Merge branch 'x86-entry-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 vsyscall updates from Thomas Gleixner:
 "Further hardening of the legacy vsyscall by providing support for
  execute only mode and switching the default to it.

  This prevents a certain class of attacks which rely on the vsyscall
  page being accessible at a fixed address in the canonical kernel
  address space"

* 'x86-entry-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  selftests/x86: Add a test for process_vm_readv() on the vsyscall page
  x86/vsyscall: Add __ro_after_init to global variables
  x86/vsyscall: Change the default vsyscall mode to xonly
  selftests/x86/vsyscall: Verify that vsyscall=none blocks execution
  x86/vsyscall: Document odd SIGSEGV error code for vsyscalls
  x86/vsyscall: Show something useful on a read fault
  x86/vsyscall: Add a new vsyscall=xonly mode
  Documentation/admin: Remove the vsyscall=native documentation

Documentation/admin-guide/kernel-parameters.txt

@@ -5100,13 +5100,12 @@
 			targets for exploits that can control RIP.
 
 			emulate     [default] Vsyscalls turn into traps and are
-			            emulated reasonably safely.
+			            emulated reasonably safely.  The vsyscall
+				    page is readable.
 
-			native      Vsyscalls are native syscall instructions.
-			            This is a little bit faster than trapping
-			            and makes a few dynamic recompilers work
-			            better than they would in emulation mode.
-			            It also makes exploits much easier to write.
+			xonly       Vsyscalls turn into traps and are
+			            emulated reasonably safely.  The vsyscall
+				    page is not readable.
 
 			none        Vsyscalls don't work at all.  This makes
 			            them quite hard to use for exploits but

arch/x86/Kconfig

@@ -2288,7 +2288,7 @@ config COMPAT_VDSO
 choice
 	prompt "vsyscall table for legacy applications"
 	depends on X86_64
-	default LEGACY_VSYSCALL_EMULATE
+	default LEGACY_VSYSCALL_XONLY
 	help
 	  Legacy user code that does not know how to find the vDSO expects
 	  to be able to issue three syscalls by calling fixed addresses in
@@ -2296,23 +2296,38 @@ choice
 	  it can be used to assist security vulnerability exploitation.
 
 	  This setting can be changed at boot time via the kernel command
-	  line parameter vsyscall=[emulate|none].
+	  line parameter vsyscall=[emulate|xonly|none].
 
 	  On a system with recent enough glibc (2.14 or newer) and no
 	  static binaries, you can say None without a performance penalty
 	  to improve security.
 
-	  If unsure, select "Emulate".
+	  If unsure, select "Emulate execution only".
 
 	config LEGACY_VSYSCALL_EMULATE
-		bool "Emulate"
+		bool "Full emulation"
 		help
-		  The kernel traps and emulates calls into the fixed
-		  vsyscall address mapping. This makes the mapping
-		  non-executable, but it still contains known contents,
-		  which could be used in certain rare security vulnerability
-		  exploits. This configuration is recommended when userspace
-		  still uses the vsyscall area.
+		  The kernel traps and emulates calls into the fixed vsyscall
+		  address mapping. This makes the mapping non-executable, but
+		  it still contains readable known contents, which could be
+		  used in certain rare security vulnerability exploits. This
+		  configuration is recommended when using legacy userspace
+		  that still uses vsyscalls along with legacy binary
+		  instrumentation tools that require code to be readable.
+
+		  An example of this type of legacy userspace is running
+		  Pin on an old binary that still uses vsyscalls.
+
+	config LEGACY_VSYSCALL_XONLY
+		bool "Emulate execution only"
+		help
+		  The kernel traps and emulates calls into the fixed vsyscall
+		  address mapping and does not allow reads.  This
+		  configuration is recommended when userspace might use the
+		  legacy vsyscall area but support for legacy binary
+		  instrumentation of legacy code is not needed.  It mitigates
+		  certain uses of the vsyscall area as an ASLR-bypassing
+		  buffer.
 
 	config LEGACY_VSYSCALL_NONE
 		bool "None"

arch/x86/entry/vsyscall/vsyscall_64.c

@@ -42,9 +42,11 @@
 #define CREATE_TRACE_POINTS
 #include "vsyscall_trace.h"
 
-static enum { EMULATE, NONE } vsyscall_mode =
+static enum { EMULATE, XONLY, NONE } vsyscall_mode __ro_after_init =
 #ifdef CONFIG_LEGACY_VSYSCALL_NONE
 	NONE;
+#elif defined(CONFIG_LEGACY_VSYSCALL_XONLY)
+	XONLY;
 #else
 	EMULATE;
 #endif
@@ -54,6 +56,8 @@ static int __init vsyscall_setup(char *str)
 	if (str) {
 		if (!strcmp("emulate", str))
 			vsyscall_mode = EMULATE;
+		else if (!strcmp("xonly", str))
+			vsyscall_mode = XONLY;
 		else if (!strcmp("none", str))
 			vsyscall_mode = NONE;
 		else
@@ -113,7 +117,8 @@ static bool write_ok_or_segv(unsigned long ptr, size_t size)
 	}
 }
 
-bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
+bool emulate_vsyscall(unsigned long error_code,
+		      struct pt_regs *regs, unsigned long address)
 {
 	struct task_struct *tsk;
 	unsigned long caller;
@@ -122,6 +127,22 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
 	long ret;
 	unsigned long orig_dx;
 
+	/* Write faults or kernel-privilege faults never get fixed up. */
+	if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
+		return false;
+
+	if (!(error_code & X86_PF_INSTR)) {
+		/* Failed vsyscall read */
+		if (vsyscall_mode == EMULATE)
+			return false;
+
+		/*
+		 * User code tried and failed to read the vsyscall page.
+		 */
+		warn_bad_vsyscall(KERN_INFO, regs, "vsyscall read attempt denied -- look up the vsyscall kernel parameter if you need a workaround");
+		return false;
+	}
+
 	/*
 	 * No point in checking CS -- the only way to get here is a user mode
 	 * trap to a high address, which means that we're in 64-bit user code.
@@ -284,7 +305,7 @@ static const char *gate_vma_name(struct vm_area_struct *vma)
 static const struct vm_operations_struct gate_vma_ops = {
 	.name = gate_vma_name,
 };
-static struct vm_area_struct gate_vma = {
+static struct vm_area_struct gate_vma __ro_after_init = {
 	.vm_start	= VSYSCALL_ADDR,
 	.vm_end		= VSYSCALL_ADDR + PAGE_SIZE,
 	.vm_page_prot	= PAGE_READONLY_EXEC,
@@ -357,12 +378,20 @@ void __init map_vsyscall(void)
 	extern char __vsyscall_page;
 	unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
 
-	if (vsyscall_mode != NONE) {
+	/*
+	 * For full emulation, the page needs to exist for real.  In
+	 * execute-only mode, there is no PTE at all backing the vsyscall
+	 * page.
+	 */
+	if (vsyscall_mode == EMULATE) {
 		__set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
 			     PAGE_KERNEL_VVAR);
 		set_vsyscall_pgtable_user_bits(swapper_pg_dir);
 	}
 
+	if (vsyscall_mode == XONLY)
+		gate_vma.vm_flags = VM_EXEC;
+
 	BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
 		     (unsigned long)VSYSCALL_ADDR);
 }

arch/x86/include/asm/vsyscall.h

@@ -13,10 +13,12 @@ extern void set_vsyscall_pgtable_user_bits(pgd_t *root);
  * Called on instruction fetch fault in vsyscall page.
  * Returns true if handled.
  */
-extern bool emulate_vsyscall(struct pt_regs *regs, unsigned long address);
+extern bool emulate_vsyscall(unsigned long error_code,
+			     struct pt_regs *regs, unsigned long address);
 #else
 static inline void map_vsyscall(void) {}
-static inline bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
+static inline bool emulate_vsyscall(unsigned long error_code,
+				    struct pt_regs *regs, unsigned long address)
 {
 	return false;
 }

arch/x86/mm/fault.c

@@ -710,6 +710,10 @@ static void set_signal_archinfo(unsigned long address,
 	 * To avoid leaking information about the kernel page
 	 * table layout, pretend that user-mode accesses to
 	 * kernel addresses are always protection faults.
+	 *
+	 * NB: This means that failed vsyscalls with vsyscall=none
+	 * will have the PROT bit.  This doesn't leak any
+	 * information and does not appear to cause any problems.
 	 */
 	if (address >= TASK_SIZE_MAX)
 		error_code |= X86_PF_PROT;
@@ -1369,16 +1373,18 @@ void do_user_addr_fault(struct pt_regs *regs,
 
 #ifdef CONFIG_X86_64
 	/*
-	 * Instruction fetch faults in the vsyscall page might need
-	 * emulation.  The vsyscall page is at a high address
-	 * (>PAGE_OFFSET), but is considered to be part of the user
-	 * address space.
+	 * Faults in the vsyscall page might need emulation.  The
+	 * vsyscall page is at a high address (>PAGE_OFFSET), but is
+	 * considered to be part of the user address space.
 	 *
 	 * The vsyscall page does not have a "real" VMA, so do this
 	 * emulation before we go searching for VMAs.
+	 *
+	 * PKRU never rejects instruction fetches, so we don't need
+	 * to consider the PF_PK bit.
 	 */
-	if ((hw_error_code & X86_PF_INSTR) && is_vsyscall_vaddr(address)) {
-		if (emulate_vsyscall(regs, address))
+	if (is_vsyscall_vaddr(address)) {
+		if (emulate_vsyscall(hw_error_code, regs, address))
 			return;
 	}
 #endif

tools/testing/selftests/x86/test_vsyscall.c

@@ -18,6 +18,7 @@
 #include <sched.h>
 #include <stdbool.h>
 #include <setjmp.h>
+#include <sys/uio.h>
 
