Re: [PATCH] io_uring: don't touch 'ctx' after installing file descriptor

[Stefano Garzarella <sgarzare@redhat.com>]

On Thu, Jul 30, 2020 at 01:47:52PM -0600, Jens Axboe wrote:
> As soon as we install the file descriptor, we have to assume that it
> can get arbitrarily closed. We currently account memory (and note that
> we did) after installing the ring fd, which means that it could be a
> potential use-after-free condition if the fd is closed right after
> being installed, but before we fiddle with the ctx.
> 
> In fact, syzbot reported this exact scenario:
> 
> BUG: KASAN: use-after-free in io_account_mem fs/io_uring.c:7397 [inline]
> BUG: KASAN: use-after-free in io_uring_create fs/io_uring.c:8369 [inline]
> BUG: KASAN: use-after-free in io_uring_setup+0x2797/0x2910 fs/io_uring.c:8400
> Read of size 1 at addr ffff888087a41044 by task syz-executor.5/18145
> 
> CPU: 0 PID: 18145 Comm: syz-executor.5 Not tainted 5.8.0-rc7-next-20200729-syzkaller #0
> 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+0x18f/0x20d lib/dump_stack.c:118
>  print_address_description.constprop.0.cold+0xae/0x497 mm/kasan/report.c:383
>  __kasan_report mm/kasan/report.c:513 [inline]
>  kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
>  io_account_mem fs/io_uring.c:7397 [inline]
>  io_uring_create fs/io_uring.c:8369 [inline]
>  io_uring_setup+0x2797/0x2910 fs/io_uring.c:8400
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> RIP: 0033:0x45c429
> Code: 8d b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 5b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007f8f121d0c78 EFLAGS: 00000246 ORIG_RAX: 00000000000001a9
> RAX: ffffffffffffffda RBX: 0000000000008540 RCX: 000000000045c429
> RDX: 0000000000000000 RSI: 0000000020000040 RDI: 0000000000000196
> RBP: 000000000078bf38 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 000000000078bf0c
> R13: 00007fff86698cff R14: 00007f8f121d19c0 R15: 000000000078bf0c
> 
> Move the accounting of the ring used locked memory before we get and
> install the ring file descriptor.

Maybe we can add:
Fixes: 309758254ea6 ("io_uring: report pinned memory usage")

The patch LGTM:
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

Thanks,
Stefano

> 
> Cc: stable@vger.kernel.org
> Reported-by: syzbot+9d46305e76057f30c74e@syzkaller.appspotmail.com
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index fabf0b692384..33702f3b5af8 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -8329,6 +8329,15 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p,
>  		ret = -EFAULT;
>  		goto err;
>  	}
> +
> +	/*
> +	 * Account memory _before_ installing the file descriptor. Once
> +	 * the descriptor is installed, it can get closed at any time.
> +	 */
> +	io_account_mem(ctx, ring_pages(p->sq_entries, p->cq_entries),
> +		       ACCT_LOCKED);
> +	ctx->limit_mem = limit_mem;
> +
>  	/*
>  	 * Install ring fd as the very last thing, so we don't risk someone
>  	 * having closed it before we finish setup
> @@ -8338,9 +8347,6 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p,
>  		goto err;
>  
>  	trace_io_uring_create(ret, ctx, p->sq_entries, p->cq_entries, p->flags);
> -	io_account_mem(ctx, ring_pages(p->sq_entries, p->cq_entries),
> -		       ACCT_LOCKED);
> -	ctx->limit_mem = limit_mem;
>  	return ret;
>  err:
>  	io_ring_ctx_wait_and_kill(ctx);
> 
> -- 
> Jens Axboe
>