[S390] vdso: clock_gettime of CLOCK_THREAD_CPUTIME_ID with noexec=on

The combination of noexec=on and a clock_gettime call with clock id
CLOCK_THREAD_CPUTIME_ID is broken. The vdso code switches to the
access register mode to get access to the per-cpu data structure to
execute the magic ectg instruction. After the ectg instruction the
code always switches back to the primary mode but for noexec=on the
correct mode is the secondary mode. The effect of the bug is that the
user space program looses the access to all mappings without PROT_EXEC,
e.g. the stack. The problem is fixed by restoring the mode that has
been active before the switch to the access register mode.

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

1 files changed, 9 insertions, 2 deletions

diff --git a/arch/s390/kernel/vdso64/clock_gettime.S b/arch/s390/kernel/vdso64/clock_gettime.S
index 79dbfee831ec..49106c6e6f88 100644
--- a/arch/s390/kernel/vdso64/clock_gettime.S
+++ b/arch/s390/kernel/vdso64/clock_gettime.S
@@ -88,10 +88,17 @@ __kernel_clock_gettime:
         llilh   %r4,0x0100
         sar     %a4,%r4
         lghi    %r4,0
+        epsw    %r5,0
         sacf    512                             /* Magic ectg instruction */
         .insn   ssf,0xc80100000000,__VDSO_ECTG_BASE(4),__VDSO_ECTG_USER(4),4
-        sacf    0
-        sar     %a4,%r2
+        tml     %r5,0x4000
+        jo      11f
+        tml     %r5,0x8000
+        jno     10f
+        sacf    256
+        j       11f
+10:     sacf    0
+11:     sar     %a4,%r2
         algr    %r1,%r0                         /* r1 = cputime as TOD value */
         mghi    %r1,1000                        /* convert to nanoseconds */
         srlg    %r1,%r1,12                      /* r1 = cputime in nanosec */