Re: kernel BUG at kernel/cred.c:434!

From: Paul Moore
Date: Mon Apr 15 2019 - 10:48:46 EST

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On Mon, Apr 15, 2019 at 9:43 AM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> Well, acct("/proc/self/attr/current") doesn't look like a good idea, but I do
> not know where should we put the additional check... And probably
> "echo /proc/self/attr/current > /proc/sys/kernel/core_pattern" can hit the
> same problem, do_coredump() does override_creds() too.
>
> May be just add
>
> if (current->cred != current->real_cred)
> return -EACCES;
>
> into proc_pid_attr_write(), I dunno.

Is the problem that do_acct_process() is calling override_creds() and
the returned/old credentials are being freed before do_acct_process()
can reinstall the creds via revert_creds()? Presumably because the
process accounting is causing the credentials to be replaced?

> On 04/12, Casey Schaufler wrote:
> >
> > On 4/11/2019 11:21 PM, chengjian (D) wrote:
> >
> > Added LSM and SELinux lists.
> >
> >
> > >Hi.
> > >
> > >
> > >syzkaller reported the following BUG:
> > >
> > >[ 73.146973] kernel BUG at kernel/cred.c:434!
> > >[ 73.150231] invalid opcode: 0000 [#1] SMP KASAN PTI
> > >[ 73.151928] CPU: 2 PID: 4058 Comm: syz-executor.6 Not tainted
> > >5.1.0-rc4-00062-g2d06b235815e-dirty #2
> > >[ 73.155174] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> > >rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
> > >[ 73.159798] RIP: 0010:commit_creds+0xadb/0xe50
> > >[ 73.161426] Code: 8b 5b 20 48 c1 ea 03 0f b6 04 02 84 c0 74 08 3c 03 0f
> > >8e 06 03 00 00 39 5d 20 0f 85 ff fa ff ff e9 0c fb ff ff e8 05 a2 25 00
> > ><0f> 0b 48 c7 c7 80 56 c0 83 e8 95 22 b1 00 e8 f2 a1 25 00 0f 0b 48
> > >[ 73.167852] RSP: 0000:ffff88836e65f5d0 EFLAGS: 00010293
> > >[ 73.169636] RAX: ffff8883767b0000 RBX: ffff88837f111300 RCX:
> > >ffffffff8124b5db
> > >[ 73.171962] RDX: 0000000000000000 RSI: ffffffff83c9b140 RDI:
> > >ffff88837f111300
> > >[ 73.174310] RBP: ffff888376610400 R08: 0000000000000000 R09:
> > >0000000000000004
> > >[ 73.176646] R10: 0000000000000001 R11: ffffed107c655acf R12:
> > >ffff8883767b0000
> > >[ 73.178527] Process accounting resumed
> > >[ 73.179021] R13: ffff88837f111900 R14: ffff88837f111300 R15:
> > >ffff8883767b0ac0
> > >[ 73.179029] FS: 00007f2d207f9700(0000) GS:ffff8883e3280000(0000)
> > >knlGS:0000000000000000
> > >[ 73.179034] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > >[ 73.179039] CR2: 00007f1500bd36c0 CR3: 00000003df304003 CR4:
> > >00000000000206e0
> > >[ 73.179047] Call Trace:
> > >[ 73.190461] selinux_setprocattr+0x2ea/0x8f0
> > >[ 73.191925] ? ptrace_parent_sid+0x530/0x530
> > >[ 73.193436] ? proc_pid_attr_write+0x185/0x5a0
> > >[ 73.194967] security_setprocattr+0xa1/0x100
> > >[ 73.196408] proc_pid_attr_write+0x307/0x5a0
> > >[ 73.197869] ? mem_read+0x40/0x40
> > >[ 73.199013] __vfs_write+0x81/0x100
> > >[ 73.200222] __kernel_write+0xf8/0x330
> > >[ 73.201562] do_acct_process+0xca5/0x1340
> > >[ 73.202969] ? __ia32_sys_acct+0x1e0/0x1e0
> > >[ 73.204498] ? find_held_lock+0x2f/0x1e0
> > >[ 73.205857] ? rcu_irq_exit+0xec/0x2c0
> > >[ 73.207160] ? lock_downgrade+0x630/0x630
> > >[ 73.208541] acct_pin_kill+0x63/0x150
