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Date:   Tue, 25 Jul 2023 22:06:22 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Juergen Gross <jgross@suse.com>
Cc:     Nathan Chancellor <nathan@kernel.org>,
        Bagas Sanjaya <bagasdotme@gmail.com>,
        Jan Beulich <jbeulich@suse.com>,
        "David S. Miller" <davem@davemloft.net>,
        sander44 <ionut_n2001@yahoo.com>,
        Linux Xen <xen-devel@lists.xenproject.org>,
        Linux BPF <bpf@vger.kernel.org>,
        Linux Networking <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Regressions <regressions@lists.linux.dev>,
        gustavoars@kernel.org
Subject: Re: Fwd: UBSAN: index 1 is out of range for type
 'xen_netif_rx_sring_entry [1]'
Message-ID: <202307252204.09AC0089@keescook>
References: <7e3841ce-011d-5ba6-9dae-7b14e07b5c4b@gmail.com>
 <20230723000657.GA878540@dev-arch.thelio-3990X>
 <e208365f-dbc6-06d1-ccc9-3b2e945a0bff@suse.com>
 <73019154-6a34-f714-96b5-b43038569feb@suse.com>
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In-Reply-To: <73019154-6a34-f714-96b5-b43038569feb@suse.com>
Precedence: bulk

On Tue, Jul 25, 2023 at 03:34:26PM +0200, Juergen Gross wrote:
> On 25.07.23 15:24, Juergen Gross wrote:
> > On 23.07.23 02:06, Nathan Chancellor wrote:
> > > On Sat, Jul 22, 2023 at 07:21:05AM +0700, Bagas Sanjaya wrote:
> > > > Hi,
> > > > 
> > > > I notice a regression report on Bugzilla [1]. Quoting from it:
> > > > 
> > > > > Hi Kernel Team,
> > > > > 
> > > > > I rebuild today latest version from mainline repo.
> > > > > And i notice issue regarding xen-netfront.c.
> > > > > 
> > > > > Error:
> > > > > [??? 3.477400] ================================================================================
> > > > > [??? 3.477633] UBSAN: array-index-out-of-bounds in
> > > > > drivers/net/xen-netfront.c:1291:3
> > > > > [??? 3.477858] index 1 is out of range for type 'xen_netif_rx_sring_entry [1]'
> > > > > [??? 3.478085] CPU: 0 PID: 700 Comm: NetworkManager Not
> > > > > tainted 6.5.0-rc2-1-generation1 #3
> > > > > [??? 3.478088] Hardware name: Intel Corporation
> > > > > W2600CR/W2600CR, BIOS SE5C600.86B.02.06.0007.082420181029
> > > > > 01/13/2022
> > > > > [??? 3.478090] Call Trace:
> > > > > [??? 3.478092]? <IRQ>
> > > > > [??? 3.478097]? dump_stack_lvl+0x48/0x70
> > > > > [??? 3.478105]? dump_stack+0x10/0x20
> > > > > [??? 3.478107]? __ubsan_handle_out_of_bounds+0xc6/0x110
> > > > > [??? 3.478114]? xennet_poll+0xa94/0xac0
> > > > > [??? 3.478118]? ? generic_smp_call_function_single_interrupt+0x13/0x20
> > > > > [??? 3.478125]? __napi_poll+0x33/0x200
> > > > > [??? 3.478131]? net_rx_action+0x181/0x2e0
> > > > > [??? 3.478135]? __do_softirq+0xd9/0x346
> > > > > [??? 3.478139]? do_softirq.part.0+0x41/0x80
> > > > > [??? 3.478144]? </IRQ>
> > > > > [??? 3.478145]? <TASK>
> > > > > [??? 3.478146]? __local_bh_enable_ip+0x72/0x80
> > > > > [??? 3.478149]? _raw_spin_unlock_bh+0x1d/0x30
> > > > > [??? 3.478151]? xennet_open+0x75/0x160
> > > > > [??? 3.478154]? __dev_open+0x105/0x1d0
> > > > > [??? 3.478156]? __dev_change_flags+0x1b5/0x230
> > > > > [??? 3.478158]? dev_change_flags+0x27/0x80
> > > > > [??? 3.478160]? do_setlink+0x3d2/0x12b0
> > > > > [??? 3.478164]? ? __nla_validate_parse+0x5b/0xdb0
> > > > > [??? 3.478169]? __rtnl_newlink+0x6f6/0xb10
> > > > > [??? 3.478173]? ? rtnl_newlink+0x2f/0x80
