Re: [PATCH] io_uring: prevent io_put_identity() from freeing a static identity

[Tadeusz Struk <tadeusz.struk@linaro.org>]

On 11/3/21 18:21, Tadeusz Struk wrote:
> Note: this applies to 5.10 stable only. It doesn't trigger on anything
> above 5.10 as the code there has been substantially reworked. This also
> doesn't apply to any stable kernel below 5.10 afaict.
> 
> Syzbot found a bug: KASAN: invalid-free in io_dismantle_req
> https://syzkaller.appspot.com/bug?id=123d9a852fc88ba573ffcb2dbcf4f9576c3b0559
> 
> The test submits bunch of io_uring writes and exits, which then triggers
> uring_task_cancel() and io_put_identity(), which in some corner cases,
> tries to free a static identity. This causes a panic as shown in the
> trace below:
> 
>   BUG: KASAN: double-free or invalid-free in kfree+0xd5/0x310
>   CPU: 0 PID: 4618 Comm: repro Not tainted 5.10.76-05281-g4944ec82ebb9-dirty #17
>   Call Trace:
>    dump_stack_lvl+0x1b2/0x21b
>    print_address_description+0x8d/0x3b0
>    kasan_report_invalid_free+0x58/0x130
>    ____kasan_slab_free+0x14b/0x170
>    __kasan_slab_free+0x11/0x20
>    slab_free_freelist_hook+0xcc/0x1a0
>    kfree+0xd5/0x310
>    io_dismantle_req+0x9b0/0xd90
>    io_do_iopoll+0x13a4/0x23e0
>    io_iopoll_try_reap_events+0x116/0x290
>    io_uring_cancel_task_requests+0x197d/0x1ee0
>    io_uring_flush+0x170/0x6d0
>    filp_close+0xb0/0x150
>    put_files_struct+0x1d4/0x350
>    exit_files+0x80/0xa0
>    do_exit+0x6d9/0x2390
>    do_group_exit+0x16a/0x2d0
>    get_signal+0x133e/0x1f80
>    arch_do_signal+0x7b/0x610
>    exit_to_user_mode_prepare+0xaa/0xe0
>    syscall_exit_to_user_mode+0x24/0x40
>    do_syscall_64+0x3d/0x70
>    entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
>   Allocated by task 4611:
>    ____kasan_kmalloc+0xcd/0x100
>    __kasan_kmalloc+0x9/0x10
>    kmem_cache_alloc_trace+0x208/0x390
>    io_uring_alloc_task_context+0x57/0x550
>    io_uring_add_task_file+0x1f7/0x290
>    io_uring_create+0x2195/0x3490
>    __x64_sys_io_uring_setup+0x1bf/0x280
>    do_syscall_64+0x31/0x70
>    entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
>   The buggy address belongs to the object at ffff88810732b500
>    which belongs to the cache kmalloc-192 of size 192
>   The buggy address is located 88 bytes inside of
>    192-byte region [ffff88810732b500, ffff88810732b5c0)
>   Kernel panic - not syncing: panic_on_warn set ...
> 
> This issue bisected to this commit:
> commit 186725a80c4e ("io_uring: fix skipping disabling sqo on exec")
> 
> Simple reverting the offending commit doesn't work as it hits some
> other, related issues like:
> 
> /* sqo_dead check is for when this happens after cancellation */
> WARN_ON_ONCE(ctx->sqo_task == current && !ctx->sqo_dead &&
> 	     !xa_load(&tctx->xa, (unsigned long)file));
> 
>   ------------[ cut here ]------------
>   WARNING: CPU: 1 PID: 5622 at fs/io_uring.c:8960 io_uring_flush+0x5bc/0x6d0
>   Modules linked in:
>   CPU: 1 PID: 5622 Comm: repro Not tainted 5.10.76-05281-g4944ec82ebb9-dirty #16
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-6.fc35 04/01/2014
>   RIP: 0010:io_uring_flush+0x5bc/0x6d0
>   Call Trace:
>   filp_close+0xb0/0x150
>   put_files_struct+0x1d4/0x350
>   reset_files_struct+0x88/0xa0
>   bprm_execve+0x7f2/0x9f0
>   do_execveat_common+0x46f/0x5d0
>   __x64_sys_execve+0x92/0xb0
>   do_syscall_64+0x31/0x70
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Changing __io_uring_task_cancel() to call io_disable_sqo_submit() directly,
> as the comment suggests, only if __io_uring_files_cancel() is not executed
> seems to fix the issue.
> 
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: <io-uring@vger.kernel.org>
> Cc: <linux-fsdevel@vger.kernel.org>
> Cc: <linux-kernel@vger.kernel.org>
> Cc: <stable@vger.kernel.org>
> Reported-by: syzbot+6055980d041c8ac23307@syzkaller.appspotmail.com
> Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
> ---
>   fs/io_uring.c | 21 +++++++++++++++++----
>   1 file changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 0736487165da..fcf9ffe9b209 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -8882,20 +8882,18 @@ void __io_uring_task_cancel(void)
>   	struct io_uring_task *tctx = current->io_uring;
>   	DEFINE_WAIT(wait);
>   	s64 inflight;
> +	int canceled = 0;
>   
>   	/* make sure overflow events are dropped */
>   	atomic_inc(&tctx->in_idle);
>   
> -	/* trigger io_disable_sqo_submit() */
> -	if (tctx->sqpoll)
> -		__io_uring_files_cancel(NULL);
> -
>   	do {
>   		/* read completions before cancelations */
>   		inflight = tctx_inflight(tctx);
>   		if (!inflight)
>   			break;
>   		__io_uring_files_cancel(NULL);
> +		canceled = 1;
>   
>   		prepare_to_wait(&tctx->wait, &wait, TASK_UNINTERRUPTIBLE);
>   
> @@ -8909,6 +8907,21 @@ void __io_uring_task_cancel(void)
>   		finish_wait(&tctx->wait, &wait);
>   	} while (1);
>   
> +	/*
> +	 * trigger io_disable_sqo_submit()
> +	 * if not already done by __io_uring_files_cancel()
> +	 */
> +	if (tctx->sqpoll && !canceled) {
> +		struct file *file;
> +		unsigned long index;
> +
> +		xa_for_each(&tctx->xa, index, file) {
> +			struct io_ring_ctx *ctx = file->private_data;
> +
> +			io_disable_sqo_submit(ctx);
> +		}
> +	}
> +
>   	atomic_dec(&tctx->in_idle);
>   
>   	io_uring_remove_task_files(tctx);
> 

Hi,
Any comments on this one? It needs to be ACK'ed by the maintainer before
it is applied to 5.10 stable.

-- 
Thanks,
Tadeusz