s390: fix system call single stepping

When single stepping an svc instruction on s390, the kernel is entered
with a PER program check interruption. The program check handler than
jumps to the system call handler by reloading the PSW. The code didn't
set GPR13 to the thread pointer in struct task_struct. This made the
kernel access invalid memory while trying to fetch the syscall function
address. Fix this by always assigned GPR13 after .Lsysc_per.

Fixes: 0b0ed657fe00 ("s390: remove critical section cleanup from entry.S")
Reported-and-tested-by: Christian Borntraeger <[email protected]>
Signed-off-by: Sven Schnelle <[email protected]>
Reviewed-by: Heiko Carstens <[email protected]>
Signed-off-by: Heiko Carstens <[email protected]>

diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index 496f74d984732990a0da1e8a10e916c5c88773e9..969b35b177dd4fc673f2ea1ee2fb4069270ad1ad 100644 (file)
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -378,9 +378,9 @@ ENTRY(system_call)
        stmg    %r8,%r15,__LC_SAVE_AREA_SYNC
        BPOFF
        lg      %r12,__LC_CURRENT
-       lghi    %r13,__TASK_thread
        lghi    %r14,_PIF_SYSCALL
 .Lsysc_per:
+       lghi    %r13,__TASK_thread
        lg      %r15,__LC_KERNEL_STACK
        la      %r11,STACK_FRAME_OVERHEAD(%r15) # pointer to pt_regs
        UPDATE_VTIME %r8,%r9,__LC_SYNC_ENTER_TIMER