[PATCH v2] x86: test that userspace stack is in fact NX

[Alexey Dobriyan <adobriyan@gmail.com>]

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 in either SIGSEGV or
  SIGTRAP handler.

Note: trying to execute _every_ int3 on a 8 MiB stack takes 30-40 seconds
even on fast machine which is too much for kernel selftesting
(not for LTP!) so only 1 int3 per page is tried.

Tested on F37 kernel and on a custom kernel which does

	vm_flags |= VM_EXEC;

to stack VMA.

Report from the buggy kernel:

	$ ./nx_stack_32
	stack min ff007000
	stack max ff807000
	FAIL    executable page on the stack: eip ff806001

	$ ./nx_stack_64
	stack min 7ffe65bb0000
	stack max 7ffe663b0000
	FAIL    executable page on the stack: rip 7ffe663af001

Changes since v1:

	i386 support
	nice pretty printing of test result
	cld in the SIGSEGV handler for robustness
	SIGSTKSZ is recommended not MINSIGSTKSZ
	better comments

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

 tools/testing/selftests/x86/Makefile   |    4 
 tools/testing/selftests/x86/nx_stack.c |  212 +++++++++++++++++++++++++++++++++
 2 files changed, 216 insertions(+)

--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -14,6 +14,7 @@ TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt test_mremap
 			check_initial_reg_state sigreturn iopl ioperm \
 			test_vsyscall mov_ss_trap \
 			syscall_arg_fault fsgsbase_restore sigaltstack
+TARGETS_C_BOTHBITS += nx_stack
 TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
@@ -109,3 +110,6 @@ $(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_32: CFLAGS += -Wl,-z,noexecstack
+$(OUTPUT)/nx_stack_64: CFLAGS += -Wl,-z,noexecstack
new file mode 100644
--- /dev/null
+++ b/tools/testing/selftests/x86/nx_stack.c
@@ -0,0 +1,212 @@
+/*
+ * Copyright (c) 2023 Alexey Dobriyan <adobriyan@gmail.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
+ * because I don't want to bother with PT_GNU_STACK detection.
+ *
+ * Fill the stack with int3's and then try to execute some of them:
+ * SIGSEGV -- good, SIGTRAP -- bad.
+ *
+ * Regular stack is completely overwritten before testing.
+ * Test doesn't exit SIGSEGV handler after first fault at int3.
+ */
+#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 tweak in the signal handler).
+ */
+void make_stack1(void);
+asm(
+".pushsection .text\n"
+".globl make_stack1\n"
+".align 16\n"
+"make_stack1:\n"
+	"mov $0xcc, %al\n"
+#if defined __amd64__
+	"mov %rsp, %rdi\n"
+	"mov $-1, %rcx\n"
+#elif defined __i386__
+	"mov %esp, %edi\n"
+	"mov $-1, %ecx\n"
+#else
+#error
+#endif
+	"std\n"
+	"rep stosb\n"
+	/* unreachable */
+	"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.
+ */
+void make_stack2(uint64_t p);
+asm(
+".pushsection .text\n"
+".globl make_stack2\n"
+".align 16\n"
+"make_stack2:\n"
+	"mov $0xcc, %al\n"
+#if defined __amd64__
+	"mov $-1, %rcx\n"
+#elif defined __i386__
+	"mov $-1, %ecx\n"
+#else
+#error
+#endif
+	"cld\n"
+	"rep stosb\n"
+	/* unreachable */
+	"hlt\n"
+".type make_stack2,@function\n"
+".size make_stack2,.-make_stack2\n"
+".popsection\n"
+);
+
+static volatile int test_state = 0;
+static volatile unsigned long stack_min_addr;
+
+#if defined __amd64__
+#define RDI	REG_RDI
+#define RIP	REG_RIP
+#define RIP_STRING "rip"
+#elif defined __i386__
+#define RDI	REG_EDI
+#define RIP	REG_EIP
+#define RIP_STRING "eip"
+#else
+#error
+#endif
+
+static void sigsegv(int _, siginfo_t *__, void *uc_)
+{
+	/*
+	 * Some Linux versions didn't clear DF before entering signal
+	 * handler. make_stack1() doesn't have a chance to clear DF
+	 * either so we clear it by hand here.
+	 */
+	asm volatile ("cld" ::: "memory");
+
+	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[RDI];
+		if (1) {
+			printf("stack min %lx\n", stack_min_addr);
+		}
+		uc->uc_mcontext.gregs[RIP] = (uintptr_t)&make_stack2;
+		test_state = 1;
+	} else if (test_state == 1) {
+		/* Stack has been cleared from top to bottom. */
+		unsigned long stack_max_addr = uc->uc_mcontext.gregs[RDI];
+		if (1) {
+			printf("stack max %lx\n", stack_max_addr);
+		}
+		/* Start faulting pages on stack and see what happens. */
+		uc->uc_mcontext.gregs[RIP] = stack_max_addr - PAGE_SIZE;
+		test_state = 2;
+	} else if (test_state == 2) {
+		/* Stack page is NX -- good, test next page. */
+		uc->uc_mcontext.gregs[RIP] -= PAGE_SIZE;
+		if (uc->uc_mcontext.gregs[RIP] == stack_min_addr) {
+			/* One more SIGSEGV and test ends. */
+			test_state = 3;
+		}
+	} else {
+		printf("PASS\tAll stack pages are NX\n");
+		_exit(EXIT_SUCCESS);
+	}
+}
+
+static void sigtrap(int _, siginfo_t *__, void *uc_)
+{
+	const ucontext_t *uc = uc_;
+	unsigned long rip = uc->uc_mcontext.gregs[RIP];
+	printf("FAIL\texecutable page on the stack: " RIP_STRING " %lx\n", rip);
+	_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);
+	}
+	{
+		/*
+		 * We don't know now much stack SIGSEGV handler uses.
+		 * Bump this by 1 page every time someone complains,
+		 * or rewrite it in assembly.
+		 */
+		const size_t len = SIGSTKSZ;
+		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();
+	/*
+	 * Unreachable, but if _this_ int3 is ever reached, it's a bug somewhere.
+	 * Fold it into main SIGTRAP pathway.
+	 */
+	__builtin_trap();
+}