Re: [syzbot] WARNING in mntput_no_expire (2)

From: Christian Brauner
Date: Sun Apr 04 2021 - 12:41:00 EST

On Sun, Apr 04, 2021 at 03:56:02PM +0000, Al Viro wrote:
> On Sun, Apr 04, 2021 at 01:34:45PM +0200, Christian Brauner wrote:
>
> > Sorry for not replying to your earlier mail but I've been debugging this
> > too. My current theory is that it's related to LOOKUP_ROOT_GRABBED when
> > LOOKUP_CACHED is specified _possibly_ with an interaction how
> > create_io_thread() is created with CLONE_FS. The reproducer requires you
> > either have called pivot_root() or chroot() in order for the failure to
> > happen. So I think the fact that we skip legitimize_root() when
> > LOOKUP_CACHED is set might figure into this. I can keep digging.
> >
>
> > Funny enough I already placed a printk statement into the place you
> > wanted one too so I just amended mine. Here's what you get:
> >
> > If pivot pivot_root() is used before the chroot() you get:
> >
> > [ 637.464555] AAAA: count(-1) | mnt_mntpoint(/) | mnt->mnt.mnt_root(/) | id(579) | dev(tmpfs)
> >
> > if you only call chroot, i.e. make the pivot_root() branch a simple
> > if (true) you get:
> >
> > [ 955.206117] AAAA: count(-2) | mnt_mntpoint(/) | mnt->mnt.mnt_root(/) | id(580) | dev(tmpfs)
>
> Very interesting. What happens if you call loop() twice? And now I wonder
> whether it's root or cwd, actually... Hmm...
>
> How about this:
> fd = open("/proc/self/mountinfo", 0);
> mkdir("./newroot/foo", 0777);
> mount("./newroot/foo", "./newroot/foo", 0, MS_BIND, NULL);
> chroot("./newroot");
> chdir("/foo");
> while (1) {
> static char buf[4096];
> int n = read(fd, buf, 4096);
> if (n <= 0)
> break;
> write(1, buf, n);
> }
> close(fd);
> drop_caps();
> loop();
> as the end of namespace_sandbox_proc(), instead of
> chroot("./newroot");
> chdir("/");
> drop_caps();
> loop();
> sequence we have there?

Uhum, well then we oops properly with a null-deref.