 #ifdef __x86_64__
 # define VSYS(x) (x)
@@ -49,21 +50,21 @@ static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
 }
 
 /* vsyscalls and vDSO */
-bool should_read_vsyscall = false;
+bool vsyscall_map_r = false, vsyscall_map_x = false;
 
 typedef long (*gtod_t)(struct timeval *tv, struct timezone *tz);
-gtod_t vgtod = (gtod_t)VSYS(0xffffffffff600000);
+const gtod_t vgtod = (gtod_t)VSYS(0xffffffffff600000);
 gtod_t vdso_gtod;
 
 typedef int (*vgettime_t)(clockid_t, struct timespec *);
 vgettime_t vdso_gettime;
 
 typedef long (*time_func_t)(time_t *t);
-time_func_t vtime = (time_func_t)VSYS(0xffffffffff600400);
+const time_func_t vtime = (time_func_t)VSYS(0xffffffffff600400);
 time_func_t vdso_time;
 
 typedef long (*getcpu_t)(unsigned *, unsigned *, void *);
-getcpu_t vgetcpu = (getcpu_t)VSYS(0xffffffffff600800);
+const getcpu_t vgetcpu = (getcpu_t)VSYS(0xffffffffff600800);
 getcpu_t vdso_getcpu;
 
 static void init_vdso(void)
@@ -107,7 +108,7 @@ static int init_vsys(void)
 	maps = fopen("/proc/self/maps", "r");
 	if (!maps) {
 		printf("[WARN]\tCould not open /proc/self/maps -- assuming vsyscall is r-x\n");
-		should_read_vsyscall = true;
+		vsyscall_map_r = true;
 		return 0;
 	}
 
@@ -133,12 +134,8 @@ static int init_vsys(void)
 		}
 
 		printf("\tvsyscall permissions are %c-%c\n", r, x);
-		should_read_vsyscall = (r == 'r');
-		if (x != 'x') {
-			vgtod = NULL;
-			vtime = NULL;
-			vgetcpu = NULL;
-		}
+		vsyscall_map_r = (r == 'r');
+		vsyscall_map_x = (x == 'x');
 
 		found = true;
 		break;
@@ -148,10 +145,8 @@ static int init_vsys(void)
 
 	if (!found) {
 		printf("\tno vsyscall map in /proc/self/maps\n");
-		should_read_vsyscall = false;
-		vgtod = NULL;
-		vtime = NULL;
-		vgetcpu = NULL;
+		vsyscall_map_r = false;
+		vsyscall_map_x = false;
 	}
 
 	return nerrs;
@@ -183,9 +178,13 @@ static inline long sys_getcpu(unsigned * cpu, unsigned * node,
 }
 
 static jmp_buf jmpbuf;
+static volatile unsigned long segv_err;
 
 static void sigsegv(int sig, siginfo_t *info, void *ctx_void)
 {
+	ucontext_t *ctx = (ucontext_t *)ctx_void;
+
+	segv_err =  ctx->uc_mcontext.gregs[REG_ERR];
 	siglongjmp(jmpbuf, 1);
 }
 
@@ -238,7 +237,7 @@ static int test_gtod(void)
 		err(1, "syscall gettimeofday");
 	if (vdso_gtod)
 		ret_vdso = vdso_gtod(&tv_vdso, &tz_vdso);
-	if (vgtod)
+	if (vsyscall_map_x)
 		ret_vsys = vgtod(&tv_vsys, &tz_vsys);
 	if (sys_gtod(&tv_sys2, &tz_sys) != 0)
 		err(1, "syscall gettimeofday");
@@ -252,7 +251,7 @@ static int test_gtod(void)
 		}
 	}
 
-	if (vgtod) {
+	if (vsyscall_map_x) {
 		if (ret_vsys == 0) {
 			nerrs += check_gtod(&tv_sys1, &tv_sys2, &tz_sys, "vsyscall", &tv_vsys, &tz_vsys);
 		} else {
@@ -273,7 +272,7 @@ static int test_time(void) {
 	t_sys1 = sys_time(&t2_sys1);
 	if (vdso_time)
 		t_vdso = vdso_time(&t2_vdso);
-	if (vtime)
+	if (vsyscall_map_x)
 		t_vsys = vtime(&t2_vsys);
 	t_sys2 = sys_time(&t2_sys2);
 	if (t_sys1 < 0 || t_sys1 != t2_sys1 || t_sys2 < 0 || t_sys2 != t2_sys2) {
@@ -294,7 +293,7 @@ static int test_time(void) {
 		}
 	}
 