> > > > > [??? 3.478177]? rtnl_newlink+0x48/0x80
> > > > > [??? 3.478180]? rtnetlink_rcv_msg+0x170/0x430
> > > > > [??? 3.478183]? ? fib6_clean_node+0xad/0x190
> > > > > [??? 3.478188]? ? __pfx_rtnetlink_rcv_msg+0x10/0x10
> > > > > [??? 3.478191]? netlink_rcv_skb+0x5d/0x110
> > > > > [??? 3.478195]? rtnetlink_rcv+0x15/0x30
> > > > > [??? 3.478198]? netlink_unicast+0x247/0x390
> > > > > [??? 3.478200]? netlink_sendmsg+0x25e/0x4e0
> > > > > [??? 3.478202]? sock_sendmsg+0xaf/0xc0
> > > > > [??? 3.478204]? ____sys_sendmsg+0x2a9/0x350
> > > > > [??? 3.478206]? ___sys_sendmsg+0x9a/0xf0
> > > > > [??? 3.478212]? ? _copy_from_iter+0x80/0x4a0
> > > > > [??? 3.478217]? __sys_sendmsg+0x89/0xf0
> > > > > [??? 3.478220]? __x64_sys_sendmsg+0x1d/0x30
> > > > > [??? 3.478222]? do_syscall_64+0x5c/0x90
> > > > > [??? 3.478226]? ? do_syscall_64+0x68/0x90
> > > > > [??? 3.478228]? ? ksys_write+0xe6/0x100
> > > > > [??? 3.478232]? ? exit_to_user_mode_prepare+0x49/0x220
> > > > > [??? 3.478236]? ? syscall_exit_to_user_mode+0x1b/0x50
> > > > > [??? 3.478240]? ? do_syscall_64+0x68/0x90
> > > > > [??? 3.478242]? ? do_syscall_64+0x68/0x90
> > > > > [??? 3.478243]? ? irqentry_exit_to_user_mode+0x9/0x30
> > > > > [??? 3.478246]? ? irqentry_exit+0x43/0x50
> > > > > [??? 3.478248]? ? sysvec_xen_hvm_callback+0x4b/0xd0
> > > > > [??? 3.478250]? entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> > > > > [??? 3.478253] RIP: 0033:0x7f973c244e4d
> > > > > [??? 3.478268] Code: 28 89 54 24 1c 48 89 74 24 10 89 7c 24
> > > > > 08 e8 ca ee ff ff 8b 54 24 1c 48 8b 74 24 10 41 89 c0 8b 7c
> > > > > 24 08 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 33 44 89
> > > > > c7 48 89 44 24 08 e8 fe ee ff ff 48
> > > > > [??? 3.478270] RSP: 002b:00007fff4777f470 EFLAGS: 00000293
> > > > > ORIG_RAX: 000000000000002e
> > > > > [??? 3.478273] RAX: ffffffffffffffda RBX: 00005583087c6480
> > > > > RCX: 00007f973c244e4d
> > > > > [??? 3.478274] RDX: 0000000000000000 RSI: 00007fff4777f4c0
> > > > > RDI: 000000000000000c
> > > > > [??? 3.478276] RBP: 00007fff4777f4c0 R08: 0000000000000000
> > > > > R09: 0000000000000000
> > > > > [??? 3.478277] R10: 0000000000000000 R11: 0000000000000293
> > > > > R12: 00005583087c6480
> > > > > [??? 3.478279] R13: 00007fff4777f668 R14: 00007fff4777f65c
> > > > > R15: 0000000000000000
> > > > > [??? 3.478283]? </TASK>
> > > > > [??? 3.478284] ================================================================================
> > > > > [??? 3.685513] ================================================================================
> > > > > [??? 3.685751] UBSAN: array-index-out-of-bounds in
> > > > > drivers/net/xen-netfront.c:485:7
> > > > > [??? 3.686111] index 1 is out of range for type 'xen_netif_tx_sring_entry [1]'
> > > > > [??? 3.686379] CPU: 1 PID: 697 Comm: avahi-daemon Not
> > > > > tainted 6.5.0-rc2-1-generation1 #3
> > > > > [??? 3.686381] Hardware name: Intel Corporation
> > > > > W2600CR/W2600CR, BIOS SE5C600.86B.02.06.0007.082420181029
> > > > > 01/13/2022
> > > > > [??? 3.686385] Call Trace:
> > > > > [??? 3.686388]? <TASK>
> > > > > [??? 3.686391]? dump_stack_lvl+0x48/0x70
> > > > > [??? 3.686399]? dump_stack+0x10/0x20
> > > > > [??? 3.686399]? __ubsan_handle_out_of_bounds+0xc6/0x110
> > > > > [??? 3.686403]? xennet_tx_setup_grant+0x1f7/0x230
