seccomp: Make syscall skipping and nr changes more consistent

This fixes two issues that could cause incompatibility between
kernel versions:

 - If a tracer uses SECCOMP_RET_TRACE to select a syscall number
   higher than the largest known syscall, emulate the unknown
   vsyscall by returning -ENOSYS.  (This is unlikely to make a
   noticeable difference on x86-64 due to the way the system call
   entry works.)

 - On x86-64 with vsyscall=emulate, skipped vsyscalls were buggy.

This updates the documentation accordingly.
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Will Drewry <wad@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>

Documentation/prctl/seccomp_filter.txt

@@ -95,12 +95,15 @@ SECCOMP_RET_KILL:
 
 SECCOMP_RET_TRAP:
 	Results in the kernel sending a SIGSYS signal to the triggering
-	task without executing the system call.  The kernel will
-	rollback the register state to just before the system call
-	entry such that a signal handler in the task will be able to
-	inspect the ucontext_t->uc_mcontext registers and emulate
-	system call success or failure upon return from the signal
-	handler.
+	task without executing the system call.  siginfo->si_call_addr
+	will show the address of the system call instruction, and
+	siginfo->si_syscall and siginfo->si_arch will indicate which
+	syscall was attempted.  The program counter will be as though
+	the syscall happened (i.e. it will not point to the syscall
+	instruction).  The return value register will contain an arch-
+	dependent value -- if resuming execution, set it to something
+	sensible.  (The architecture dependency is because replacing
+	it with -ENOSYS could overwrite some useful information.)
 
 	The SECCOMP_RET_DATA portion of the return value will be passed
 	as si_errno.
@@ -123,6 +126,18 @@ SECCOMP_RET_TRACE:
 	the BPF program return value will be available to the tracer
 	via PTRACE_GETEVENTMSG.
 
+	The tracer can skip the system call by changing the syscall number
+	to -1.  Alternatively, the tracer can change the system call
+	requested by changing the system call to a valid syscall number.  If
+	the tracer asks to skip the system call, then the system call will
+	appear to return the value that the tracer puts in the return value
+	register.
+
+	The seccomp check will not be run again after the tracer is
+	notified.  (This means that seccomp-based sandboxes MUST NOT
+	allow use of ptrace, even of other sandboxed processes, without
+	extreme care; ptracers can use this mechanism to escape.)
+
 SECCOMP_RET_ALLOW:
 	Results in the system call being executed.
 
@@ -161,3 +176,50 @@ architecture supports both ptrace_event and seccomp, it will be able to
 support seccomp filter with minor fixup: SIGSYS support and seccomp return
 value checking.  Then it must just add CONFIG_HAVE_ARCH_SECCOMP_FILTER
 to its arch-specific Kconfig.
+
+
+
+Caveats
+-------
+
+The vDSO can cause some system calls to run entirely in userspace,
+leading to surprises when you run programs on different machines that
+fall back to real syscalls.  To minimize these surprises on x86, make
+sure you test with
+/sys/devices/system/clocksource/clocksource0/current_clocksource set to
+something like acpi_pm.
+
+On x86-64, vsyscall emulation is enabled by default.  (vsyscalls are
+legacy variants on vDSO calls.)  Currently, emulated vsyscalls will honor seccomp, with a few oddities:
+
+- A return value of SECCOMP_RET_TRAP will set a si_call_addr pointing to
+  the vsyscall entry for the given call and not the address after the
+  'syscall' instruction.  Any code which wants to restart the call
+  should be aware that (a) a ret instruction has been emulated and (b)
+  trying to resume the syscall will again trigger the standard vsyscall
+  emulation security checks, making resuming the syscall mostly
+  pointless.
+
+- A return value of SECCOMP_RET_TRACE will signal the tracer as usual,
+  but the syscall may not be changed to another system call using the
+  orig_rax register. It may only be changed to -1 order to skip the
+  currently emulated call. Any other change MAY terminate the process.
+  The rip value seen by the tracer will be the syscall entry address;
+  this is different from normal behavior.  The tracer MUST NOT modify
+  rip or rsp.  (Do not rely on other changes terminating the process.
+  They might work.  For example, on some kernels, choosing a syscall
+  that only exists in future kernels will be correctly emulated (by
+  returning -ENOSYS).
+
+To detect this quirky behavior, check for addr & ~0x0C00 ==
+0xFFFFFFFFFF600000.  (For SECCOMP_RET_TRACE, use rip.  For
+SECCOMP_RET_TRAP, use siginfo->si_call_addr.)  Do not check any other
+condition: future kernels may improve vsyscall emulation and current
+kernels in vsyscall=native mode will behave differently, but the
+instructions at 0xF...F600{0,4,8,C}00 will not be system calls in these
+cases.
+
+Note that modern systems are unlikely to use vsyscalls at all -- they
+are a legacy feature and they are considerably slower than standard
+syscalls.  New code will use the vDSO, and vDSO-issued system calls
+are indistinguishable from normal system calls.

