[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [qemu-s390x] [PATCH v1 2/2] s390x/tcg: Store only the necessary amou

From: Stefan Liebler
Subject: Re: [qemu-s390x] [PATCH v1 2/2] s390x/tcg: Store only the necessary amount of doublewords for STFLE
Date: Mon, 3 Jun 2019 12:38:53 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1

On 5/31/19 4:56 PM, David Hildenbrand wrote:
The PoP (z14, 7-382) says:
     Doublewords to the right of the doubleword in which the
     highest-numbered facility bit is assigned for a model
     may or may not be stored.

However, stack protection in certain binaries can't deal with that.
"gzip" example code:

f1b4:       a7 08 00 03             lhi     %r0,3
f1b8:       b2 b0 f0 a0             stfle   160(%r15)
f1bc:       e3 20 f0 b2 00 90       llgc    %r2,178(%r15)
f1c2:       c0 2b 00 00 00 01       nilf    %r2,1
f1c8:       b2 4f 00 10             ear     %r1,%a0
f1cc:       b9 14 00 22             lgfr    %r2,%r2
f1d0:       eb 11 00 20 00 0d       sllg    %r1,%r1,32
f1d6:       b2 4f 00 11             ear     %r1,%a1
f1da:       d5 07 f0 b8 10 28       clc     184(8,%r15),40(%r1)
f1e0:       a7 74 00 06             jne     f1ec <file_read@@Base+0x1bc>
f1e4:       eb ef f1 30 00 04       lmg     %r14,%r15,304(%r15)
f1ea:       07 fe                   br      %r14
f1ec:       c0 e5 ff ff 9d 6e       brasl   %r14,2cc8 <address@hidden>

In QEMU, we currently have:
     max_bytes = 24
the code asks for (3 + 1) doublewords == 32 bytes.

If we write 32 bytes instead of only 24, and return "2 + 1" doublewords
("one less than the number of doulewords needed to contain all of the
  facility bits"), the example code detects a stack corruption.

In my opinion, the code is wrong. However, it seems to work fine on
real machines. So let's limit storing to the minimum of the requested
and the maximum doublewords.
Hi David,

Thanks for catching this. I've reported the "gzip" example to Ilya and indeed, r0 is setup too large. He will fix it in gzip.

You've mentioned, that this is detected in certain binaries.
Can you please share those occurrences.
Perhaps on future machines with further facility bits, stfle will write beyond the provided doubleword-array on a real machine or in qemu.


Cc: Stefan Liebler <address@hidden>
Cc: Andreas Krebbel <address@hidden>
Signed-off-by: David Hildenbrand <address@hidden>
  target/s390x/misc_helper.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/s390x/misc_helper.c b/target/s390x/misc_helper.c
index 34476134a4..b561c5781b 100644
--- a/target/s390x/misc_helper.c
+++ b/target/s390x/misc_helper.c
@@ -678,7 +678,7 @@ uint32_t HELPER(stfle)(CPUS390XState *env, uint64_t addr)
      max_bytes = ROUND_UP(used_stfl_bytes, 8);
-    for (i = 0; i < count_bytes; ++i) {
+    for (i = 0; i < MIN(count_bytes, max_bytes); ++i) {
          cpu_stb_data_ra(env, addr + i, stfl_bytes[i], ra);

reply via email to

[Prev in Thread] Current Thread [Next in Thread]