s390/ptrace: race of single stepping vs signal delivery

The current single step code is racy in regard to concurrent delivery
of signals. If a signal is delivered after a PER program check occurred
but before the TIF_PER_TRAP bit has been checked in entry[64].S the code
clears TIF_PER_TRAP and then calls do_signal. This is wrong, if the
instruction completed (or has been suppressed) a SIGTRAP should be
delivered to the debugger in any case. Only if the instruction has been
nullified the SIGTRAP may not be send.

The new logic always sets TIF_PER_TRAP if the program check indicates PER
tracing but removes it again for all program checks that are nullifying.
The effect is that for each change in the PSW address we now get a
single SIGTRAP.

Reported-by: Andreas Arnez <arnez@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

4 files changed, 21 insertions, 14 deletions

diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index ef46f66bc0d6..f954b37740c0 100644
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -231,12 +231,12 @@ sysc_work:
         jo      sysc_mcck_pending
         tm      __TI_flags+3(%r12),_TIF_NEED_RESCHED
         jo      sysc_reschedule
+        tm      __TI_flags+3(%r12),_TIF_PER_TRAP
+        jo      sysc_singlestep
         tm      __TI_flags+3(%r12),_TIF_SIGPENDING
         jo      sysc_sigpending
         tm      __TI_flags+3(%r12),_TIF_NOTIFY_RESUME
         jo      sysc_notify_resume
-        tm      __TI_flags+3(%r12),_TIF_PER_TRAP
-        jo      sysc_singlestep
         j       sysc_return             # beware of critical section cleanup
 
 #
@@ -259,7 +259,6 @@ sysc_mcck_pending:
 # _TIF_SIGPENDING is set, call do_signal
 #
 sysc_sigpending:
-        ni      __TI_flags+3(%r12),255-_TIF_PER_TRAP # clear TIF_PER_TRAP
         lr      %r2,%r11                # pass pointer to pt_regs
         l       %r1,BASED(.Ldo_signal)
         basr    %r14,%r1                # call do_signal
@@ -286,7 +285,7 @@ sysc_notify_resume:
 # _TIF_PER_TRAP is set, call do_per_trap
 #
 sysc_singlestep:
-        ni      __TI_flags+3(%r12),255-(_TIF_SYSCALL | _TIF_PER_TRAP)
+        ni      __TI_flags+3(%r12),255-_TIF_PER_TRAP
         lr      %r2,%r11                # pass pointer to pt_regs
         l       %r1,BASED(.Ldo_per_trap)
         la      %r14,BASED(sysc_return)
diff --git a/arch/s390/kernel/entry64.S b/arch/s390/kernel/entry64.S
index e42842a3072b..7a2d22dda9ef 100644
--- a/arch/s390/kernel/entry64.S
+++ b/arch/s390/kernel/entry64.S
@@ -262,12 +262,12 @@ sysc_work:
         jo      sysc_mcck_pending
         tm      __TI_flags+7(%r12),_TIF_NEED_RESCHED
         jo      sysc_reschedule
+        tm      __TI_flags+7(%r12),_TIF_PER_TRAP
+        jo      sysc_singlestep
         tm      __TI_flags+7(%r12),_TIF_SIGPENDING
         jo      sysc_sigpending
         tm      __TI_flags+7(%r12),_TIF_NOTIFY_RESUME
         jo      sysc_notify_resume
-        tm      __TI_flags+7(%r12),_TIF_PER_TRAP
-        jo      sysc_singlestep
         j       sysc_return             # beware of critical section cleanup
 
 #
@@ -288,7 +288,6 @@ sysc_mcck_pending:
 # _TIF_SIGPENDING is set, call do_signal
 #
 sysc_sigpending:
-        ni      __TI_flags+7(%r12),255-_TIF_PER_TRAP # clear TIF_PER_TRAP
         lgr     %r2,%r11                # pass pointer to pt_regs
         brasl   %r14,do_signal
         tm      __TI_flags+7(%r12),_TIF_SYSCALL
@@ -313,7 +312,7 @@ sysc_notify_resume:
 # _TIF_PER_TRAP is set, call do_per_trap
 #
 sysc_singlestep:
-        ni      __TI_flags+7(%r12),255-(_TIF_SYSCALL | _TIF_PER_TRAP)
+        ni      __TI_flags+7(%r12),255-_TIF_PER_TRAP
         lgr     %r2,%r11                # pass pointer to pt_regs
         larl    %r14,sysc_return
         jg      do_per_trap
diff --git a/arch/s390/kernel/signal.c b/arch/s390/kernel/signal.c
index d1259d875074..c3ff70a7b247 100644
--- a/arch/s390/kernel/signal.c
+++ b/arch/s390/kernel/signal.c
@@ -461,6 +461,8 @@ void do_signal(struct pt_regs *regs)
                         /* Restart system call with magic TIF bit. */
                         regs->gprs[2] = regs->orig_gpr2;
                         set_thread_flag(TIF_SYSCALL);
+                        if (test_thread_flag(TIF_SINGLE_STEP))
+                                set_thread_flag(TIF_PER_TRAP);
                         break;
                 }
         }
diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index 870a644895f0..42601d6e166f 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -277,10 +277,16 @@ static inline int do_exception(struct pt_regs *regs, int access)
         unsigned int flags;
         int fault;
 
+        tsk = current;
+        /*
+         * The instruction that caused the program check has
+         * been nullified. Don't signal single step via SIGTRAP.
+         */
+        clear_tsk_thread_flag(tsk, TIF_PER_TRAP);
+
         if (notify_page_fault(regs))
                 return 0;
 
-        tsk = current;
         mm = tsk->mm;
         trans_exc_code = regs->int_parm_long;
 
@@ -376,11 +382,6 @@ retry:
                         goto retry;
                 }
         }
-        /*
-         * The instruction that caused the program check will
-         * be repeated. Don't signal single step via SIGTRAP.
-         */
-        clear_tsk_thread_flag(tsk, TIF_PER_TRAP);
         fault = 0;
 out_up:
         up_read(&mm->mmap_sem);
@@ -427,6 +428,12 @@ void __kprobes do_asce_exception(struct pt_regs *regs)
         struct vm_area_struct *vma;
         unsigned long trans_exc_code;
 
+        /*
+         * The instruction that caused the program check has
+         * been nullified. Don't signal single step via SIGTRAP.
+         */
+        clear_tsk_thread_flag(current, TIF_PER_TRAP);
+
         trans_exc_code = regs->int_parm_long;
         if (unlikely(!user_space_fault(trans_exc_code) || in_atomic() || !mm))
                 goto no_context;