arch/x86/kernel/vsyscall_64.c

@@ -136,19 +136,6 @@ static int addr_to_vsyscall_nr(unsigned long addr)
 	return nr;
 }
 
-#ifdef CONFIG_SECCOMP
-static int vsyscall_seccomp(struct task_struct *tsk, int syscall_nr)
-{
-	if (!seccomp_mode(&tsk->seccomp))
-		return 0;
-	task_pt_regs(tsk)->orig_ax = syscall_nr;
-	task_pt_regs(tsk)->ax = syscall_nr;
-	return __secure_computing(syscall_nr);
-}
-#else
-#define vsyscall_seccomp(_tsk, _nr) 0
-#endif
-
 static bool write_ok_or_segv(unsigned long ptr, size_t size)
 {
 	/*
@@ -181,10 +168,9 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
 {
 	struct task_struct *tsk;
 	unsigned long caller;
-	int vsyscall_nr;
+	int vsyscall_nr, syscall_nr, tmp;
 	int prev_sig_on_uaccess_error;
 	long ret;
-	int skip;
 
 	/*
 	 * No point in checking CS -- the only way to get here is a user mode
@@ -216,56 +202,84 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
 	}
 
 	tsk = current;
-	/*
-	 * With a real vsyscall, page faults cause SIGSEGV.  We want to
-	 * preserve that behavior to make writing exploits harder.
-	 */
-	prev_sig_on_uaccess_error = current_thread_info()->sig_on_uaccess_error;
-	current_thread_info()->sig_on_uaccess_error = 1;
 
 	/*
+	 * Check for access_ok violations and find the syscall nr.
+	 *
 	 * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
 	 * 64-bit, so we don't need to special-case it here.  For all the
 	 * vsyscalls, NULL means "don't write anything" not "write it at
 	 * address 0".
 	 */
-	ret = -EFAULT;
-	skip = 0;
 	switch (vsyscall_nr) {
 	case 0:
-		skip = vsyscall_seccomp(tsk, __NR_gettimeofday);
-		if (skip)
-			break;
-
 		if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
-		    !write_ok_or_segv(regs->si, sizeof(struct timezone)))
-			break;
+		    !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
+			ret = -EFAULT;
+			goto check_fault;
+		}
+
+		syscall_nr = __NR_gettimeofday;
+		break;
+
+	case 1:
+		if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
+			ret = -EFAULT;
+			goto check_fault;
+		}
+
+		syscall_nr = __NR_time;
+		break;
+
+	case 2:
+		if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
+		    !write_ok_or_segv(regs->si, sizeof(unsigned))) {
+			ret = -EFAULT;
+			goto check_fault;
+		}
+
+		syscall_nr = __NR_getcpu;
+		break;
+	}
+
+	/*
+	 * Handle seccomp.  regs->ip must be the original value.
+	 * See seccomp_send_sigsys and Documentation/prctl/seccomp_filter.txt.
+	 *
+	 * We could optimize the seccomp disabled case, but performance
+	 * here doesn't matter.
+	 */
+	regs->orig_ax = syscall_nr;
+	regs->ax = -ENOSYS;
+	tmp = secure_computing(syscall_nr);
+	if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
+		warn_bad_vsyscall(KERN_DEBUG, regs,
+				  "seccomp tried to change syscall nr or ip");
+		do_exit(SIGSYS);
+	}
+	if (tmp)
+		goto do_ret;  /* skip requested */
 
