linux-kernel - autofs crash with latest linux-next

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Date:   Mon, 12 Oct 2020 09:54:39 +0200
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org
Subject: autofs crash with latest linux-next

Hi,

on s390 i see the following crash with linux-next:

[ 4525.432605] Unable to handle kernel pointer dereference in virtual kernel address space
[ 4525.432612] Failing address: 0000000000000000 TEID: 0000000000000483
[ 4525.432613] Fault in home space mode while using kernel ASCE.
[ 4525.432616] AS:00000000cf048007 R3:00000001fffec007 S:00000001ffff1800 P:000000000000003d 
[ 4525.432640] Oops: 0004 ilc:3 [#1] SMP 
[ 4525.432644] Modules linked in: dm_crypt encrypted_keys lcs ctcm fsm nfsv3 nfs_acl nfs lockd grace quota_v2 quota_tree tun overlay ntfs exfat vfat fat sctp vfio_pci irqbypass vfio_virqfd scsi_debug vhost_vsock vmw_vsock_virtio_transport_common vsock vhost vhost_iotlb vfio_ap kvm loop nft_counter bridge stp llc dm_service_time nft_ct nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip_set nf_tables nfnetlink sunrpc dm_multipath dm_mod scsi_dh_rdac scsi_dh_emc scsi_dh_alua s390_trng vfio_ccw vfio_mdev mdev vfio_iommu_type1 vfio zcrypt_cex4 eadm_sch sch_fq_codel ip_tables x_tables ghash_s390 prng aes_s390 des_s390 libdes sha3_512_s390 sha3_256_s390 sha512_s390 sha256_s390 sha1_s390 sha_common pkey zcrypt rng_core autofs4 [last unloaded: dummy_del_mod]
[ 4525.432691] CPU: 9 PID: 1050921 Comm: find Tainted: G           OE     5.9.0-20201011.rc8.git0.d67bc7812221.300.fc32.s390x+next #1
[ 4525.432693] Hardware name: IBM 3906 M04 704 (LPAR)
[ 4525.432694] Krnl PSW : 0704d00180000000 00000000cde29f70 (__kernel_write+0x1a0/0x2a0)
[ 4525.432702]            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
[ 4525.432704] Krnl GPRS: 0000000100067343 0000000000000000 0000000000000130 0000000000000001
[ 4525.432705]            0000000000000006 000000005f82be2f 0000000000000130 000000008c6ab568
[ 4525.432728]            0000000084441f00 0000000000000000 0000000000000130 0000000084441f00
[ 4525.432729]            0000000081476000 0000000000000001 00000000cde29ef4 000003e002f5b6f0
[ 4525.432735] Krnl Code: 00000000cde29f62: a7280000		lhi	%r2,0
                          00000000cde29f66: a7f4ff9d		brc	15,00000000cde29ea0
                         #00000000cde29f6a: e310f0f00004	lg	%r1,240(%r15)
                         >00000000cde29f70: e31090000024	stg	%r1,0(%r9)
                          00000000cde29f76: 9104b044		tm	68(%r11),4
                          00000000cde29f7a: a784000f		brc	8,00000000cde29f98
                          00000000cde29f7e: e31003400004	lg	%r1,832
                          00000000cde29f84: b904002a		lgr	%r2,%r10
[ 4525.432748] Call Trace:
[ 4525.432750]  [<00000000cde29f70>] __kernel_write+0x1a0/0x2a0 
[ 4525.432752] ([<00000000cde29ef4>] __kernel_write+0x124/0x2a0)
[ 4525.432756]  [<000003ff80004cfa>] autofs_write+0x5a/0x140 [autofs4] 
[ 4525.432758]  [<000003ff80005262>] autofs_notify_daemon.constprop.0+0x10a/0x1c8 [autofs4] 
[ 4525.432760]  [<000003ff80005872>] autofs_wait+0x552/0x718 [autofs4] 
[ 4525.432762]  [<000003ff800033ca>] autofs_mount_wait+0x5a/0xb0 [autofs4] 
[ 4525.432764]  [<000003ff800048b2>] autofs_d_automount+0x102/0x278 [autofs4] 
[ 4525.432766]  [<00000000cde398fe>] __traverse_mounts+0x9e/0x270 
[ 4525.432768]  [<00000000cde3e7ee>] step_into+0x1de/0x280 
[ 4525.432770]  [<00000000cde3f000>] open_last_lookups+0xb8/0x3f8 
[ 4525.432772]  [<00000000cde3f726>] path_openat+0x86/0x1d0 
[ 4525.432773]  [<00000000cde425b0>] do_filp_open+0x78/0x118 
[ 4525.432776]  [<00000000cde278d0>] do_sys_openat2+0xa8/0x168 
[ 4525.432778]  [<00000000cde27cfa>] __s390x_sys_openat+0x6a/0x98 
[ 4525.432781]  [<00000000ce64f2e8>] system_call+0xdc/0x2a4 
[ 4525.432782] Last Breaking-Event-Address:
[ 4525.432783]  [<00000000cde29efc>] __kernel_write+0x12c/0x2a0

This seems to be caused by the result of merging 0fb702791bf ("autofs:
use __kernel_write() for the autofs pipe writing") and 4d03e3cc5982
("fs: don't allow kernel reads and writes without iter
ops"). __kernel_write() gets now called with a NULL pointer as pos
argument, but __kernel_write expects a valid pointer as it
fetches/stores the pos value there. Is there a fix pending somewhere?

Thanks
Sven