1 files changed, 245 insertions, 0 deletions
diff --git a/tools/testing/selftests/x86/fsgsbase_restore.c b/tools/testing/selftests/x86/fsgsbase_restore.c
new file mode 100644
index 000000000000..6fffadc51579
--- /dev/null
+++ b/tools/testing/selftests/x86/fsgsbase_restore.c
@@ -0,0 +1,245 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * fsgsbase_restore.c, test ptrace vs fsgsbase
+ * Copyright (c) 2020 Andy Lutomirski
+ *
+ * This test case simulates a tracer redirecting tracee execution to
+ * a function and then restoring tracee state using PTRACE_GETREGS and
+ * PTRACE_SETREGS.  This is similar to what gdb does when doing
+ * 'p func()'.  The catch is that this test has the called function
+ * modify a segment register.  This makes sure that ptrace correctly
+ * restores segment state when using PTRACE_SETREGS.
+ *
+ * This is not part of fsgsbase.c, because that test is 64-bit only.
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <string.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+#include <err.h>
+#include <sys/user.h>
+#include <asm/prctl.h>
+#include <sys/prctl.h>
+#include <asm/ldt.h>
+#include <sys/mman.h>
+#include <stddef.h>
+#include <sys/ptrace.h>
+#include <sys/wait.h>
+#include <stdint.h>
+
+#define EXPECTED_VALUE 0x1337f00d
+
+#ifdef __x86_64__
+# define SEG "%gs"
+#else
+# define SEG "%fs"
+#endif
+
+static unsigned int dereference_seg_base(void)
+{
+	int ret;
+	asm volatile ("mov %" SEG ":(0), %0" : "=rm" (ret));
+	return ret;
+}
+
+static void init_seg(void)
+{
+	unsigned int *target = mmap(
+		NULL, sizeof(unsigned int),
+		PROT_READ | PROT_WRITE,
+		MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0);
+	if (target == MAP_FAILED)
+		err(1, "mmap");
+
+	*target = EXPECTED_VALUE;
+
+	printf("\tsegment base address = 0x%lx\n", (unsigned long)target);
+
+	struct user_desc desc = {
+		.entry_number    = 0,
+		.base_addr       = (unsigned int)(uintptr_t)target,
+		.limit           = sizeof(unsigned int) - 1,
+		.seg_32bit       = 1,
+		.contents        = 0, /* Data, grow-up */
+		.read_exec_only  = 0,
+		.limit_in_pages  = 0,
+		.seg_not_present = 0,
+		.useable         = 0
+	};
+	if (syscall(SYS_modify_ldt, 1, &desc, sizeof(desc)) == 0) {
+		printf("\tusing LDT slot 0\n");
+		asm volatile ("mov %0, %" SEG :: "rm" ((unsigned short)0x7));
+	} else {
+		/* No modify_ldt for us (configured out, perhaps) */
+
+		struct user_desc *low_desc = mmap(
+			NULL, sizeof(desc),
+			PROT_READ | PROT_WRITE,
+			MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0);
+		memcpy(low_desc, &desc, sizeof(desc));
+
+		low_desc->entry_number = -1;
+
+		/* 32-bit set_thread_area */
+		long ret;
+		asm volatile ("int $0x80"
+			      : "=a" (ret), "+m" (*low_desc)
+			      : "a" (243), "b" (low_desc)
+#ifdef __x86_64__
+			      : "r8", "r9", "r10", "r11"
+#endif
+			);
+		memcpy(&desc, low_desc, sizeof(desc));
+		munmap(low_desc, sizeof(desc));
+
+		if (ret != 0) {
+			printf("[NOTE]\tcould not create a segment -- can't test anything\n");
+			exit(0);
+		}
+		printf("\tusing GDT slot %d\n", desc.entry_number);
+
+		unsigned short sel = (unsigned short)((desc.entry_number << 3) | 0x3);
+		asm volatile ("mov %0, %" SEG :: "rm" (sel));
+	}
+}
+
+static void tracee_zap_segment(void)
+{
+	/*
+	 * The tracer will redirect execution here.  This is meant to
+	 * work like gdb's 'p func()' feature.  The tricky bit is that
+	 * we modify a segment register in order to make sure that ptrace
+	 * can correctly restore segment registers.
+	 */
+	printf("\tTracee: in tracee_zap_segment()\n");
+
+	/*
+	 * Write a nonzero selector with base zero to the segment register.
+	 * Using a null selector would defeat the test on AMD pre-Zen2
+	 * CPUs, as such CPUs don't clear the base when loading a null
+	 * selector.