+	/*
+	 * With a real vsyscall, page faults cause SIGSEGV.  We want to
+	 * preserve that behavior to make writing exploits harder.
+	 */
+	prev_sig_on_uaccess_error = current_thread_info()->sig_on_uaccess_error;
+	current_thread_info()->sig_on_uaccess_error = 1;
+
+	ret = -EFAULT;
+	switch (vsyscall_nr) {
+	case 0:
 		ret = sys_gettimeofday(
 			(struct timeval __user *)regs->di,
 			(struct timezone __user *)regs->si);
 		break;
 
 	case 1:
-		skip = vsyscall_seccomp(tsk, __NR_time);
-		if (skip)
-			break;
-
-		if (!write_ok_or_segv(regs->di, sizeof(time_t)))
-			break;
-
 		ret = sys_time((time_t __user *)regs->di);
 		break;
 
 	case 2:
-		skip = vsyscall_seccomp(tsk, __NR_getcpu);
-		if (skip)
-			break;
-
-		if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
-		    !write_ok_or_segv(regs->si, sizeof(unsigned)))
-			break;
-
 		ret = sys_getcpu((unsigned __user *)regs->di,
 				 (unsigned __user *)regs->si,
 				 NULL);
@@ -274,12 +288,7 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
 
 	current_thread_info()->sig_on_uaccess_error = prev_sig_on_uaccess_error;
 
-	if (skip) {
-		if ((long)regs->ax <= 0L) /* seccomp errno emulation */
-			goto do_ret;
-		goto done; /* seccomp trace/trap */
-	}
-
+check_fault:
 	if (ret == -EFAULT) {
 		/* Bad news -- userspace fed a bad pointer to a vsyscall. */
 		warn_bad_vsyscall(KERN_INFO, regs,
@@ -302,7 +311,6 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
 	/* Emulate a ret instruction. */
 	regs->ip = caller;
 	regs->sp += 8;
-done:
 	return true;
 
 sigsegv:

kernel/seccomp.c

@@ -396,25 +396,29 @@ int __secure_computing(int this_syscall)
 #ifdef CONFIG_SECCOMP_FILTER
 	case SECCOMP_MODE_FILTER: {
 		int data;
+		struct pt_regs *regs = task_pt_regs(current);
 		ret = seccomp_run_filters(this_syscall);
 		data = ret & SECCOMP_RET_DATA;
 		ret &= SECCOMP_RET_ACTION;
 		switch (ret) {
 		case SECCOMP_RET_ERRNO:
 			/* Set the low-order 16-bits as a errno. */
-			syscall_set_return_value(current, task_pt_regs(current),
+			syscall_set_return_value(current, regs,
 						 -data, 0);
 			goto skip;
 		case SECCOMP_RET_TRAP:
 			/* Show the handler the original registers. */
-			syscall_rollback(current, task_pt_regs(current));
+			syscall_rollback(current, regs);
 			/* Let the filter pass back 16 bits of data. */
 			seccomp_send_sigsys(this_syscall, data);
 			goto skip;
 		case SECCOMP_RET_TRACE:
 			/* Skip these calls if there is no tracer. */
-			if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP))
+			if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP)) {
+				syscall_set_return_value(current, regs,
+							 -ENOSYS, 0);
 				goto skip;
+			}
 			/* Allow the BPF to provide the event message */
 			ptrace_event(PTRACE_EVENT_SECCOMP, data);
 			/*
@@ -425,6 +429,9 @@ int __secure_computing(int this_syscall)
 			 */
 			if (fatal_signal_pending(current))
 				break;
+			if (syscall_get_nr(current, regs) < 0)
+				goto skip;  /* Explicit request to skip. */
+
 			return 0;
 		case SECCOMP_RET_ALLOW:
 			return 0;