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SubjectRe: [PATCH v2 1/5] KVM: s390: vsie: Fix region 1 ASCE sanity shadow address checks
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On 07.04.20 09:52, Christian Borntraeger wrote:
>
>
> On 07.04.20 09:49, David Hildenbrand wrote:
>> On 07.04.20 09:33, Christian Borntraeger wrote:
>>>
>>> On 03.04.20 17:30, David Hildenbrand wrote:
>>>> In case we have a region 1 ASCE, our shadow/g3 address can have any value.
>>>> Unfortunately, (-1UL << 64) is undefined and triggers sometimes,
>>>> rejecting valid shadow addresses when trying to walk our shadow table
>>>> hierarchy.
>>>>
>>>> The result is that the prefix cannot get mapped and will loop basically
>>>> forever trying to map it (-EAGAIN loop).
>>>>
>>>> After all, the broken check is only a sanity check, our table shadowing
>>>> code in kvm_s390_shadow_tables() already checks these conditions, injecting
>>>> proper translation exceptions. Turn it into a WARN_ON_ONCE().
>>>
>>> After some testing I now triggered this warning:
>>>
>>> [ 541.633114] ------------[ cut here ]------------
>>> [ 541.633128] WARNING: CPU: 38 PID: 2812 at arch/s390/mm/gmap.c:799 gmap_shadow_pgt_lookup+0x98/0x1a0
>>> [ 541.633129] Modules linked in: vhost_net vhost macvtap macvlan tap kvm xt_CHECKSUM xt_MASQUERADE nf_nat_tftp nf_conntrack_tftp xt_CT tun bridge stp llc xt_tcpudp ip6t_REJECT nf_reject_ipv6 ip6t_rpfilter ipt_REJECT nf_reject_ipv4 xt_conntrack ip6table_nat ip6table_mangle ip6table_raw ip6table_security iptable_nat nf_nat iptable_mangle iptable_raw iptable_security nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip_set nfnetlink ip6table_filter ip6_tables iptable_filter rpcrdma sunrpc rdma_ucm rdma_cm iw_cm ib_cm configfs mlx5_ib s390_trng ghash_s390 prng aes_s390 ib_uverbs des_s390 ib_core libdes sha3_512_s390 genwqe_card sha3_256_s390 vfio_ccw crc_itu_t vfio_mdev sha512_s390 mdev vfio_iommu_type1 sha1_s390 vfio eadm_sch zcrypt_cex4 sch_fq_codel ip_tables x_tables mlx5_core sha256_s390 sha_common pkey zcrypt rng_core autofs4
>>> [ 541.633164] CPU: 38 PID: 2812 Comm: CPU 0/KVM Not tainted 5.6.0+ #354
>>> [ 541.633166] Hardware name: IBM 3906 M04 704 (LPAR)
>>> [ 541.633167] Krnl PSW : 0704d00180000000 00000014e05dc454 (gmap_shadow_pgt_lookup+0x9c/0x1a0)
>>> [ 541.633169] R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:1 PM:0 RI:0 EA:3
>>> [ 541.633171] Krnl GPRS: 0000000000000000 0000001f00000000 0000001f487b8000 ffffffff80000000
>>> [ 541.633172] ffffffffffffffff 000003e003defa18 000003e003defa1c 000003e003defa18
>>> [ 541.633173] fffffffffffff000 000003e003defa18 000003e003defa28 0000001f70e06300
>>> [ 541.633174] 0000001f43484000 00000000043ed200 000003e003def978 000003e003def920
>>> [ 541.633203] Krnl Code: 00000014e05dc448: b9800038 ngr %r3,%r8
>>> 00000014e05dc44c: a7840014 brc 8,00000014e05dc474
>>> #00000014e05dc450: af000000 mc 0,0
>>> >00000014e05dc454: a728fff5 lhi %r2,-11
>>> 00000014e05dc458: a7180000 lhi %r1,0
>>> 00000014e05dc45c: b2fa0070 niai 7,0
>>> 00000014e05dc460: 4010b04a sth %r1,74(%r11)
>>> 00000014e05dc464: b9140022 lgfr %r2,%r2
>>> [ 541.633215] Call Trace:
>>> [ 541.633218] [<00000014e05dc454>] gmap_shadow_pgt_lookup+0x9c/0x1a0
>>> [ 541.633257] [<000003ff804c57d6>] kvm_s390_shadow_fault+0x66/0x1e8 [kvm]
>>> [ 541.633265] [<000003ff804c72dc>] vsie_run+0x43c/0x710 [kvm]
>>> [ 541.633273] [<000003ff804c85ca>] kvm_s390_handle_vsie+0x632/0x750 [kvm]
>>> [ 541.633281] [<000003ff804c123c>] kvm_s390_handle_b2+0x84/0x4e0 [kvm]
>>> [ 541.633289] [<000003ff804b46b2>] kvm_handle_sie_intercept+0x172/0xcb8 [kvm]
>>> [ 541.633297] [<000003ff804b18a8>] __vcpu_run+0x658/0xc90 [kvm]
>>> [ 541.633305] [<000003ff804b2920>] kvm_arch_vcpu_ioctl_run+0x248/0x858 [kvm]
>>> [ 541.633313] [<000003ff8049d454>] kvm_vcpu_ioctl+0x284/0x7b0 [kvm]
>>> [ 541.633316] [<00000014e087d5ae>] ksys_ioctl+0xae/0xe8
>>> [ 541.633318] [<00000014e087d652>] __s390x_sys_ioctl+0x2a/0x38
>>> [ 541.633323] [<00000014e0ff02a2>] system_call+0x2a6/0x2c8
>>> [ 541.633323] Last Breaking-Event-Address:
>>> [ 541.633334] [<000003ff804983e0>] kvm_running_vcpu+0x3ea9ee997d8/0x3ea9ee99950 [kvm]
>>> [ 541.633335] ---[ end trace f69b6021855ea189 ]---
>>>
>>>
>>> Unfortunately no dump at that point in time.
>>> I have other tests which are clearly fixed by this patch, so we should propbably go forward anyway.
>>> Question is, is this just another bug we need to fix or is the assumption that somebody else checked
>>> all conditions so we can warn false?
>>
>> Yeah, I think it is via
>>
>> kvm_s390_shadow_fault()->gmap_shadow_pgt_lookup()->gmap_table_walk()
>>
>> where we just peek if there is already something shadowed. If not, we go
>> via the full kvm_s390_shadow_tables() path.
>>
>> So we could either do sanity checks in gmap_shadow_pgt_lookup(), or
>> rather drop the WARN_ON_ONCE. I think the latter makes sense, now that
>> we understood the problem.
>
> Ok, so I will drop the WARN_ON_ONCE and fixup the commit message.
>

Perfect, thanks!

--
Thanks,

David / dhildenb

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