+	 */
+	unsigned short sel;
+	asm volatile ("mov %%ss, %0\n\t"
+		      "mov %0, %" SEG
+		      : "=rm" (sel));
+
+	pid_t pid = getpid(), tid = syscall(SYS_gettid);
+
+	printf("\tTracee is going back to sleep\n");
+	syscall(SYS_tgkill, pid, tid, SIGSTOP);
+
+	/* Should not get here. */
+	while (true) {
+		printf("[FAIL]\tTracee hit unreachable code\n");
+		pause();
+	}
+}
+
+int main()
+{
+	printf("\tSetting up a segment\n");
+	init_seg();
+
+	unsigned int val = dereference_seg_base();
+	if (val != EXPECTED_VALUE) {
+		printf("[FAIL]\tseg[0] == %x; should be %x\n", val, EXPECTED_VALUE);
+		return 1;
+	}
+	printf("[OK]\tThe segment points to the right place.\n");
+
+	pid_t chld = fork();
+	if (chld < 0)
+		err(1, "fork");
+
+	if (chld == 0) {
+		prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0, 0);
+
+		if (ptrace(PTRACE_TRACEME, 0, 0, 0) != 0)
+			err(1, "PTRACE_TRACEME");
+
+		pid_t pid = getpid(), tid = syscall(SYS_gettid);
+
+		printf("\tTracee will take a nap until signaled\n");
+		syscall(SYS_tgkill, pid, tid, SIGSTOP);
+
+		printf("\tTracee was resumed.  Will re-check segment.\n");
+
+		val = dereference_seg_base();
+		if (val != EXPECTED_VALUE) {
+			printf("[FAIL]\tseg[0] == %x; should be %x\n", val, EXPECTED_VALUE);
+			exit(1);
+		}
+
+		printf("[OK]\tThe segment points to the right place.\n");
+		exit(0);
+	}
+
+	int status;
+
+	/* Wait for SIGSTOP. */
+	if (waitpid(chld, &status, 0) != chld || !WIFSTOPPED(status))
+		err(1, "waitpid");
+
+	struct user_regs_struct regs;
+
+	if (ptrace(PTRACE_GETREGS, chld, NULL, &regs) != 0)
+		err(1, "PTRACE_GETREGS");
+
+#ifdef __x86_64__
+	printf("\tChild GS=0x%lx, GSBASE=0x%lx\n", (unsigned long)regs.gs, (unsigned long)regs.gs_base);
+#else
+	printf("\tChild FS=0x%lx\n", (unsigned long)regs.xfs);
+#endif
+
+	struct user_regs_struct regs2 = regs;
+#ifdef __x86_64__
+	regs2.rip = (unsigned long)tracee_zap_segment;
+	regs2.rsp -= 128;	/* Don't clobber the redzone. */
+#else
+	regs2.eip = (unsigned long)tracee_zap_segment;
+#endif
+
+	printf("\tTracer: redirecting tracee to tracee_zap_segment()\n");
+	if (ptrace(PTRACE_SETREGS, chld, NULL, &regs2) != 0)
+		err(1, "PTRACE_GETREGS");
+	if (ptrace(PTRACE_CONT, chld, NULL, NULL) != 0)
+		err(1, "PTRACE_GETREGS");
+
+	/* Wait for SIGSTOP. */
+	if (waitpid(chld, &status, 0) != chld || !WIFSTOPPED(status))
+		err(1, "waitpid");
+
+	printf("\tTracer: restoring tracee state\n");
+	if (ptrace(PTRACE_SETREGS, chld, NULL, &regs) != 0)
+		err(1, "PTRACE_GETREGS");
+	if (ptrace(PTRACE_DETACH, chld, NULL, NULL) != 0)
+		err(1, "PTRACE_GETREGS");
+
+	/* Wait for SIGSTOP. */
+	if (waitpid(chld, &status, 0) != chld)
+		err(1, "waitpid");
+
+	if (WIFSIGNALED(status)) {
+		printf("[FAIL]\tTracee crashed\n");
+		return 1;
+	}
+
+	if (!WIFEXITED(status)) {
+		printf("[FAIL]\tTracee stopped for an unexpected reason: %d\n", status);
+		return 1;
+	}
+
+	int exitcode = WEXITSTATUS(status);
+	if (exitcode != 0) {
+		printf("[FAIL]\tTracee reported failure\n");
+		return 1;
+	}
+
+	printf("[OK]\tAll is well.\n");
+	return 0;
+}