> > > > > [??? 3.686403]? ? __pfx_xennet_tx_setup_grant+0x10/0x10
> > > > > [??? 3.686403]? gnttab_foreach_grant_in_range+0x5c/0x100
> > > > > [??? 3.686415]? xennet_start_xmit+0x428/0x990
> > > > > [??? 3.686415]? ? kmem_cache_alloc_node+0x1b1/0x3b0
> > > > > [??? 3.686415]? dev_hard_start_xmit+0x68/0x1e0
> > > > > [??? 3.686415]? sch_direct_xmit+0x10b/0x350
> > > > > [??? 3.686415]? __dev_queue_xmit+0x512/0xda0
> > > > > [??? 3.686439]? ? ___neigh_create+0x6cb/0x970
> > > > > [??? 3.686439]? neigh_resolve_output+0x118/0x1e0
> > > > > [??? 3.686446]? ip_finish_output2+0x181/0x540
> > > > > [??? 3.686450]? ? netif_rx_internal+0x46/0x140
> > > > > [??? 3.686456]? __ip_finish_output+0xb6/0x180
> > > > > [??? 3.686456]? ? dev_loopback_xmit+0x86/0x110
> > > > > [??? 3.686456]? ip_finish_output+0x29/0x100
> > > > > [??? 3.686456]? ip_mc_output+0x95/0x2e0
> > > > > [??? 3.686456]? ? __pfx_ip_finish_output+0x10/0x10
> > > > > [??? 3.686456]? ip_send_skb+0x9f/0xb0
> > > > > [??? 3.686456]? udp_send_skb+0x158/0x380
> > > > > [??? 3.686475]? udp_sendmsg+0xb84/0xf20
> > > > > [??? 3.686475]? ? do_sys_poll+0x3a1/0x5f0
> > > > > [??? 3.686483]? ? __pfx_ip_generic_getfrag+0x10/0x10
> > > > > [??? 3.686483]? inet_sendmsg+0x76/0x80
> > > > > [??? 3.686483]? ? inet_sendmsg+0x76/0x80
> > > > > [??? 3.686483]? sock_sendmsg+0xa8/0xc0
> > > > > [??? 3.686483]? ? _copy_from_user+0x30/0xa0
> > > > > [??? 3.686483]? ____sys_sendmsg+0x2a9/0x350
> > > > > [??? 3.686483]? ___sys_sendmsg+0x9a/0xf0
> > > > > [??? 3.686483]? __sys_sendmsg+0x89/0xf0
> > > > > [??? 3.686483]? __x64_sys_sendmsg+0x1d/0x30
> > > > > [??? 3.686483]? do_syscall_64+0x5c/0x90
> > > > > [??? 3.686483]? ? exit_to_user_mode_prepare+0x49/0x220
> > > > > [??? 3.686483]? ? syscall_exit_to_user_mode+0x1b/0x50
> > > > > [??? 3.686483]? ? do_syscall_64+0x68/0x90
> > > > > [??? 3.686483]? ? syscall_exit_to_user_mode+0x1b/0x50
> > > > > [??? 3.686483]? ? do_syscall_64+0x68/0x90
> > > > > [??? 3.686483]? entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> > > > > [??? 3.686483] RIP: 0033:0x7ff365942e13
> > > > > [??? 3.686483] Code: 8b 15 b9 a1 00 00 f7 d8 64 89 02 48 c7
> > > > > c0 ff ff ff ff eb b8 0f 1f 00 64 8b 04 25 18 00 00 00 85 c0
> > > > > 75 14 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 55 c3 0f
> > > > > 1f 40 00 48 83 ec 28 89 54 24 1c 48
> > > > > [??? 3.686483] RSP: 002b:00007ffc7bf1ca78 EFLAGS: 00000246
> > > > > ORIG_RAX: 000000000000002e
> > > > > [??? 3.686483] RAX: ffffffffffffffda RBX: 00005596bd24c900
> > > > > RCX: 00007ff365942e13
> > > > > [??? 3.686483] RDX: 0000000000000000 RSI: 00007ffc7bf1cb30
> > > > > RDI: 000000000000000c
> > > > > [??? 3.686483] RBP: 000000000000000c R08: 0000000000000004
> > > > > R09: 0000000000000019
> > > > > [??? 3.686483] R10: 00007ff365a1ca94 R11: 0000000000000246
> > > > > R12: 00007ffc7bf1cb30
> > > > > [??? 3.686483] R13: 0000000000000002 R14: 00005596bd235f9c
> > > > > R15: 0000000000000000
> > > > > [??? 3.686483]? </TASK>
> > > > > [??? 3.686483] ================================================================================
> > > > > [??? 3.686858] ================================================================================
> > > > > [??? 3.687190] UBSAN: array-index-out-of-bounds in
> > > > > drivers/net/xen-netfront.c:413:4
