x86/syscalls: Split the x32 syscalls into their own table

For unfortunate historical reasons, the x32 syscalls and the x86_64
syscalls are not all numbered the same.  As an example, ioctl() is nr 16 on
x86_64 but 514 on x32.

This has potentially nasty consequences, since it means that there are two
valid RAX values to do ioctl(2) and two invalid RAX values.  The valid
values are 16 (i.e. ioctl(2) using the x86_64 ABI) and (514 | 0x40000000)
(i.e. ioctl(2) using the x32 ABI).

The invalid values are 514 and (16 | 0x40000000).  514 will enter the
"COMPAT_SYSCALL_DEFINE3(ioctl, ...)" entry point with in_compat_syscall()
and in_x32_syscall() returning false, whereas (16 | 0x40000000) will enter
the native entry point with in_compat_syscall() and in_x32_syscall()
returning true.  Both are bogus, and both will exercise code paths in the
kernel and in any running seccomp filters that really ought to be
unreachable.

Splitting out the x32 syscalls into their own tables, allows both bogus
invocations to return -ENOSYS.  I've checked glibc, musl, and Bionic, and
all of them appear to call syscalls with their correct numbers, so this
change should have no effect on them.

There is an added benefit going forward: new syscalls that need special
handling on x32 can share the same number on x32 and x86_64.  This means
that the special syscall range 512-547 can be treated as a legacy wart
instead of something that may need to be extended in the future.

Also add a selftest to verify the new behavior.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/208024256b764312598f014ebfb0a42472c19354.1562185330.git.luto@kernel.org

2 files changed, 90 insertions, 1 deletions

diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index fa07d526fe39..07b6387f28cf 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -17,7 +17,7 @@ TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt test_mremap
 TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
-TARGETS_C_64BIT_ONLY := fsgsbase sysret_rip
+TARGETS_C_64BIT_ONLY := fsgsbase sysret_rip syscall_numbering
 # Some selftests require 32bit support enabled also on 64bit systems
 TARGETS_C_32BIT_NEEDED := ldt_gdt ptrace_syscall
 
diff --git a/tools/testing/selftests/x86/syscall_numbering.c b/tools/testing/selftests/x86/syscall_numbering.c
new file mode 100644
index 000000000000..d6b09cb1aa2c
--- /dev/null
+++ b/tools/testing/selftests/x86/syscall_numbering.c
@@ -0,0 +1,89 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * syscall_arg_fault.c - tests faults 32-bit fast syscall stack args
+ * Copyright (c) 2018 Andrew Lutomirski
+ */
+
+#define _GNU_SOURCE
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdbool.h>
+#include <errno.h>
+#include <unistd.h>
+#include <syscall.h>
+
+static int nerrs;
+
+#define X32_BIT 0x40000000UL
+
+static void check_enosys(unsigned long nr, bool *ok)
+{
+	/* If this fails, a segfault is reasonably likely. */
+	fflush(stdout);
+
+	long ret = syscall(nr, 0, 0, 0, 0, 0, 0);
+	if (ret == 0) {
+		printf("[FAIL]\tsyscall %lu succeeded, but it should have failed\n", nr);
+		*ok = false;
+	} else if (errno != ENOSYS) {
+		printf("[FAIL]\tsyscall %lu had error code %d, but it should have reported ENOSYS\n", nr, errno);
+		*ok = false;
+	}
+}
+
+static void test_x32_without_x32_bit(void)
+{
+	bool ok = true;
+
+	/*
+	 * Syscalls 512-547 are "x32" syscalls.  They are intended to be
+	 * called with the x32 (0x40000000) bit set.  Calling them without
+	 * the x32 bit set is nonsense and should not work.
+	 */
+	printf("[RUN]\tChecking syscalls 512-547\n");
+	for (int i = 512; i <= 547; i++)
+		check_enosys(i, &ok);
+
+	/*
+	 * Check that a handful of 64-bit-only syscalls are rejected if the x32
+	 * bit is set.
+	 */
+	printf("[RUN]\tChecking some 64-bit syscalls in x32 range\n");
+	check_enosys(16 | X32_BIT, &ok);	/* ioctl */
+	check_enosys(19 | X32_BIT, &ok);	/* readv */
+	check_enosys(20 | X32_BIT, &ok);	/* writev */
+
+	/*
+	 * Check some syscalls with high bits set.
+	 */
+	printf("[RUN]\tChecking numbers above 2^32-1\n");
+	check_enosys((1UL << 32), &ok);
+	check_enosys(X32_BIT | (1UL << 32), &ok);
+
+	if (!ok)
+		nerrs++;
+	else
+		printf("[OK]\tThey all returned -ENOSYS\n");
+}
+
+int main()
+{
+	/*
+	 * Anyone diagnosing a failure will want to know whether the kernel
+	 * supports x32.  Tell them.
+	 */
+	printf("\tChecking for x32...");
+	fflush(stdout);
+	if (syscall(39 | X32_BIT, 0, 0, 0, 0, 0, 0) >= 0) {
+		printf(" supported\n");
+	} else if (errno == ENOSYS) {
+		printf(" not supported\n");
+	} else {
+		printf(" confused\n");
+	}
+
+	test_x32_without_x32_bit();
+
+	return nerrs ? 1 : 0;
+}