linux-kernel - Re: [PATCH] fs/aio: fix uaf in sys_io

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Date: Mon, 4 Mar 2024 08:15:15 -0800
From: Bart Van Assche <bvanassche@....org>
To: Edward Adam Davis <eadavis@...com>,
 syzbot+b91eb2ed18f599dd3c31@...kaller.appspotmail.com
Cc: bcrl@...ck.org, brauner@...nel.org, jack@...e.cz, linux-aio@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: Re: [PATCH] fs/aio: fix uaf in sys_io_cancel

On 3/3/24 04:21, Edward Adam Davis wrote:
> The aio poll work aio_poll_complete_work() need to be synchronized with syscall
> io_cancel(). Otherwise, when poll work executes first, syscall may access the
> released aio_kiocb object.
> 
> Fixes: 54cbc058d86b ("fs/aio: Make io_cancel() generate completions again")
> Reported-and-tested-by: syzbot+b91eb2ed18f599dd3c31@...kaller.appspotmail.com
> Signed-off-by: Edward Adam Davis <eadavis@...com>
> ---
>   fs/aio.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 28223f511931..0fed22ed9eb8 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1762,9 +1762,8 @@ static void aio_poll_complete_work(struct work_struct *work)
>   	} /* else, POLLFREE has freed the waitqueue, so we must complete */
>   	list_del_init(&iocb->ki_list);
>   	iocb->ki_res.res = mangle_poll(mask);
> -	spin_unlock_irq(&ctx->ctx_lock);
> -
>   	iocb_put(iocb);
> +	spin_unlock_irq(&ctx->ctx_lock);
>   }
>   
>   /* assumes we are called with irqs disabled */
> @@ -2198,7 +2197,6 @@ SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, struct iocb __user *, iocb,
>   			break;
>   		}
>   	}
> -	spin_unlock_irq(&ctx->ctx_lock);
>   
>   	/*
>   	 * The result argument is no longer used - the io_event is always
> @@ -2206,6 +2204,7 @@ SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, struct iocb __user *, iocb,
>   	 */
>   	if (ret == 0 && kiocb->rw.ki_flags & IOCB_AIO_RW)
>   		aio_complete_rw(&kiocb->rw, -EINTR);
> +	spin_unlock_irq(&ctx->ctx_lock);
>   
>   	percpu_ref_put(&ctx->users);

I'm not enthusiast about the above patch because it increases the amount
of code executed with the ctx_lock held. Wouldn't something like the
untested patch below be a better solution?

Thanks,

Bart.


diff --git a/fs/aio.c b/fs/aio.c
index 28223f511931..c6fb10321e48 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -2177,6 +2177,7 @@ SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, 
struct iocb __user *, iocb,
  	struct kioctx *ctx;
  	struct aio_kiocb *kiocb;
  	int ret = -EINVAL;
+	bool is_cancelled_rw = false;
  	u32 key;
  	u64 obj = (u64)(unsigned long)iocb;

@@ -2193,6 +2194,7 @@ SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, 
struct iocb __user *, iocb,
  	/* TODO: use a hash or array, this sucks. */
  	list_for_each_entry(kiocb, &ctx->active_reqs, ki_list) {
  		if (kiocb->ki_res.obj == obj) {
+			is_cancelled_rw = kiocb->rw.ki_flags & IOCB_AIO_RW;
  			ret = kiocb->ki_cancel(&kiocb->rw);
  			list_del_init(&kiocb->ki_list);
  			break;
@@ -2204,7 +2206,7 @@ SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, 
struct iocb __user *, iocb,
  	 * The result argument is no longer used - the io_event is always
  	 * delivered via the ring buffer.
  	 */
-	if (ret == 0 && kiocb->rw.ki_flags & IOCB_AIO_RW)
+	if (ret == 0 && is_cancelled_rw)
  		aio_complete_rw(&kiocb->rw, -EINTR);

  	percpu_ref_put(&ctx->users);