[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>

diff --git a/arch/s390/kernel/vdso64/clock_gettime.S b/arch/s390/kernel/vdso64/clock_gettime.S
index 79dbfee..49106c6 100644 (file)
--- 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 */