[S390] Mark kernel text section read-only.

Set read-only flag in the page table entries for the kernel image text
section. This will catch all instruction caused corruptions withing the
text section.
Instruction replacement via kprobes still works, since it bypasses now
dynamic address translation.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 23 insertions, 4 deletions

diff --git a/arch/s390/kernel/kprobes.c b/arch/s390/kernel/kprobes.c
index b2e1dc89a8c6..a466bab6677e 100644
--- a/arch/s390/kernel/kprobes.c
+++ b/arch/s390/kernel/kprobes.c
@@ -155,15 +155,34 @@ void __kprobes get_instruction_type(struct arch_specific_insn *ainsn)
 static int __kprobes swap_instruction(void *aref)
 {
         struct ins_replace_args *args = aref;
+        u32 *addr;
+        u32 instr;
         int err = -EFAULT;
 
+        /*
+         * Text segment is read-only, hence we use stura to bypass dynamic
+         * address translation to exchange the instruction. Since stura
+         * always operates on four bytes, but we only want to exchange two
+         * bytes do some calculations to get things right. In addition we
+         * shall not cross any page boundaries (vmalloc area!) when writing
+         * the new instruction.
+         */
+        addr = (u32 *)ALIGN((unsigned long)args->ptr, 4);
+        if ((unsigned long)args->ptr & 2)
+                instr = ((*addr) & 0xffff0000) | args->new;
+        else
+                instr = ((*addr) & 0x0000ffff) | args->new << 16;
+
         asm volatile(
-                "0: mvc  0(2,%2),0(%3)\n"
-                "1: la   %0,0\n"
+                "       lra     %1,0(%1)\n"
+                "0:     stura   %2,%1\n"
+                "1:     la      %0,0\n"
                 "2:\n"
                 EX_TABLE(0b,2b)
-                : "+d" (err), "=m" (*args->ptr)
-                : "a" (args->ptr), "a" (&args->new), "m" (args->new));
+                : "+d" (err)
+                : "a" (addr), "d" (instr)
+                : "memory", "cc");
+
         return err;
 }