> > >[ 73.209816] pin_kill+0x16d/0x7c0
> > >[ 73.210934] ? lockdep_hardirqs_on+0x5e0/0x5e0
> > >[ 73.212452] ? xas_start+0x155/0x510
> > >[ 73.213705] ? pin_insert+0x50/0x50
> > >[ 73.214903] ? finish_wait+0x270/0x270
> > >[ 73.216213] ? cpumask_next+0x57/0x90
> > >[ 73.217442] ? mnt_pin_kill+0x68/0x1d0
> > >[ 73.218851] mnt_pin_kill+0x68/0x1d0
> > >[ 73.220398] cleanup_mnt+0x11b/0x150
> > >[ 73.221970] task_work_run+0x136/0x1b0
> > >[ 73.223427] do_exit+0x830/0x2ca0
> > >[ 73.224586] ? trace_hardirqs_off+0x3b/0x180
> > >[ 73.226088] ? mm_update_next_owner+0x6a0/0x6a0
> > >[ 73.227622] ? find_held_lock+0x2f/0x1e0
> > >[ 73.228954] ? get_signal+0x2cf/0x1c00
> > >[ 73.230236] ? lock_downgrade+0x630/0x630
> > >[ 73.231628] ? rwlock_bug.part.0+0x90/0x90
> > >[ 73.233020] do_group_exit+0x106/0x2f0
> > >[ 73.234330] get_signal+0x325/0x1c00
> > >[ 73.235571] do_signal+0x97/0x1670
> > >[ 73.236739] ? do_send_specific+0x12d/0x220
> > >[ 73.238213] ? lock_downgrade+0x630/0x630
> > >[ 73.239566] ? setup_sigcontext+0x820/0x820
> > >[ 73.240982] ? check_kill_permission+0x4a/0x510
> > >[ 73.242509] ? do_send_specific+0x156/0x220
> > >[ 73.243905] ? do_tkill+0x1c4/0x260
> > >[ 73.245081] ? do_send_specific+0x220/0x220
> > >[ 73.246514] ? trace_hardirqs_on_thunk+0x1a/0x1c
> > >[ 73.248061] ? exit_to_usermode_loop+0x97/0x1d0
> > >[ 73.249619] exit_to_usermode_loop+0x108/0x1d0
> > >[ 73.251129] do_syscall_64+0x461/0x580
> > >[ 73.252454] entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > >[ 73.254219] RIP: 0033:0x462eb9
> > >[ 73.255327] Code: Bad RIP value.
> > >[ 73.256539] RSP: 002b:00007f2d207f8c58 EFLAGS: 00000246 ORIG_RAX:
> > >00000000000000c8
> > >[ 73.259454] RAX: 0000000000000000 RBX: 000000000073bf00 RCX:
> > >0000000000462eb9
> > >[ 73.262309] RDX: 0000000000000000 RSI: 000000000000001e RDI:
> > >0000000000000005
> > >[ 73.265064] RBP: 0000000000000002 R08: 0000000000000000 R09:
> > >0000000000000000
> > >[ 73.267774] R10: 0000000000000000 R11: 0000000000000246 R12:
> > >00007f2d207f96bc
> > >[ 73.270546] R13: 00000000004c5509 R14: 00000000007042f0 R15:
> > >00000000ffffffff
> > >[ 73.273542] Modules linked in:
> > >[ 73.274670] Dumping ftrace buffer:
> > >[ 73.275852] (ftrace buffer empty)
> > >[ 73.277187] ---[ end trace dde36a95f458175d ]---
> > >[ 73.278834] RIP: 0010:commit_creds+0xadb/0xe50
> > >[ 73.280549] Code: 8b 5b 20 48 c1 ea 03 0f b6 04 02 84 c0 74 08 3c 03 0f
> > >8e 06 03 00 00 39 5d 20 0f 85 ff fa ff ff e9 0c fb ff ff e8 05 a2 25 00
> > ><0f> 0b 48 c7 c7 80 56 c0 83 e8 95 22 b1 00 e8 f2 a1 25 00 0f 0b 48
> > >[ 73.287090] RSP: 0000:ffff88836e65f5d0 EFLAGS: 00010293
> > >[ 73.288917] RAX: ffff8883767b0000 RBX: ffff88837f111300 RCX:
> > >ffffffff8124b5db
> > >[ 73.291390] RDX: 0000000000000000 RSI: ffffffff83c9b140 RDI:
> > >ffff88837f111300
> > >[ 73.293864] RBP: ffff888376610400 R08: 0000000000000000 R09:
> > >0000000000000004
> > >[ 73.296370] R10: 0000000000000001 R11: ffffed107c655acf R12:
> > >ffff8883767b0000
> > >[ 73.299275] R13: ffff88837f111900 R14: ffff88837f111300 R15:
> > >ffff8883767b0ac0
> > >[ 73.301351] Process accounting resumed
> > >[ 73.301822] FS: 00007f2d207f9700(0000) GS:ffff8883e3280000(0000)
> > >knlGS:0000000000000000
