Date:   Sun, 1 Oct 2023 19:31:41 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86_64: test that userspace stack is in fact NX

Here is how it works:

* fault and fill the stack from rsp with int3 down until rlimit allows,
* fill upwards with int3 too, overwrite libc stuff, argv, envp,
* try to exec int3 on each page and catch it with either SIGSEGV or
  SIGTRAP handler.

Note: trying to execute _every_ int3 takes 30-40 seconds even on fast
machine, so only 1 int3 per page is tried.

Tested on F37 kernel and on custom kernel which did

	vm_flags |= VM_EXEC;

to stack VMA.

Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
---

 tools/testing/selftests/x86/Makefile   |    3 
 tools/testing/selftests/x86/nx_stack.c |  167 +++++++++++++++++++++++++++++++++
 2 files changed, 170 insertions(+)

--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -19,6 +19,7 @@ TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
 			vdso_restorer
 TARGETS_C_64BIT_ONLY := fsgsbase sysret_rip syscall_numbering \
 			corrupt_xstate_header amx lam test_shadow_stack
+TARGETS_C_64BIT_ONLY += nx_stack
 # Some selftests require 32bit support enabled also on 64bit systems
 TARGETS_C_32BIT_NEEDED := ldt_gdt ptrace_syscall
 
@@ -109,3 +110,5 @@ $(OUTPUT)/test_syscall_vdso_32: thunks_32.S
 # state.
 $(OUTPUT)/check_initial_reg_state_32: CFLAGS += -Wl,-ereal_start -static
 $(OUTPUT)/check_initial_reg_state_64: CFLAGS += -Wl,-ereal_start -static
+
+$(OUTPUT)/nx_stack_64: CFLAGS += -Wl,-z,noexecstack
new file mode 100644
--- /dev/null
+++ b/tools/testing/selftests/x86/nx_stack.c
@@ -0,0 +1,167 @@
+/*
+ * Copyright (c) 2023 Alexey Dobriyan <adobriyan@...il.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+/*
+ * Test that userspace stack is NX.
+ * Requires linking with -Wl,-z,noexecstack .
+ *
+ * Fill the stack with int3's and then try to execute some of them:
+ * SIGSEGV -- good, SIGTRAP -- bad.
+ */
+#undef _GNU_SOURCE
+#define _GNU_SOURCE
+#undef NDEBUG
+#include <assert.h>
+#include <signal.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/resource.h>
+#include <unistd.h>
+
+#define PAGE_SIZE 4096
+
+/*
+ * This is memset(rsp, 0xcc, -1); but down.
+ * It will SIGSEGV when bottom of the stack is reached.
+ * Byte-size access is important! (see rdi tweaking in the signal handler).
+ */
+void make_stack1(void);
+asm(
+".pushsection .text\n"
+".globl make_stack1\n"
+".align 16\n"
+"make_stack1:\n"
+	"mov %rsp, %rdi\n"
+	"mov $0xcc, %eax\n"
+	"mov $-1, %rcx\n"
+	"std\n"
+	"rep stosb\n"
+	"hlt\n"
+".type make_stack1,@function\n"
+".size make_stack1,.-make_stack1\n"
+".popsection\n"
+);
+
+/*
+ * memset(p, 0xcc, -1);
+ * It will SIGSEGV when top of the stack is reached.
+ * Byte-size access is important! (see rdi tweaking in the signal handler).
+ */
+void make_stack2(uint64_t p);
+asm(
+".pushsection .text\n"
+".globl make_stack2\n"
+".align 16\n"
+"make_stack2:\n"
+	"mov $0xcc, %eax\n"
+	"mov $-1, %rcx\n"
+	"cld\n"
+	"rep stosb\n"
+	"hlt\n"
+".type make_stack2,@function\n"
+".size make_stack2,.-make_stack2\n"
+".popsection\n"
+);
+
+static volatile int test_state = 0;
+static volatile uint64_t stack_min_addr;
+
+static void sigsegv(int _, siginfo_t *__, void *uc_)
+{
+	ucontext_t *uc = uc_;
+
+	if (test_state == 0) {
+		/* Stack is faulted and cleared from rsp to the lowest address. */
+		stack_min_addr = ++uc->uc_mcontext.gregs[REG_RDI];
+		if (1) {
+			printf("stack min %016lx\n", stack_min_addr);
+		}
+		uc->uc_mcontext.gregs[REG_RIP] = (uintptr_t)&make_stack2;
+		test_state = 1;
+	} else if (test_state == 1) {
+		/* Stack has been cleared from top to bottom. */
+		uint64_t stack_max_addr = uc->uc_mcontext.gregs[REG_RDI];
+		if (1) {
+			printf("stack max %016lx\n", stack_max_addr);
+		}
+		uc->uc_mcontext.gregs[REG_RIP] = stack_max_addr - PAGE_SIZE;
+		test_state = 2;
+	} else if (test_state == 2) {
+		/* SIGSEGV while trying to execute int3 on stack -- PASS. */
+		uc->uc_mcontext.gregs[REG_RIP] -= PAGE_SIZE;
+		if (uc->uc_mcontext.gregs[REG_RIP] == stack_min_addr) {
+			/* One more SIGSEGV and test ends. */
+			test_state = 3;
+		}
+	} else {
+		_exit(EXIT_SUCCESS);
+	}
+}
+
+static void sigtrap(int _, siginfo_t *__, void *uc_)
+{
+	/* SIGTRAP while trying to execute int3 on stack -- FAIL. */
+	_exit(EXIT_FAILURE);
+}
+
+int main(void)
+{
+	{
+		struct sigaction act = {};
+		sigemptyset(&act.sa_mask);
+		act.sa_flags = SA_SIGINFO;
+		act.sa_sigaction = &sigsegv;
+		int rv = sigaction(SIGSEGV, &act, NULL);
+		assert(rv == 0);
+	}
+	{
+		struct sigaction act = {};
+		sigemptyset(&act.sa_mask);
+		act.sa_flags = SA_SIGINFO;
+		act.sa_sigaction = &sigtrap;
+		int rv = sigaction(SIGTRAP, &act, NULL);
+		assert(rv == 0);
+	}
+	{
+		struct rlimit rlim;
+		int rv = getrlimit(RLIMIT_STACK, &rlim);
+		assert(rv == 0);
+		// Cap stack at time-honored 8 MiB value.
+		rlim.rlim_max = rlim.rlim_cur;
+		if (rlim.rlim_max > 8 * 1024 * 1024) {
+			rlim.rlim_max = 8 * 1024 * 1024;
+		}
+		rv = setrlimit(RLIMIT_STACK, &rlim);
+		assert(rv == 0);
+	}
+	{
+		/*
+		 * Regular stack is overwritten completely during testing.
+		 * All the useful work is done in the signal handlers.
+		 */
+		const size_t len = (MINSIGSTKSZ + PAGE_SIZE - 1) / PAGE_SIZE * PAGE_SIZE;
+		void *p = mmap(NULL, len, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+		assert(p != MAP_FAILED);
+		stack_t ss = {};
+		ss.ss_sp = p;
+		ss.ss_size = len;
+		int rv = sigaltstack(&ss, NULL);
+		assert(rv == 0);
+	}
+	make_stack1();
+	__builtin_trap();
+}

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