> > > > > [??? 3.687501] index 1 is out of range for type 'xen_netif_tx_sring_entry [1]'
> > > > > [??? 3.687800] CPU: 18 PID: 0 Comm: swapper/18 Not tainted
> > > > > 6.5.0-rc2-1-generation1 #3
> > > > > [??? 3.687804] Hardware name: Intel Corporation
> > > > > W2600CR/W2600CR, BIOS SE5C600.86B.02.06.0007.082420181029
> > > > > 01/13/2022
> > > > > [??? 3.687806] Call Trace:
> > > > > [??? 3.687808]? <IRQ>
> > > > > [??? 3.687812]? dump_stack_lvl+0x48/0x70
> > > > > [??? 3.687819]? dump_stack+0x10/0x20
> > > > > [??? 3.687821]? __ubsan_handle_out_of_bounds+0xc6/0x110
> > > > > [??? 3.687827]? xennet_tx_buf_gc+0x34a/0x440
> > > > > [??? 3.687831]? xennet_handle_tx.constprop.0+0x49/0x90
> > > > > [??? 3.687834]? xennet_tx_interrupt+0x32/0x70
> > > > > [??? 3.687837]? __handle_irq_event_percpu+0x4f/0x1b0
> > > > > [??? 3.687842]? handle_irq_event+0x39/0x80
> > > > > [??? 3.687846]? handle_edge_irq+0x8c/0x230
> > > > > [??? 3.687849]? handle_irq_desc+0x40/0x60
> > > > > [??? 3.687851]? generic_handle_irq+0x1f/0x30
> > > > > [??? 3.687854]? handle_irq_for_port+0x8e/0x180
> > > > > [??? 3.687858]? ? _raw_spin_unlock_irqrestore+0x11/0x60
> > > > > [??? 3.687861]? __evtchn_fifo_handle_events+0x221/0x330
> > > > > [??? 3.687866]? evtchn_fifo_handle_events+0xe/0x20
> > > > > [??? 3.687869]? __xen_evtchn_do_upcall+0x72/0xd0
> > > > > [??? 3.687873]? xen_hvm_evtchn_do_upcall+0xe/0x20
> > > > > [??? 3.687876]? __sysvec_xen_hvm_callback+0x53/0x70
> > > > > [??? 3.687880]? sysvec_xen_hvm_callback+0x8d/0xd0
> > > > > [??? 3.687884]? </IRQ>
> > > > > [??? 3.687885]? <TASK>
> > > > > [??? 3.687886]? asm_sysvec_xen_hvm_callback+0x1b/0x20
> > > > > [??? 3.687891] RIP: 0010:pv_native_safe_halt+0xb/0x10
> > > > > [??? 3.687896] Code: 0b 66 66 2e 0f 1f 84 00 00 00 00 00 0f
> > > > > 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 eb 07
> > > > > 0f 00 2d 49 cc 33 00 fb f4 <c3> cc cc cc cc 90 90 90 90 90
> > > > > 90 90 90 90 90 90 90 90 90 90 90 55
> > > > > [??? 3.687898] RSP: 0000:ffffad85c0147e08 EFLAGS: 00000246
> > > > > [??? 3.687901] RAX: ffffffffa00d39a0 RBX: 0000000000000002
> > > > > RCX: 0000000000000000
> > > > > [??? 3.687902] RDX: 0000000000000002 RSI: ffffffffa14d28e0
> > > > > RDI: ffff920446abda00
> > > > > [??? 3.687904] RBP: ffffad85c0147e18 R08: 0000000000000000
> > > > > R09: 0000000000000000
> > > > > [??? 3.687905] R10: 0000000000000000 R11: 0000000000000000
> > > > > R12: 0000000000000002
> > > > > [??? 3.687906] R13: 0000000000000002 R14: 0000000000000002
> > > > > R15: ffffffffa14d29c8
> > > > > [??? 3.687909]? ? __pfx_intel_idle_hlt+0x10/0x10
> > > > > [??? 3.687913]? ? intel_idle_hlt+0xc/0x40
> > > > > [??? 3.687916]? cpuidle_enter_state+0xa0/0x730
> > > > > [??? 3.687920]? cpuidle_enter+0x2e/0x50
> > > > > [??? 3.687924]? call_cpuidle+0x23/0x60
> > > > > [??? 3.687928]? do_idle+0x207/0x260
> > > > > [??? 3.687932]? cpu_startup_entry+0x1d/0x20
> > > > > [??? 3.687934]? start_secondary+0x129/0x160
> > > > > [??? 3.687939]? secondary_startup_64_no_verify+0x17e/0x18b
> > > > > [??? 3.687945]? </TASK>
> > > > > [??? 3.687946] ================================================================================
> > > > > [??? 4.624607] bridge: filtering via arp/ip/ip6tables is no
> > > > > longer available by default. Update your scripts to load
> > > > > br_netfilter if you need this.
