[tip: x86/mm] selftests/x86/mm: Add new test that userspace stack is in fact NX
From: tip-bot2 for Alexey Dobriyan
Date: Tue Oct 03 2023 - 15:12:14 EST
The following commit has been merged into the x86/mm branch of tip:
Commit-ID: 802e87cc464613441f9098ebf940b1895fe3f5e5
Gitweb: https://git.kernel.org/tip/802e87cc464613441f9098ebf940b1895fe3f5e5
Author: Alexey Dobriyan <adobriyan@xxxxxxxxx>
AuthorDate: Tue, 03 Oct 2023 19:18:43 +03:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Tue, 03 Oct 2023 21:00:45 +02:00
selftests/x86/mm: Add new 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 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
Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/4cef8266-ad6d-48af-a5f1-fc2b6a8eb422@p183
---
tools/testing/selftests/x86/Makefile | 4 +-
tools/testing/selftests/x86/nx_stack.c | 212 ++++++++++++++++++++++++-
2 files changed, 216 insertions(+)
create mode 100644 tools/testing/selftests/x86/nx_stack.c
diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index 7e8c937..0b872c0 100644
--- 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
diff --git a/tools/testing/selftests/x86/nx_stack.c b/tools/testing/selftests/x86/nx_stack.c
new file mode 100644
index 0000000..ea4a4e2
--- /dev/null
+++ b/tools/testing/selftests/x86/nx_stack.c
@@ -0,0 +1,212 @@
+/*
+ * Copyright (c) 2023 Alexey Dobriyan <adobriyan@xxxxxxxxx>
+ *
+ * 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();
+}