f1-vm login: [ 395.046971][ T5856] general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
[ 395.049716][ T5856] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[ 395.052847][ T5856] CPU: 1 PID: 5856 Comm: iou-wrk-5851 Tainted: G E 5.12.0-rc5-020f68e042a19f59784f0962004d848181d13b9e #46
[ 395.056594][ T5856] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009)/LXD, BIOS 0.0.0 02/06/2015
[ 395.058962][ T5856] RIP: 0010:inode_permission+0x4e/0x530
[ 395.060362][ T5856] Code: ff 89 de e8 34 42 a9 ff 85 db 0f 85 ef 00 00 00 e8 87 40 a9 ff 4c 8d 7d 02 48 b8 00 00 00 00 00 fc ff df 4c 89 fa 48 c1 ea 03 <0f> b6 14 02 4c 89 f8 83 e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 c2
[ 395.065442][ T5856] RSP: 0018:ffffc9000681f640 EFLAGS: 00010246
[ 395.067274][ T5856] RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff84ac11fc
[ 395.069834][ T5856] RDX: 0000000000000000 RSI: ffff888013210000 RDI: 0000000000000002
[ 395.072527][ T5856] RBP: 0000000000000000 R08: 0000000000000001 R09: ffffed1000044f22
[ 395.075287][ T5856] R10: ffff888031c105d3 R11: ffffed1000044f21 R12: ffffc9000681f8e0
[ 395.077026][ T5856] R13: 0000000000000001 R14: ffffffff8e441e60 R15: 0000000000000002
[ 395.079001][ T5856] FS: 00007f136886f800(0000) GS:ffff888015a80000(0000) knlGS:0000000000000000
[ 395.081181][ T5856] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 395.082659][ T5856] CR2: 000055ecfefe8e70 CR3: 00000000236b7000 CR4: 0000000000350ee0
[ 395.084727][ T5856] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 395.087117][ T5856] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 395.089613][ T5856] Call Trace:
[ 395.090570][ T5856] link_path_walk.part.0+0x790/0xc10
[ 395.091735][ T5856] ? write_comp_data+0x2a/0x90
[ 395.092931][ T5856] ? __sanitizer_cov_trace_pc+0x1d/0x50
[ 395.094093][ T5856] ? walk_component+0x6a0/0x6a0
[ 395.095021][ T5856] ? percpu_counter_add_batch+0xbc/0x180
[ 395.096255][ T5856] path_openat+0x269/0x2790
[ 395.097305][ T5856] ? path_lookupat.isra.0+0x530/0x530
[ 395.098391][ T5856] ? ret_from_fork+0x1f/0x30
[ 395.099431][ T5856] ? lockdep_hardirqs_on_prepare+0x400/0x400
[ 395.101326][ T5856] do_filp_open+0x208/0x270
[ 395.102894][ T5856] ? rcu_read_lock_bh_held+0xb0/0xb0
[ 395.104525][ T5856] ? may_open_dev+0xf0/0xf0
[ 395.105968][ T5856] ? do_raw_spin_lock+0x125/0x2e0
[ 395.107426][ T5856] ? write_comp_data+0x2a/0x90
[ 395.108651][ T5856] ? __sanitizer_cov_trace_pc+0x1d/0x50
[ 395.110033][ T5856] ? _raw_spin_unlock+0x29/0x40
[ 395.111397][ T5856] ? alloc_fd+0x499/0x640
[ 395.112783][ T5856] io_openat2+0x1d1/0x8f0
[ 395.114237][ T5856] ? __lock_acquire+0x1847/0x5850
[ 395.115982][ T5856] ? io_req_complete_post+0xa90/0xa90
[ 395.117347][ T5856] ? write_comp_data+0x2a/0x90
[ 395.118428][ T5856] ? io_issue_sqe+0x5ac0/0x5ac0
[ 395.119497][ T5856] io_issue_sqe+0x2a2/0x5ac0
[ 395.120707][ T5856] ? io_poll_complete.constprop.0+0x100/0x100
[ 395.130417][ T5856] ? find_held_lock+0x2d/0x110
[ 395.139772][ T5856] ? io_worker_handle_work+0x55d/0x13b0
[ 395.149051][ T5856] ? lock_downgrade+0x690/0x690
[ 395.158262][ T5856] ? rwlock_bug.part.0+0x90/0x90
[ 395.171997][ T5856] ? io_issue_sqe+0x5ac0/0x5ac0
[ 395.180513][ T5856] io_wq_submit_work+0x1da/0x640
[ 395.193646][ T5856] ? io_issue_sqe+0x5ac0/0x5ac0
[ 395.202240][ T5856] io_worker_handle_work+0x70a/0x13b0
[ 395.210276][ T5856] ? rwlock_bug.part.0+0x90/0x90
[ 395.218389][ T5856] io_wqe_worker+0x2ec/0xd00
[ 395.226152][ T5856] ? io_worker_handle_work+0x13b0/0x13b0
[ 395.238492][ T5856] ? ret_from_fork+0x8/0x30
[ 395.245595][ T5856] ? lock_downgrade+0x690/0x690
[ 395.252630][ T5856] ? do_raw_spin_lock+0x125/0x2e0
[ 395.259801][ T5856] ? rwlock_bug.part.0+0x90/0x90
[ 395.271613][ T5856] ? write_comp_data+0x2a/0x90
[ 395.279000][ T5856] ? _raw_spin_unlock_irq+0x24/0x50
[ 395.285999][ T5856] ? io_worker_handle_work+0x13b0/0x13b0
[ 395.297727][ T5856] ret_from_fork+0x1f/0x30
[ 395.304546][ T5856] Modules linked in: efi_pstore(E) efivarfs(E)
[ 395.311931][ T5851] general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#2] PREEMPT SMP KASAN
[ 395.313449][ T5856] ---[ end trace e9d61a560dd12328 ]---