> > > > > [??? 4.629153] Bridge firewalling registered
> > > > > [??? 4.745355] Initializing XFRM netlink socket
> > > > > [??? 4.794107] loop8: detected capacity change from 0 to 8
> > > > > [??? 7.104544] rfkill: input handler disabled
> > > > > [?? 26.445163] ================================================================================
> > > > > [?? 26.445171] UBSAN: array-index-out-of-bounds in
> > > > > drivers/net/xen-netfront.c:807:4
> > > > > [?? 26.445175] index 109 is out of range for type
> > > > > 'xen_netif_tx_sring_entry [1]'
> > > > > [?? 26.445178] CPU: 8 PID: 1729 Comm: sshd Not tainted
> > > > > 6.5.0-rc2-1-generation1 #3
> > > > > [?? 26.445180] Hardware name: Intel Corporation
> > > > > W2600CR/W2600CR, BIOS SE5C600.86B.02.06.0007.082420181029
> > > > > 01/13/2022
> > > > > [?? 26.445181] Call Trace:
> > > > > [?? 26.445185]? <TASK>
> > > > > [?? 26.445185]? dump_stack_lvl+0x48/0x70
> > > > > [?? 26.445185]? dump_stack+0x10/0x20
> > > > > [?? 26.445200]? __ubsan_handle_out_of_bounds+0xc6/0x110
> > > > > [?? 26.445206]? xennet_start_xmit+0x932/0x990
> > > > > [?? 26.445211]? dev_hard_start_xmit+0x68/0x1e0
> > > > > [?? 26.445216]? sch_direct_xmit+0x10b/0x350
> > > > > [?? 26.445220]? __dev_queue_xmit+0x512/0xda0
> > > > > [?? 26.445224]? ip_finish_output2+0x261/0x540
> > > > > [?? 26.445225]? __ip_finish_output+0xb6/0x180
> > > > > [?? 26.445225]? ip_finish_output+0x29/0x100
> > > > > [?? 26.445234]? ip_output+0x73/0x120
> > > > > [?? 26.445234]? ? __pfx_ip_finish_output+0x10/0x10
> > > > > [?? 26.445238]? ip_local_out+0x61/0x70
> > > > > [?? 26.445238]? __ip_queue_xmit+0x18d/0x470
> > > > > [?? 26.445238]? ip_queue_xmit+0x15/0x30
> > > > > [?? 26.445238]? __tcp_transmit_skb+0xb39/0xcc0
> > > > > [?? 26.445238]? tcp_write_xmit+0x595/0x1570
> > > > > [?? 26.445238]? ? _copy_from_iter+0x80/0x4a0
> > > > > [?? 26.445256]? __tcp_push_pending_frames+0x37/0x110
> > > > > [?? 26.445259]? tcp_push+0x123/0x190
> > > > > [?? 26.445260]? tcp_sendmsg_locked+0xafe/0xed0
> > > > > [?? 26.445264]? tcp_sendmsg+0x2c/0x50
> > > > > [?? 26.445268]? inet_sendmsg+0x42/0x80
> > > > > [?? 26.445268]? sock_write_iter+0x160/0x180
> > > > > [?? 26.445274]? vfs_write+0x397/0x440
> > > > > [?? 26.445274]? ksys_write+0xc9/0x100
> > > > > [?? 26.445274]? __x64_sys_write+0x19/0x30
> > > > > [?? 26.445274]? do_syscall_64+0x5c/0x90
> > > > > [?? 26.445287]? ? syscall_exit_to_user_mode+0x1b/0x50
> > > > > [?? 26.445290]? ? do_syscall_64+0x68/0x90
> > > > > [?? 26.445290]? ? do_syscall_64+0x68/0x90
> > > > > [?? 26.445294]? ? do_syscall_64+0x68/0x90
> > > > > [?? 26.445294]? ? syscall_exit_to_user_mode+0x1b/0x50
> > > > > [?? 26.445298]? ? do_syscall_64+0x68/0x90
> > > > > [?? 26.445300]? ? exc_page_fault+0x94/0x1b0
> > > > > [?? 26.445302]? entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> > > > > [?? 26.445306] RIP: 0033:0x7f26c4c3d473
> > > > > [?? 26.445318] Code: 8b 15 21 2a 0e 00 f7 d8 64 89 02 48 c7
> > > > > c0 ff ff ff ff eb b7 0f 1f 00 64 8b 04 25 18 00 00 00 85 c0
> > > > > 75 14 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 55 c3 0f
