sunrpc: fix crash when cache_head become valid before update

I was investigating a crash in our Virtuozzo7 kernel which happened in
in svcauth_unix_set_client. I found out that we access m_client field
in ip_map structure, which was received from sunrpc_cache_lookup (we
have a bit older kernel, now the code is in sunrpc_cache_add_entry), and
these field looks uninitialized (m_client == 0x74 don't look like a
pointer) but in the cache_head in flags we see 0x1 which is CACHE_VALID.

It looks like the problem appeared from our previous fix to sunrpc (1):
commit 4ecd55ea07 ("sunrpc: fix cache_head leak due to queued
request")

And we've also found a patch already fixing our patch (2):
commit d58431eacb ("sunrpc: don't mark uninitialised items as VALID.")

Though the crash is eliminated, I think the core of the problem is not
completely fixed:

Neil in the patch (2) makes cache_head CACHE_NEGATIVE, before
cache_fresh_locked which was added in (1) to fix crash. These way
cache_is_valid won't say the cache is valid anymore and in
svcauth_unix_set_client the function cache_check will return error
instead of 0, and we don't count entry as initialized.

But it looks like we need to remove cache_fresh_locked completely in
sunrpc_cache_lookup:

In (1) we've only wanted to make cache_fresh_unlocked->cache_dequeue so
that cache_requests with no readers also release corresponding
cache_head, to fix their leak.  We with Vasily were not sure if
cache_fresh_locked and cache_fresh_unlocked should be used in pair or
not, so we've guessed to use them in pair.

Now we see that we don't want the CACHE_VALID bit set here by
cache_fresh_locked, as "valid" means "initialized" and there is no
initialization in sunrpc_cache_add_entry. Both expiry_time and
last_refresh are not used in cache_fresh_unlocked code-path and also not
required for the initial fix.

So to conclude cache_fresh_locked was called by mistake, and we can just
safely remove it instead of crutching it with CACHE_NEGATIVE. It looks
ideologically better for me. Hope I don't miss something here.

