s390: fix kernel crash due to linkage stack instructions

The kernel currently crashes with a low-address-protection exception
if a user space process executes an instruction that tries to use the
linkage stack. Set the base-ASTE origin and the subspace-ASTE origin
of the dispatchable-unit-control-table to point to a dummy ASTE.
Set up control register 15 to point to an empty linkage stack with no
room left.

A user space process with a linkage stack instruction will still crash
but with a different exception which is correctly translated to a
segmentation fault instead of a kernel oops.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/arch/s390/kernel/head64.S b/arch/s390/kernel/head64.S
index b9e25ae2579c..d7c00507568a 100644
--- a/arch/s390/kernel/head64.S
+++ b/arch/s390/kernel/head64.S
@@ -59,7 +59,7 @@ ENTRY(startup_continue)
         .quad   0                       # cr12: tracing off
         .quad   0                       # cr13: home space segment table
         .quad   0xc0000000              # cr14: machine check handling off
-        .quad   0                       # cr15: linkage stack operations
+        .quad   .Llinkage_stack         # cr15: linkage stack operations
 .Lpcmsk:.quad   0x0000000180000000
 .L4malign:.quad 0xffffffffffc00000
 .Lscan2g:.quad  0x80000000 + 0x20000 - 8        # 2GB + 128K - 8
@@ -67,12 +67,15 @@ ENTRY(startup_continue)
 .Lparmaddr:
         .quad   PARMAREA
         .align  64
-.Lduct: .long   0,0,0,0,.Lduald,0,0,0
+.Lduct: .long   0,.Laste,.Laste,0,.Lduald,0,0,0
         .long   0,0,0,0,0,0,0,0
+.Laste: .quad   0,0xffffffffffffffff,0,0,0,0,0,0
         .align  128
 .Lduald:.rept   8
         .long   0x80000000,0,0,0        # invalid access-list entries
         .endr
+.Llinkage_stack:
+        .long   0,0,0x89000000,0,0,0,0x8a000000,0
 
 ENTRY(_ehead)