> > >[ 73.301827] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > >[ 73.301832] CR2: 0000000000462e8f CR3: 0000000003a18002 CR4:
> > >00000000000206e0
> > >[ 73.301850] Kernel panic - not syncing: Fatal exception
> > >[ 73.310719] Process accounting resumed
> > >[ 73.311515] Process accounting resumed
> > >[ 73.318916] Dumping ftrace buffer:
> > >[ 73.318921] (ftrace buffer empty)
> > >[ 73.318945] Kernel Offset: disabled
> > >[ 73.328061] Rebooting in 10 seconds..
> > >
> > >
> > >425 int commit_creds(struct cred *new)
> > >426 {
> > >427 struct task_struct *task = current;
> > >428 const struct cred *old = task->real_cred;
> > >429
> > >430 kdebug("commit_creds(%p{%d,%d})", new,
> > >431 atomic_read(&new->usage),
> > >432 read_cred_subscribers(new));
> > >433
> > >434 BUG_ON(task->cred != old); // BUG here
> > >
> > >
> > >I find that the call chain which triggered the BUG is :
> > >
> > >do_exit
> > > |-=> acct_process
> > > | -=> do_acct_process
> > > | -=> orig_cred = override_creds(file->f_cred); // cred =
> > >ffff8883c1878900/real_cred = ffff8883c1878900
> > > | -=> if (file_start_write_trylock(file))
> > >{__kernel_write(file, &ac, sizeof(acct_t), &pos);}
> > > | -=> __kernel_write+0xf8/0x330
> > > | -=> __vfs_write+0x81/0x100
> > > | -=> proc_pid_attr_write+0x307/0x5a0
> > > | -=> security_setprocattr+0xa1/0x100
> > > | -=>selinux_setprocattr+0x2ea/0x8f0
> > > | -=>commit_creds+0xd97/0x1080 // cred = ffff888379a79c00/real_cred =
> > >ffff888379a79c00
> > > | -=> revert_creds(orig_cred); // cred =
> > >ffff8883c1878900/real_cred = ffff888379a79c00
> > > |-=> task_work_run
> > > -=> cleanup_mnt+0x11b/0x150
> > > -=> mnt_pin_kill+0x68/0x1d0
> > > -=> pin_kill+0x16d/0x7c0
> > > -=> acct_pin_kill+0x2e/0x100
> > > -=> do_acct_process+0x1a0/0x1340
> > > -=> override_creds+0x18a/0x1c0 // cred =
> > >ffff8883c1878900/real_cred = ffff888379a79c00
> > > -=> if (file_start_write_trylock(file))
> > >{__kernel_write(file, &ac, sizeof(acct_t), &pos);}
> > > -=> ......
> > >-=>commit_creds+0xd97/0x1080 // new = ffff888379a6bb00, cred =
> > >ffff8883c1878900, real_ceed = ffff888379a79c00 BUG here
> > > -=> revert_creds(orig_cred);
> > >
> > >
> > >Syzkaller Report Testcase:
> > >
> > >cat crash.log
> > >
> > >04:05:28 executing program 3:
> > >clone(0x24100, 0x0, 0x0, 0x0, 0x0)
> > >r0 = gettid()
> > >perf_event_open(&(0x7f00000000c0)={0x2, 0x70, 0x4, 0x2, 0x0, 0x0, 0x0,
> > >0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
> > >0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
> > >0x0, 0x0, 0x0, 0x0, @perf_bp={0x0}}, 0x0, 0xffffffffffffffff,
> > >0xffffffffffffffff, 0x0)
> > >acct(&(0x7f0000000000)='./file0\x00')
> > >r1 = openat$smack_thread_current(0xffffffffffffff9c,
> > >&(0x7f0000000000)='/proc/thread-self/attr/current\x00', 0x2, 0x0)
> > >fcntl$setown(r1, 0x8, r0)
> > >rt_tgsigqueueinfo(r0, r0, 0x1e, &(0x7f00000002c0))
> > >tkill(r0, 0x1e)
> > >
> > >
> > >Reproduce this BUG:
> > >./syz-execprog -executor=./syz-executor -repeat=0 -procs=16 -cover=0
> > >./crash.log
> > >
> > >
> > >Can anyone help me ?
> > >
> > >
> > >Thanks,
> > >
> > > Cheng Jian.
> > >
> > >
>

--
paul moore
www.paul-moore.com

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