> > > > > 1f 40 00 48 83 ec 28 48 89 54 24 18
> > > > > [?? 26.445321] RSP: 002b:00007ffdee7b5528 EFLAGS: 00000246
> > > > > ORIG_RAX: 0000000000000001
> > > > > [?? 26.445321] RAX: ffffffffffffffda RBX: 0000000000000700
> > > > > RCX: 00007f26c4c3d473
> > > > > [?? 26.445321] RDX: 0000000000000700 RSI: 000055567032e230
> > > > > RDI: 0000000000000004
> > > > > [?? 26.445321] RBP: 0000555670313d70 R08: fffffffffffffff0
> > > > > R09: 0000000000000000
> > > > > [?? 26.445321] R10: 0000000000000000 R11: 0000000000000246
> > > > > R12: 000055566fcb2768
> > > > > [?? 26.445321] R13: 0000000000000000 R14: 0000000000000004
> > > > > R15: 000055566fc67a80
> > > > > [?? 26.445332]? </TASK>
> > > > > [?? 26.445333] ================================================================================
> > > > 
> > > > See Bugzilla for the full thread and attached dmesg.
> > > > 
> > > > Anyway, I'm adding it to regzbot:
> > > > 
> > > > #regzbot introduced: 8446066bf8c1f9f
> > > > https://bugzilla.kernel.org/show_bug.cgi?id=217693
> > > > 
> > > > Thanks.
> > > > 
> > > > [1]: https://bugzilla.kernel.org/show_bug.cgi?id=217693
> > > 
> > > I doubt it is 8446066bf8c1f9f that causes this. Based on the comment
> > > next to the 'ring[1]' in DEFINE_RING_TYPES() in
> > > include/xen/interface/io/ring.h, this is probably caused/exposed by
> > > commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3") in
> > > 6.5-rc1, which causes that array to no longer be a flexible array but an
> > > array with one element, which would cause UBSAN to complain about an
> > > array access past index one. Adding Kees and Gustavo.
> > 
> > I agree.
> > 
> > > 
> > > Unfortunately, it seems this file is vendored from Xen, so I assume it
> > > would need to be fixed there then pulled into Linux:
> > > 
> > > https://github.com/xen-project/xen/tree/master/xen/include/public/io/ring.h
> > 
> > No, I don't think it will be possible to change this in the Xen tree easily.
> > 
> > Especially the public Xen headers are meant to be compatible with a large
> > variety of compilers, including rather old ones.
> > 
> > This means that ring[1] can't be easily swapped with ring[], as that would
> > cause compile time errors with some compilers.
> > 
> > Just modifying the Linux side header is an option, though, as we don't need
> > the same wide range of supported compilers as Xen.
> > 
> > I'll send a patch for that purpose.
> 
> Oh, in fact there is a way in Xen to do that correctly. It schould be enough to
> use ring[XEN_FLEX_ARRAY_DIM], which will do the right thing.
> 
> So I'll write a Xen patch first, after all.

Perfect! I went to go look, and yes, this is good:

/* Define a variable length array (depends on compiler). */
#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L
#define XEN_FLEX_ARRAY_DIM
#elif defined(__GNUC__)
#define XEN_FLEX_ARRAY_DIM  0
#else
#define XEN_FLEX_ARRAY_DIM  1 /* variable size */
#endif

Be careful, of course, going from [1] to [], if anything is using
sizeof() on the structure.

Thanks for fixing this!

-Kees

-- 
Kees Cook