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>

3 files changed, 137 insertions, 60 deletions

diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt
index 597c3c581375..1e469ef75778 100644
--- a/Documentation/prctl/seccomp_filter.txt
+++ b/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.
diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 8d141b309046..b2e58a248b3b 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/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 @@ do_ret:
         /* Emulate a ret instruction. */
         regs->ip = caller;
         regs->sp += 8;
-done:
         return true;
 
 sigsegv:
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index ee376beedaf9..5af44b593770 100644
--- a/kernel/seccomp.c
+++ b/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;