powerpc/kernel: Change the do_syscall_trace_enter() API

The API for calling do_syscall_trace_enter() is currently sensible
enough, it just returns the (modified) syscall number.

However once we enable seccomp filter it will get more complicated. When
seccomp filter runs, the seccomp kernel code (via SECCOMP_RET_ERRNO), or
a ptracer (via SECCOMP_RET_TRACE), may reject the syscall and *may* or may
*not* set a return value in r3.

That means the assembler that calls do_syscall_trace_enter() can not
blindly return ENOSYS, it needs to only return ENOSYS if a return value
has not already been set.

There is no way to implement that logic with the current API. So change
the do_syscall_trace_enter() API to make it deal with the return code
juggling, and the assembler can then just return whatever return code it
is given.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Kees Cook <keescook@chromium.org>

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 67ecdf61f4e3f4d63d2e39e528c829d473ca6b50..2405631e91a29db579c75a2b5f33d18d2f7972f6 100644 (file)
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S
@@ -458,6 +458,10 @@ syscall_dotrace:
        lwz     r7,GPR7(r1)
        lwz     r8,GPR8(r1)
        REST_NVGPRS(r1)
+
+       cmplwi  r0,NR_syscalls
+       /* Return code is already in r3 thanks to do_syscall_trace_enter() */
+       bge-    ret_from_syscall
        b       syscall_dotrace_cont
 
 syscall_exit_work:

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index ee15d3c62e269fa06a017de887952ec58a06207a..a94f155db78e83d67fca2e967ba43585f18aeda8 100644 (file)
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -151,8 +151,7 @@ END_FW_FTR_SECTION_IFSET(FW_FEATURE_SPLPAR)
        CURRENT_THREAD_INFO(r11, r1)
        ld      r10,TI_FLAGS(r11)
        andi.   r11,r10,_TIF_SYSCALL_DOTRACE
-       bne     syscall_dotrace
-.Lsyscall_dotrace_cont:
+       bne     syscall_dotrace         /* does not return */
        cmpldi  0,r0,NR_syscalls
        bge-    syscall_enosys
 
@@ -246,22 +245,34 @@ syscall_dotrace:
        bl      save_nvgprs
        addi    r3,r1,STACK_FRAME_OVERHEAD
        bl      do_syscall_trace_enter
+
        /*
-        * Restore argument registers possibly just changed.
-        * We use the return value of do_syscall_trace_enter
-        * for the call number to look up in the table (r0).
+        * We use the return value of do_syscall_trace_enter() as the syscall
+        * number. If the syscall was rejected for any reason do_syscall_trace_enter()
+        * returns an invalid syscall number and the test below against
+        * NR_syscalls will fail.
         */
        mr      r0,r3
+
+       /* Restore argument registers just clobbered and/or possibly changed. */
        ld      r3,GPR3(r1)
        ld      r4,GPR4(r1)
        ld      r5,GPR5(r1)
        ld      r6,GPR6(r1)
        ld      r7,GPR7(r1)
        ld      r8,GPR8(r1)
+
+       /* Repopulate r9 and r10 for the system_call path */
        addi    r9,r1,STACK_FRAME_OVERHEAD
        CURRENT_THREAD_INFO(r10, r1)
        ld      r10,TI_FLAGS(r10)
-       b       .Lsyscall_dotrace_cont
+
+       cmpldi  r0,NR_syscalls
+       blt+    system_call
+
+       /* Return code is already in r3 thanks to do_syscall_trace_enter() */
+       b       .Lsyscall_exit
+
 
 syscall_enosys:
        li      r3,-ENOSYS

diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
index f21897b420576c63330eb5c62982ba0729229a85..7484221bb3f8fa3fcfb3b3a4323722fd9afe2a20 100644 (file)
--- a/arch/powerpc/kernel/ptrace.c
+++ b/arch/powerpc/kernel/ptrace.c
@@ -1762,26 +1762,42 @@ long arch_ptrace(struct task_struct *child, long request,
        return ret;
 }
 
-/*
- * We must return the syscall number to actually look up in the table.
- * This can be -1L to skip running any syscall at all.
+/**
+ * do_syscall_trace_enter() - Do syscall tracing on kernel entry.
+ * @regs: the pt_regs of the task to trace (current)
+ *
+ * Performs various types of tracing on syscall entry. This includes seccomp,
+ * ptrace, syscall tracepoints and audit.
+ *
+ * The pt_regs are potentially visible to userspace via ptrace, so their
+ * contents is ABI.
+ *
+ * One or more of the tracers may modify the contents of pt_regs, in particular
+ * to modify arguments or even the syscall number itself.
+ *
+ * It's also possible that a tracer can choose to reject the system call. In
+ * that case this function will return an illegal syscall number, and will put
+ * an appropriate return value in regs->r3.
+ *
+ * Return: the (possibly changed) syscall number.
  */
 long do_syscall_trace_enter(struct pt_regs *regs)
 {
-       long ret = 0;
+       bool abort = false;
 
        user_exit();
 
        secure_computing_strict(regs->gpr[0]);
 
-       if (test_thread_flag(TIF_SYSCALL_TRACE) &&
-           tracehook_report_syscall_entry(regs))
+       if (test_thread_flag(TIF_SYSCALL_TRACE)) {
                /*
-                * Tracing decided this syscall should not happen.
-                * We'll return a bogus call number to get an ENOSYS
-                * error, but leave the original number in regs->gpr[0].
+                * The tracer may decide to abort the syscall, if so tracehook
+                * will return !0. Note that the tracer may also just change
+                * regs->gpr[0] to an invalid syscall number, that is handled
+                * below on the exit path.
                 */
-               ret = -1L;
+               abort = tracehook_report_syscall_entry(regs) != 0;
+       }
 
        if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
                trace_sys_enter(regs, regs->gpr[0]);
@@ -1798,7 +1814,17 @@ long do_syscall_trace_enter(struct pt_regs *regs)
                                    regs->gpr[5] & 0xffffffff,
                                    regs->gpr[6] & 0xffffffff);
 
-       return ret ?: regs->gpr[0];
+       if (abort || regs->gpr[0] >= NR_syscalls) {
+               /*
+                * If we are aborting explicitly, or if the syscall number is
+                * now invalid, set the return value to -ENOSYS.
+                */
+               regs->gpr[3] = -ENOSYS;
+               return -1;
+       }
+
+       /* Return the possibly modified but valid syscall number */
+       return regs->gpr[0];
 }
 
 void do_syscall_trace_leave(struct pt_regs *regs)