-	if (vtime) {
+	if (vsyscall_map_x) {
 		if (t_vsys < 0 || t_vsys != t2_vsys) {
 			printf("[FAIL]\tvsyscall failed (ret:%ld output:%ld)\n", t_vsys, t2_vsys);
 			nerrs++;
@@ -330,7 +329,7 @@ static int test_getcpu(int cpu)
 	ret_sys = sys_getcpu(&cpu_sys, &node_sys, 0);
 	if (vdso_getcpu)
 		ret_vdso = vdso_getcpu(&cpu_vdso, &node_vdso, 0);
-	if (vgetcpu)
+	if (vsyscall_map_x)
 		ret_vsys = vgetcpu(&cpu_vsys, &node_vsys, 0);
 
 	if (ret_sys == 0) {
@@ -369,7 +368,7 @@ static int test_getcpu(int cpu)
 		}
 	}
 
-	if (vgetcpu) {
+	if (vsyscall_map_x) {
 		if (ret_vsys) {
 			printf("[FAIL]\tvsyscall getcpu() failed\n");
 			nerrs++;
@@ -410,20 +409,88 @@ static int test_vsys_r(void)
 		can_read = false;
 	}
 
-	if (can_read && !should_read_vsyscall) {
+	if (can_read && !vsyscall_map_r) {
 		printf("[FAIL]\tWe have read access, but we shouldn't\n");
 		return 1;
-	} else if (!can_read && should_read_vsyscall) {
+	} else if (!can_read && vsyscall_map_r) {
 		printf("[FAIL]\tWe don't have read access, but we should\n");
 		return 1;
+	} else if (can_read) {
+		printf("[OK]\tWe have read access\n");
+	} else {
+		printf("[OK]\tWe do not have read access: #PF(0x%lx)\n",
+		       segv_err);
+	}
+#endif
+
+	return 0;
+}
+
+static int test_vsys_x(void)
+{
+#ifdef __x86_64__
+	if (vsyscall_map_x) {
+		/* We already tested this adequately. */
+		return 0;
+	}
+
+	printf("[RUN]\tMake sure that vsyscalls really page fault\n");
+
+	bool can_exec;
+	if (sigsetjmp(jmpbuf, 1) == 0) {
+		vgtod(NULL, NULL);
+		can_exec = true;
+	} else {
+		can_exec = false;
+	}
+
+	if (can_exec) {
+		printf("[FAIL]\tExecuting the vsyscall did not page fault\n");
+		return 1;
+	} else if (segv_err & (1 << 4)) { /* INSTR */
+		printf("[OK]\tExecuting the vsyscall page failed: #PF(0x%lx)\n",
+		       segv_err);
 	} else {
-		printf("[OK]\tgot expected result\n");
+		printf("[FAILT]\tExecution failed with the wrong error: #PF(0x%lx)\n",
+		       segv_err);
+		return 1;
 	}
 #endif
 
 	return 0;
 }
 
+static int test_process_vm_readv(void)
+{
+#ifdef __x86_64__
+	char buf[4096];
+	struct iovec local, remote;
+	int ret;
+
+	printf("[RUN]\tprocess_vm_readv() from vsyscall page\n");
+
+	local.iov_base = buf;
+	local.iov_len = 4096;
+	remote.iov_base = (void *)0xffffffffff600000;
+	remote.iov_len = 4096;
+	ret = process_vm_readv(getpid(), &local, 1, &remote, 1, 0);
+	if (ret != 4096) {
+		printf("[OK]\tprocess_vm_readv() failed (ret = %d, errno = %d)\n", ret, errno);
+		return 0;
+	}
+
+	if (vsyscall_map_r) {
+		if (!memcmp(buf, (const void *)0xffffffffff600000, 4096)) {
+			printf("[OK]\tIt worked and read correct data\n");
+		} else {
+			printf("[FAIL]\tIt worked but returned incorrect data\n");
+			return 1;
+		}
+	}
+#endif
+
+	return 0;
+}
 
 #ifdef __x86_64__
 #define X86_EFLAGS_TF (1UL << 8)
@@ -455,7 +522,7 @@ static int test_emulation(void)
 	time_t tmp;
 	bool is_native;
 
-	if (!vtime)
+	if (!vsyscall_map_x)
 		return 0;
 
 	printf("[RUN]\tchecking that vsyscalls are emulated\n");
@@ -497,6 +564,9 @@ int main(int argc, char **argv)
 
 	sethandler(SIGSEGV, sigsegv, 0);
 	nerrs += test_vsys_r();
+	nerrs += test_vsys_x();
+
+	nerrs += test_process_vm_readv();
 
 #ifdef __x86_64__
 	nerrs += test_emulation();