perf: use-after-free in perf_release

[Dmitry Vyukov]

Hello,

I've got the following use-after-free report while running syzkaller
fuzzer on 86292b33d4b79ee03e2f43ea0381ef85f077c760. Note that the task
is freed right in copy_process due to some error, but it's referenced
by another thread in perf subsystem.

==================================================================
BUG: KASAN: use-after-free in atomic_dec_and_test
arch/x86/include/asm/atomic.h:123 [inline] at addr ffff880079c30158
BUG: KASAN: use-after-free in put_task_struct
include/linux/sched/task.h:93 [inline] at addr ffff880079c30158
BUG: KASAN: use-after-free in put_ctx+0xcf/0x110
kernel/events/core.c:1131 at addr ffff880079c30158
Write of size 4 by task syz-executor6/25698
CPU: 2 PID: 25698 Comm: syz-executor6 Not tainted 4.10.0+ #302
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:16 [inline]
 dump_stack+0x2fb/0x3fd lib/dump_stack.c:52
 kasan_object_err+0x1c/0x90 mm/kasan/report.c:166
 print_address_description mm/kasan/report.c:208 [inline]
 kasan_report_error mm/kasan/report.c:292 [inline]
 kasan_report.part.2+0x1b0/0x460 mm/kasan/report.c:314
 kasan_report+0x21/0x30 mm/kasan/report.c:301
 check_memory_region_inline mm/kasan/kasan.c:326 [inline]
 check_memory_region+0x139/0x190 mm/kasan/kasan.c:333
 kasan_check_write+0x14/0x20 mm/kasan/kasan.c:344
 atomic_dec_and_test arch/x86/include/asm/atomic.h:123 [inline]
 put_task_struct include/linux/sched/task.h:93 [inline]
 put_ctx+0xcf/0x110 kernel/events/core.c:1131
 perf_event_release_kernel+0x3ad/0xc90 kernel/events/core.c:4322
 perf_release+0x37/0x50 kernel/events/core.c:4338
 __fput+0x332/0x800 fs/file_table.c:209
 ____fput+0x15/0x20 fs/file_table.c:245
 task_work_run+0x197/0x260 kernel/task_work.c:116
 exit_task_work include/linux/task_work.h:21 [inline]
 do_exit+0xb38/0x29c0 kernel/exit.c:880
 do_group_exit+0x149/0x420 kernel/exit.c:984
 get_signal+0x7e0/0x1820 kernel/signal.c:2318
 do_signal+0xd2/0x2190 arch/x86/kernel/signal.c:808
 exit_to_usermode_loop+0x200/0x2a0 arch/x86/entry/common.c:157
 syscall_return_slowpath arch/x86/entry/common.c:191 [inline]
 do_syscall_64+0x6fc/0x930 arch/x86/entry/common.c:286
 entry_SYSCALL64_slow_path+0x25/0x25
RIP: 0033:0x4458d9
RSP: 002b:00007f3f07187cf8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 00000000007080c8 RCX: 00000000004458d9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000007080c8
RBP: 00000000007080a8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007f3f071889c0 R15: 00007f3f07188700
Object at ffff880079c30140, in cache task_struct size: 5376
Allocated:
PID = 25681
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
 save_stack+0x43/0xd0 mm/kasan/kasan.c:513
 set_track mm/kasan/kasan.c:525 [inline]
 kasan_kmalloc+0xaa/0xd0 mm/kasan/kasan.c:616
 kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:555
 kmem_cache_alloc_node+0x122/0x6f0 mm/slab.c:3662
 alloc_task_struct_node kernel/fork.c:153 [inline]
 dup_task_struct kernel/fork.c:495 [inline]
 copy_process.part.38+0x19c8/0x4aa0 kernel/fork.c:1560
 copy_process kernel/fork.c:1531 [inline]
 _do_fork+0x200/0x1010 kernel/fork.c:1994
 SYSC_clone kernel/fork.c:2104 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2098
 do_syscall_64+0x2e8/0x930 arch/x86/entry/common.c:281
 return_from_SYSCALL_64+0x0/0x7a
Freed:
PID = 25681
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
 save_stack+0x43/0xd0 mm/kasan/kasan.c:513
 set_track mm/kasan/kasan.c:525 [inline]
 kasan_slab_free+0x6f/0xb0 mm/kasan/kasan.c:589
 __cache_free mm/slab.c:3514 [inline]
 kmem_cache_free+0x71/0x240 mm/slab.c:3774
 free_task_struct kernel/fork.c:158 [inline]
 free_task+0x151/0x1d0 kernel/fork.c:370
 copy_process.part.38+0x18e5/0x4aa0 kernel/fork.c:1931
 copy_process kernel/fork.c:1531 [inline]
 _do_fork+0x200/0x1010 kernel/fork.c:1994
 SYSC_clone kernel/fork.c:2104 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2098
 do_syscall_64+0x2e8/0x930 arch/x86/entry/common.c:281
 return_from_SYSCALL_64+0x0/0x7a