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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20190603090635.10631-2-david@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Mon, 03 Jun 2019 09:07:41 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH v2 01/22] s390x/tcg: Store only the necessary amount of doublewords for STFLE X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Stefan Liebler , Thomas Huth , Denys Vlasenko , Andreas Krebbel , Cornelia Huck , Richard Henderson , Pino Toscano , Christian Borntraeger , qemu-s390x@nongnu.org, Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 03.06.19 11:06, 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 > 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 <__stack_chk_fail@plt> > > 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. > > Cc: Stefan Liebler > Cc: Andreas Krebbel > Reviewed-by: Richard Henderson > Signed-off-by: David Hildenbrand > --- > target/s390x/misc_helper.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/target/s390x/misc_helper.c b/target/s390x/misc_helper.c > index 34476134a4..10aa617cf9 100644 > --- a/target/s390x/misc_helper.c > +++ b/target/s390x/misc_helper.c > @@ -678,7 +678,13 @@ uint32_t HELPER(stfle)(CPUS390XState *env, uint64_t addr) > > prepare_stfl(); > max_bytes = ROUND_UP(used_stfl_bytes, 8); > - for (i = 0; i < count_bytes; ++i) { > + > + /* > + * The PoP says that doublewords beyond the highest-numbered facility > + * bit may or may not be stored. However, existing hardware appears to > + * not store the words, and existing software depend on that. > + */ > + for (i = 0; i < MIN(count_bytes, max_bytes); ++i) { > cpu_stb_data_ra(env, addr + i, stfl_bytes[i], ra); > } > > Not intended to be included in this series, please ignore :) -- Thanks, David / dhildenb