:34:15AM +0000, Miodrag Dinic wrote:

Hi David,

Sorry for a late response, please find answers in-lined:

quoted

quoted

If this parameter is omitted, kernel behavior remains the same as it
was before this patch is applied.

Do other architectures have a similar hack?

If arm{,64} and x86 don't need this, what would make MIPS so special
that we have to carry this around?

Yes, there are similar workarounds. Just a couple lines above
nonxstack description in the documentation there are :
      noexec          [IA-64]

      noexec          [X86]
                      On X86-32 available only on PAE configured kernels.
                      noexec=on: enable non-executable mappings (default)
                      noexec=off: disable non-executable mappings
...

      noexec32        [X86-64]
                      This affects only 32-bit executables.
                      noexec32=on: enable non-executable mappings (default)
                              read doesn't imply executable mappings
                      noexec32=off: disable non-executable mappings
                              read implies executable mappings

quoted

quoted

This functionality is convenient during debugging and is especially
useful for Android development where non-exec stack is required.

Why not just set the PT_GNU_STACK flags correctly in the first place?

We do have PT_GNU_STACK flags set correctly, this feature is required to
workaround CPU revisions which do not have RIXI support.

RIXI support can be discovered programatically from CP0_Config3.RXI
(cpu_has_rixi in asm/cpu-features.h), so I don't follow why CPUs without
RIXI would require a kernel parameter.

Cheers
James

Kind regards,
Miodrag
________________________________________
From: David Daney [ddaney@caviumnetworks.com]
Sent: Tuesday, November 21, 2017 9:53 PM
To: Aleksandar Markovic; linux-mips@linux-mips.org
Cc: Miodrag Dinic; Aleksandar Markovic; Andrew Morton; DengCheng Zhu; Ding Tianhong; Douglas Leung; Frederic Weisbecker; Goran Ferenc; Ingo Molnar; James Cowgill; James Hogan; Jonathan Corbet; linux-doc@vger.kernel.org; linux-kernel@vger.kernel.org; Marc Zyngier; Matt Redfearn; Mimi Zohar; Paul Burton; Paul E. McKenney; Petar Jovanovic; Raghu Gandham; Ralf Baechle; Thomas Gleixner; Tom Saeger
Subject: Re: [PATCH v2] MIPS: Add nonxstack=on|off kernel parameter

On 11/21/2017 05:56 AM, Aleksandar Markovic wrote:

quoted

From: Miodrag Dinic <miodrag.dinic@mips.com>

Add a new kernel parameter to override the default behavior related
to the decision whether to set up stack as non-executable in function
mips_elf_read_implies_exec().

The new parameter is used to control non executable stack and heap,
regardless of PT_GNU_STACK entry. This does apply to both stack and
heap, despite the name.

Allowed values:

nonxstack=on  Force non-exec stack & heap
nonxstack=off Force executable stack & heap

If this parameter is omitted, kernel behavior remains the same as it
was before this patch is applied.

Do other architectures have a similar hack?

If arm{,64} and x86 don't need this, what would make MIPS so special
that we have to carry this around?

quoted

This functionality is convenient during debugging and is especially
useful for Android development where non-exec stack is required.

Why not just set the PT_GNU_STACK flags correctly in the first place?

quoted

Signed-off-by: Miodrag Dinic <miodrag.dinic@mips.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@mips.com>
---
  Documentation/admin-guide/kernel-parameters.txt | 11 +++++++
  arch/mips/kernel/elf.c                          | 39 +++++++++++++++++++++++++
  2 files changed, 50 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index b74e133..99464ee 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -2614,6 +2614,17 @@
                      noexec32=off: disable non-executable mappings
                              read implies executable mappings

+     nonxstack       [MIPS]
+                     Force setting up stack and heap as non-executable or
+                     executable regardless of PT_GNU_STACK entry. Both
+                     stack and heap are affected, despite the name. Valid
+                     arguments: on, off.
+                     nonxstack=on:   Force non-executable stack and heap
+                     nonxstack=off:  Force executable stack and heap
+                     If ommited, stack and heap will or will not be set
+                     up as non-executable depending on PT_GNU_STACK
+                     entry and possibly other factors.
+
      nofpu           [MIPS,SH] Disable hardware FPU at boot time.

      nofxsr          [BUGS=X86-32] Disables x86 floating point extended

diff --git a/arch/mips/kernel/elf.c b/arch/mips/kernel/elf.c
index 731325a..28ef7f3 100644
--- a/arch/mips/kernel/elf.c
+++ b/arch/mips/kernel/elf.c

@@ -326,8 +326,47 @@ void mips_set_personality_nan(struct arch_elf_state *state)
      }
  }

+static int nonxstack = EXSTACK_DEFAULT;
+
+/*
+ * kernel parameter: nonxstack=on|off
+ *
+ *   Force setting up stack and heap as non-executable or
+ *   executable regardless of PT_GNU_STACK entry. Both
+ *   stack and heap are affected, despite the name. Valid
+ *   arguments: on, off.
+ *
+ *     nonxstack=on:   Force non-executable stack and heap
+ *     nonxstack=off:  Force executable stack and heap
+ *
+ *   If ommited, stack and heap will or will not be set
+ *   up as non-executable depending on PT_GNU_STACK
+ *   entry and possibly other factors.
+ */
+static int __init nonxstack_setup(char *str)
+{
+     if (!strcmp(str, "on"))
+             nonxstack = EXSTACK_DISABLE_X;
+     else if (!strcmp(str, "off"))
+             nonxstack = EXSTACK_ENABLE_X;
+     else
+             pr_err("Malformed nonxstack format! nonxstack=on|off\n");
+
+     return 1;
+}
+__setup("nonxstack=", nonxstack_setup);
+
  int mips_elf_read_implies_exec(void *elf_ex, int exstack)
  {
+     switch (nonxstack) {
+     case EXSTACK_DISABLE_X:
+             return 0;
+     case EXSTACK_ENABLE_X:
+             return 1;
+     default:
+             break;
+     }
+
      if (exstack != EXSTACK_DISABLE_X) {
              /* The binary doesn't request a non-executable stack */
              return 1;