s390: fix test_fp_ctl inline assembly contraints

commit bcf4dd5f9ee096bd1510f838dd4750c35df4e38b upstream.

The test_fp_ctl function is used to test if a given value is a valid
floating-point control. The inline assembly in test_fp_ctl uses an
incorrect constraint for the 'orig_fpc' variable. If the compiler
chooses the same register for 'fpc' and 'orig_fpc' the test_fp_ctl()
function always returns true. This allows user space to trigger
kernel oopses with invalid floating-point control values on the
signal stack.

This problem has been introduced with git commit 4725c86055f5bbdcdf
"s390: fix save and restore of the floating-point-control register"

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/s390/include/asm/switch_to.h b/arch/s390/include/asm/switch_to.h
index 18ea9e3f8142954926b8fea3afe79ce90193a4b3..716df3a992d0162439e1cacc77013a6d4ddc35ca 100644 (file)
--- a/arch/s390/include/asm/switch_to.h
+++ b/arch/s390/include/asm/switch_to.h
@@ -28,7 +28,7 @@ static inline int test_fp_ctl(u32 fpc)
                "       la      %0,0\n"
                "1:\n"
                EX_TABLE(0b,1b)
-               : "=d" (rc), "=d" (orig_fpc)
+               : "=d" (rc), "=&d" (orig_fpc)
                : "d" (fpc), "0" (-EINVAL));
        return rc;
 }