Here is our crash backtrace:
[13108726.326291] BUG: unable to handle kernel NULL pointer dereference at 0000000000000074
[13108726.326365] IP: [<ffffffffc01f79eb>] svcauth_unix_set_client+0x2ab/0x520 [sunrpc]
[13108726.326448] PGD 0
[13108726.326468] Oops: 0002 [#1] SMP
[13108726.326497] Modules linked in: nbd isofs xfs loop kpatch_cumulative_81_0_r1(O) xt_physdev nfnetlink_queue bluetooth rfkill ip6table_nat nf_nat_ipv6 ip_vs_wrr ip_vs_wlc ip_vs_sh nf_conntrack_netlink ip_vs_sed ip_vs_pe_sip nf_conntrack_sip ip_vs_nq ip_vs_lc ip_vs_lblcr ip_vs_lblc ip_vs_ftp ip_vs_dh nf_nat_ftp nf_conntrack_ftp iptable_raw xt_recent nf_log_ipv6 xt_hl ip6t_rt nf_log_ipv4 nf_log_common xt_LOG xt_limit xt_TCPMSS xt_tcpmss vxlan ip6_udp_tunnel udp_tunnel xt_statistic xt_NFLOG nfnetlink_log dummy xt_mark xt_REDIRECT nf_nat_redirect raw_diag udp_diag tcp_diag inet_diag netlink_diag af_packet_diag unix_diag rpcsec_gss_krb5 xt_addrtype ip6t_rpfilter ipt_REJECT nf_reject_ipv4 ip6t_REJECT nf_reject_ipv6 ebtable_nat ebtable_broute nf_conntrack_ipv6 nf_defrag_ipv6 ip6table_mangle ip6table_raw nfsv4
[13108726.327173]  dns_resolver cls_u32 binfmt_misc arptable_filter arp_tables ip6table_filter ip6_tables devlink fuse_kio_pcs ipt_MASQUERADE nf_nat_masquerade_ipv4 xt_nat iptable_nat nf_nat_ipv4 xt_comment nf_conntrack_ipv4 nf_defrag_ipv4 xt_wdog_tmo xt_multiport bonding xt_set xt_conntrack iptable_filter iptable_mangle kpatch(O) ebtable_filter ebt_among ebtables ip_set_hash_ip ip_set nfnetlink vfat fat skx_edac intel_powerclamp coretemp intel_rapl iosf_mbi kvm_intel kvm irqbypass fuse pcspkr ses enclosure joydev sg mei_me hpwdt hpilo lpc_ich mei ipmi_si shpchp ipmi_devintf ipmi_msghandler xt_ipvs acpi_power_meter ip_vs_rr nfsv3 nfsd auth_rpcgss nfs_acl nfs lockd grace fscache nf_nat cls_fw sch_htb sch_cbq sch_sfq ip_vs em_u32 nf_conntrack tun br_netfilter veth overlay ip6_vzprivnet ip6_vznetstat ip_vznetstat
[13108726.327817]  ip_vzprivnet vziolimit vzevent vzlist vzstat vznetstat vznetdev vzmon vzdev bridge pio_kaio pio_nfs pio_direct pfmt_raw pfmt_ploop1 ploop ip_tables ext4 mbcache jbd2 sd_mod crc_t10dif crct10dif_generic mgag200 i2c_algo_bit drm_kms_helper scsi_transport_iscsi 8021q syscopyarea sysfillrect garp sysimgblt fb_sys_fops mrp stp ttm llc bnx2x crct10dif_pclmul crct10dif_common crc32_pclmul crc32c_intel drm dm_multipath ghash_clmulni_intel uas aesni_intel lrw gf128mul glue_helper ablk_helper cryptd tg3 smartpqi scsi_transport_sas mdio libcrc32c i2c_core usb_storage ptp pps_core wmi sunrpc dm_mirror dm_region_hash dm_log dm_mod [last unloaded: kpatch_cumulative_82_0_r1]
[13108726.328403] CPU: 35 PID: 63742 Comm: nfsd ve: 51332 Kdump: loaded Tainted: G        W  O   ------------   3.10.0-862.20.2.vz7.73.29 #1 73.29
[13108726.328491] Hardware name: HPE ProLiant DL360 Gen10/ProLiant DL360 Gen10, BIOS U32 10/02/2018
[13108726.328554] task: ffffa0a6a41b1160 ti: ffffa0c2a74bc000 task.ti: ffffa0c2a74bc000
[13108726.328610] RIP: 0010:[<ffffffffc01f79eb>]  [<ffffffffc01f79eb>] svcauth_unix_set_client+0x2ab/0x520 [sunrpc]
[13108726.328706] RSP: 0018:ffffa0c2a74bfd80  EFLAGS: 00010246
[13108726.328750] RAX: 0000000000000001 RBX: ffffa0a6183ae000 RCX: 0000000000000000
[13108726.328811] RDX: 0000000000000074 RSI: 0000000000000286 RDI: ffffa0c2a74bfcf0
[13108726.328864] RBP: ffffa0c2a74bfe00 R08: ffffa0bab8c22960 R09: 0000000000000001
[13108726.328916] R10: 0000000000000001 R11: 0000000000000001 R12: ffffa0a32aa7f000
[13108726.328969] R13: ffffa0a6183afac0 R14: ffffa0c233d88d00 R15: ffffa0c2a74bfdb4
[13108726.329022] FS:  0000000000000000(0000) GS:ffffa0e17f9c0000(0000) knlGS:0000000000000000
[13108726.329081] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[13108726.332311] CR2: 0000000000000074 CR3: 00000026a1b28000 CR4: 00000000007607e0
[13108726.334606] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[13108726.336754] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[13108726.338908] PKRU: 00000000
[13108726.341047] Call Trace:
[13108726.343074]  [<ffffffff8a2c78b4>] ? groups_alloc+0x34/0x110
[13108726.344837]  [<ffffffffc01f5eb4>] svc_set_client+0x24/0x30 [sunrpc]
[13108726.346631]  [<ffffffffc01f2ac1>] svc_process_common+0x241/0x710 [sunrpc]
[13108726.348332]  [<ffffffffc01f3093>] svc_process+0x103/0x190 [sunrpc]
[13108726.350016]  [<ffffffffc07d605f>] nfsd+0xdf/0x150 [nfsd]
[13108726.351735]  [<ffffffffc07d5f80>] ? nfsd_destroy+0x80/0x80 [nfsd]
[13108726.353459]  [<ffffffff8a2bf741>] kthread+0xd1/0xe0
[13108726.355195]  [<ffffffff8a2bf670>] ? create_kthread+0x60/0x60
[13108726.356896]  [<ffffffff8a9556dd>] ret_from_fork_nospec_begin+0x7/0x21
[13108726.358577]  [<ffffffff8a2bf670>] ? create_kthread+0x60/0x60
[13108726.360240] Code: 4c 8b 45 98 0f 8e 2e 01 00 00 83 f8 fe 0f 84 76 fe ff ff 85 c0 0f 85 2b 01 00 00 49 8b 50 40 b8 01 00 00 00 48 89 93 d0 1a 00 00 <f0> 0f c1 02 83 c0 01 83 f8 01 0f 8e 53 02 00 00 49 8b 44 24 38
[13108726.363769] RIP  [<ffffffffc01f79eb>] svcauth_unix_set_client+0x2ab/0x520 [sunrpc]
[13108726.365530]  RSP <ffffa0c2a74bfd80>
[13108726.367179] CR2: 0000000000000074

Fixes: d58431eacb ("sunrpc: don't mark uninitialised items as VALID.")
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/cache.c

@@ -53,9 +53,6 @@ static void cache_init(struct cache_head *h, struct cache_detail *detail)
 	h->last_refresh = now;
 }
 
-static inline int cache_is_valid(struct cache_head *h);
-static void cache_fresh_locked(struct cache_head *head, time_t expiry,
-				struct cache_detail *detail);
 static void cache_fresh_unlocked(struct cache_head *head,
 				struct cache_detail *detail);
 
@@ -105,9 +102,6 @@ static struct cache_head *sunrpc_cache_add_entry(struct cache_detail *detail,
 			if (cache_is_expired(detail, tmp)) {
 				hlist_del_init_rcu(&tmp->cache_list);
 				detail->entries --;
-				if (cache_is_valid(tmp) == -EAGAIN)
-					set_bit(CACHE_NEGATIVE, &tmp->flags);
-				cache_fresh_locked(tmp, 0, detail);
 				freeme = tmp;
 				break;
 			}