[BUG] I found a syzkaller softlockup "BUG: soft lockup in __mmap_region"

[BUG] I found a syzkaller softlockup "BUG: soft lockup in __mmap_region"

[Yang Jihong]

Hello，

In my environment, a perf_mmap softlockup syzkaller problem occurs. The 
call stack is as follows:
[ 3666.984385][    C2] watchdog: BUG: soft lockup - CPU#2 stuck for 23s! 
[syz-executor.2:32404]
[ 3666.986137][    C2] Modules linked in:
[ 3666.989581][    C2] CPU: 2 PID: 32404 Comm: syz-executor.2 Not 
tainted 5.10.0+ #4
[ 3666.990697][    C2] Hardware name: linux,dummy-virt (DT)
[ 3666.992270][    C2] pstate: 80000005 (Nzcv daif -PAN -UAO -TCO BTYPE=--)
[ 3666.993787][    C2] pc : __kasan_check_write+0x0/0x40
[ 3666.994841][    C2] lr : perf_mmap+0x3c8/0xf80
[ 3666.995661][    C2] sp : ffff00001011f8f0
[ 3666.996598][    C2] x29: ffff00001011f8f0 x28: ffff0000cf644868
[ 3666.998488][    C2] x27: ffff000012cad2c0 x26: 0000000000000000
[ 3666.999888][    C2] x25: 0000000000000001 x24: ffff000012cad298
[ 3667.003511][    C2] x23: 0000000000000000 x22: ffff000012cad000
[ 3667.005504][    C2] x21: ffff0000cf644818 x20: ffff0000cf6d2400
[ 3667.006891][    C2] x19: ffff0000cf6d24c0 x18: 0000000000000000
[ 3667.008295][    C2] x17: 0000000000000000 x16: 0000000000000000
[ 3667.009528][    C2] x15: 0000000000000000 x14: 0000000000000000
[ 3667.010658][    C2] x13: 0000000000000000 x12: ffff800002023f17
[ 3667.012169][    C2] x11: 1fffe00002023f16 x10: ffff800002023f16
[ 3667.013780][    C2] x9 : dfffa00000000000 x8 : ffff00001011f8b7
[ 3667.015265][    C2] x7 : 0000000000000001 x6 : ffff800002023f16
[ 3667.016683][    C2] x5 : ffff0000c0f36400 x4 : 0000000000000000
[ 3667.018078][    C2] x3 : ffffa00010000000 x2 : ffffa000119a0000
[ 3667.019343][    C2] x1 : 0000000000000004 x0 : ffff0000cf6d24c0
[ 3667.021276][    C2] Call trace:
[ 3667.022598][    C2]  __kasan_check_write+0x0/0x40
[ 3667.023666][    C2]  __mmap_region+0x7a4/0xc90
[ 3667.024679][    C2]  __do_mmap_mm+0x600/0xa20
[ 3667.025700][    C2]  do_mmap+0x114/0x384
[ 3667.026583][    C2]  vm_mmap_pgoff+0x138/0x230
[ 3667.027532][    C2]  ksys_mmap_pgoff+0x1d8/0x570
[ 3667.028537][    C2]  __arm64_sys_mmap+0xa4/0xd0
[ 3667.029597][    C2]  el0_svc_common.constprop.0+0xf4/0x414
[ 3667.030682][    C2]  do_el0_svc+0x50/0x11c
[ 3667.031545][    C2]  el0_svc+0x20/0x30
[ 3667.032368][    C2]  el0_sync_handler+0xe4/0x1e0
[ 3667.033305][    C2]  el0_sync+0x148/0x180

I checked the perf_mmap code locally. When the problem occurred, the 
kernel kept running the goto again command:

again:
         mutex_lock(&event->mmap_mutex);
         if (event->rb) {
                 if (data_page_nr(event->rb) != nr_pages) {
                         ret = -EINVAL;
                         goto unlock;
                 }

                 if (!atomic_inc_not_zero(&event->rb->mmap_count)) {
                         /*
                          * Raced against perf_mmap_close() through
                          * perf_event_set_output(). Try again, hope for 
bette
                          * luck.
                          */
                         mutex_unlock(&event->mmap_mutex);
                         goto again;             ===> kept goto again
                 }

                 goto unlock;
         } else {
                 printk("[%d]test_mmap: perf_mmap lock, NULL\n", get_cpu());
                 put_cpu();
         }


Under what conditions would this cause?

Best Regards,
Yang