Re: [PATCH] io_uring: fix error clear of ->file_table in io_sqe_files_register()

[Bob Liu <bob.liu@oracle.com>]

On 11/10/19 11:46 PM, Jens Axboe wrote:
> syzbot reports that when using failslab and friends, we can get a double
> free in io_sqe_files_unregister():
> 
> BUG: KASAN: double-free or invalid-free in
> io_sqe_files_unregister+0x20b/0x300 fs/io_uring.c:3185
> 
> CPU: 1 PID: 8819 Comm: syz-executor452 Not tainted 5.4.0-rc6-next-20191108
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Call Trace:
>   __dump_stack lib/dump_stack.c:77 [inline]
>   dump_stack+0x197/0x210 lib/dump_stack.c:118
>   print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
>   kasan_report_invalid_free+0x65/0xa0 mm/kasan/report.c:468
>   __kasan_slab_free+0x13a/0x150 mm/kasan/common.c:450
>   kasan_slab_free+0xe/0x10 mm/kasan/common.c:480
>   __cache_free mm/slab.c:3426 [inline]
>   kfree+0x10a/0x2c0 mm/slab.c:3757
>   io_sqe_files_unregister+0x20b/0x300 fs/io_uring.c:3185
>   io_ring_ctx_free fs/io_uring.c:3998 [inline]
>   io_ring_ctx_wait_and_kill+0x348/0x700 fs/io_uring.c:4060
>   io_uring_release+0x42/0x50 fs/io_uring.c:4068
>   __fput+0x2ff/0x890 fs/file_table.c:280
>   ____fput+0x16/0x20 fs/file_table.c:313
>   task_work_run+0x145/0x1c0 kernel/task_work.c:113
>   exit_task_work include/linux/task_work.h:22 [inline]
>   do_exit+0x904/0x2e60 kernel/exit.c:817
>   do_group_exit+0x135/0x360 kernel/exit.c:921
>   __do_sys_exit_group kernel/exit.c:932 [inline]
>   __se_sys_exit_group kernel/exit.c:930 [inline]
>   __x64_sys_exit_group+0x44/0x50 kernel/exit.c:930
>   do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x43f2c8
> Code: 31 b8 c5 f7 ff ff 48 8b 5c 24 28 48 8b 6c 24 30 4c 8b 64 24 38 4c 8b
> 6c 24 40 4c 8b 74 24 48 4c 8b 7c 24 50 48 83 c4 58 c3 66 <0f> 1f 84 00 00
> 00 00 00 48 8d 35 59 ca 00 00 0f b6 d2 48 89 fb 48
> RSP: 002b:00007ffd5b976008 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 000000000043f2c8
> RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
> RBP: 00000000004bf0a8 R08: 00000000000000e7 R09: ffffffffffffffd0
> R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000001
> R13: 00000000006d1180 R14: 0000000000000000 R15: 0000000000000000
> 
> This happens if we fail allocating the file tables. For that case we do
> free the file table correctly, but we forget to set it to NULL. This
> means that ring teardown will see it as being non-NULL, and attempt to
> free it again.
> 
> Fix this by clearing the file_table pointer if we free the table.
> 
> Reported-by: syzbot+3254bc44113ae1e331ee@syzkaller.appspotmail.com
> Fixes: 65e19f54d29c ("io_uring: support for larger fixed file sets")
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 

Reviewed-by: Bob Liu <bob.liu@oracle.com>

By the way, there are many place(besides io_uring.c) which need to set pointer to NULL after free.
I saw similar fix from time to time.

Do you think a safe_free() is worth? e.g
#define SAFE_FREE(p) { if (p) { free(p); (p)=NULL; } }

> ---
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 7fc3a72e1e1e..5c10b34ceb24 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -3488,6 +3488,7 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
>  
>  	if (io_sqe_alloc_file_tables(ctx, nr_tables, nr_args)) {
>  		kfree(ctx->file_table);
> +		ctx->file_table = NULL;
>  		return -ENOMEM;
>  	}
>  
>