Re: [PATCH v2 1/5] KVM: s390: vsie: Fix region 1 ASCE sanity shadow address checks

[David Hildenbrand <david@redhat.com>]

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.

-- 
Thanks,

David / dhildenb