Re: [REPORT] Use-after-free Read in __fdget_raw in v5.10.y

[Jens Axboe <axboe@kernel.dk>]

On 5/18/22 10:34 AM, Lee Jones wrote:
> On Wed, 18 May 2022, Jens Axboe wrote:
> 
>> On 5/18/22 09:39, Lee Jones wrote:
>>> On Wed, 18 May 2022, Jens Axboe wrote:
>>>
>>>> On 5/18/22 9:14 AM, Lee Jones wrote:
>>>>> On Wed, 18 May 2022, Jens Axboe wrote:
>>>>>
>>>>>> On 5/18/22 6:54 AM, Jens Axboe wrote:
>>>>>>> On 5/18/22 6:52 AM, Jens Axboe wrote:
>>>>>>>> On 5/18/22 6:50 AM, Lee Jones wrote:
>>>>>>>>> On Tue, 17 May 2022, Jens Axboe wrote:
>>>>>>>>>
>>>>>>>>>> On 5/17/22 7:00 AM, Lee Jones wrote:
>>>>>>>>>>> On Tue, 17 May 2022, Jens Axboe wrote:
>>>>>>>>>>>
>>>>>>>>>>>> On 5/17/22 6:36 AM, Lee Jones wrote:
>>>>>>>>>>>>> On Tue, 17 May 2022, Jens Axboe wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> On 5/17/22 6:24 AM, Lee Jones wrote:
>>>>>>>>>>>>>>> On Tue, 17 May 2022, Jens Axboe wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> On 5/17/22 5:41 AM, Lee Jones wrote:
>>>>>>>>>>>>>>>>> Good afternoon Jens, Pavel, et al.,
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Not sure if you are presently aware, but there appears to be a
>>>>>>>>>>>>>>>>> use-after-free issue affecting the io_uring worker driver (fs/io-wq.c)
>>>>>>>>>>>>>>>>> in Stable v5.10.y.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> The full sysbot report can be seen below [0].
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> The C-reproducer has been placed below that [1].
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I had great success running this reproducer in an infinite loop.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> My colleague reverse-bisected the fixing commit to:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>   commit fb3a1f6c745ccd896afadf6e2d6f073e871d38ba
>>>>>>>>>>>>>>>>>   Author: Jens Axboe <axboe@kernel.dk>
>>>>>>>>>>>>>>>>>   Date:   Fri Feb 26 09:47:20 2021 -0700
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>        io-wq: have manager wait for all workers to exit
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>        Instead of having to wait separately on workers and manager, just have
>>>>>>>>>>>>>>>>>        the manager wait on the workers. We use an atomic_t for the reference
>>>>>>>>>>>>>>>>>        here, as we need to start at 0 and allow increment from that. Since the
>>>>>>>>>>>>>>>>>        number of workers is naturally capped by the allowed nr of processes,
>>>>>>>>>>>>>>>>>        and that uses an int, there is no risk of overflow.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>        Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>     fs/io-wq.c | 30 ++++++++++++++++++++++--------
>>>>>>>>>>>>>>>>>     1 file changed, 22 insertions(+), 8 deletions(-)
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Does this fix it:
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> commit 886d0137f104a440d9dfa1d16efc1db06c9a2c02
>>>>>>>>>>>>>>>> Author: Jens Axboe <axboe@kernel.dk>
>>>>>>>>>>>>>>>> Date:   Fri Mar 5 12:59:30 2021 -0700
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>     io-wq: fix race in freeing 'wq' and worker access
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Looks like it didn't make it into 5.10-stable, but we can certainly
>>>>>>>>>>>>>>>> rectify that.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Thanks for your quick response Jens.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> This patch doesn't apply cleanly to v5.10.y.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> This is probably why it never made it into 5.10-stable :-/
>>>>>>>>>>>>>
>>>>>>>>>>>>> Right.  It doesn't apply at all unfortunately.
>>>>>>>>>>>>>
>>>>>>>>>>>>>>> I'll have a go at back-porting it.  Please bear with me.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Let me know if you into issues with that and I can help out.
>>>>>>>>>>>>>
>>>>>>>>>>>>> I think the dependency list is too big.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Too much has changed that was never back-ported.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Actually the list of patches pertaining to fs/io-wq.c alone isn't so
>>>>>>>>>>>>> bad, I did start to back-port them all but some of the big ones have
>>>>>>>>>>>>> fs/io_uring.c changes incorporated and that list is huge (256 patches
>>>>>>>>>>>>> from v5.10 to the fixing patch mentioned above).
>>>>>>>>>>>>
>>>>>>>>>>>> The problem is that 5.12 went to the new worker setup, and this patch
>>>>>>>>>>>> landed after that even though it also applies to the pre-native workers.
>>>>>>>>>>>> Hence the dependency chain isn't really as long as it seems, probably
>>>>>>>>>>>> just a few patches backporting the change references and completions.
>>>>>>>>>>>>
>>>>>>>>>>>> I'll take a look this afternoon.
>>>>>>>>>>>
>>>>>>>>>>> Thanks Jens.  I really appreciate it.
>>>>>>>>>>
>>>>>>>>>> Can you see if this helps? Untested...
>>>>>>>>>
>>>>>>>>> What base does this apply against please?
>>>>>>>>>
>>>>>>>>> I tried Mainline and v5.10.116 and both failed.
>>>>>>>>
>>>>>>>> It's against 5.10.116, so that's puzzling. Let me double check I sent
>>>>>>>> the right one...
>>>>>>>
>>>>>>> Looks like I sent the one from the wrong directory, sorry about that.
>>>>>>> This one should be better:
>>>>>>
>>>>>> Nope, both are the right one. Maybe your mailer is mangling the patch?
>>>>>> I'll attach it gzip'ed here in case that helps.
>>>>>
>>>>> Okay, that applied, thanks.
>>>>>
>>>>> Unfortunately, I am still able to crash the kernel in the same way.
>>>>
>>>> Alright, maybe it's not enough. I can't get your reproducer to crash,
>>>> unfortunately. I'll try on a different box.
>>>
>>> You need to have fuzzing and kasan enabled.
>>
>> I do have kasan enabled. What's fuzzing?
> 
> CONFIG_KCOV

Ah ok - I don't think that's needed for this.

Looking a bit deeper at this, I'm now convinced your bisect went off the
rails at some point. Probably because this can be timing specific.

Can you try with this patch?


diff --git a/fs/io_uring.c b/fs/io_uring.c
index 4330603eae35..3ecf71151fb1 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4252,12 +4252,8 @@ static int io_statx(struct io_kiocb *req, bool force_nonblock)
 	struct io_statx *ctx = &req->statx;
 	int ret;
 
-	if (force_nonblock) {
-		/* only need file table for an actual valid fd */
-		if (ctx->dfd == -1 || ctx->dfd == AT_FDCWD)
-			req->flags |= REQ_F_NO_FILE_TABLE;
+	if (force_nonblock)
 		return -EAGAIN;
-	}
 
 	ret = do_statx(ctx->dfd, ctx->filename, ctx->flags, ctx->mask,
 		       ctx->buffer);

-- 
Jens Axboe