From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-174.mta1.migadu.com (out-174.mta1.migadu.com [95.215.58.174]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0FD472010FA for ; Wed, 27 Nov 2024 20:31:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.174 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1732739505; cv=none; b=VOGvtFNgBq3W2zeN1fPDvRAlTEfdW3OYvwqFdLEyd/KyGukFP+bVnFjI0JyTyai5Aw+bHQ0YeFh9dGaEsYog83dTwSksuG7ERw/3t9UDUr62luNVH2ejEYV7hVL8DQAgB1nxh8sa9TlFGs5g3rH3kxGMSlImAC9A+fL/fvvdrts= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1732739505; c=relaxed/simple; bh=rvLd5onfQ7DDQ8Nf9yaM4f5vbkpvO4qNaW5Mlqy4cmw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=N3p2UcBwgKE4qawIgmEGBOxyXYvEgkMQY7V3MHqJa8KPJwS6qTezi5Jaw9VbtMoKbI6qUVhcqHCnMZ2lSB5YyJ7gzi5ixFRpCW6mbgPkDYQDt2NN4QS64EgoL7fW8K0qNTGexaNOuQYjODTNu4elqE62+Wppc1T0/kIJKlNRQfo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=UohZEOHF; arc=none smtp.client-ip=95.215.58.174 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="UohZEOHF" Date: Wed, 27 Nov 2024 15:31:36 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1732739500; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=P7M5ojNoGPW7ShBlkjPkmQcgRGWWiNlgP5enVALM/EM=; b=UohZEOHFvZ0b71DYERxk+CMte4r+Dpf2ePKZREciLLH4yIgY5GjnrNhEeaTnGOBYlgTUVP Y0kA5buOaEJ7DbOxZ23OddtAB0HcQt07S7FZv49iHjNZ0DcY0v2eFJwzwXSE8N23RlTaZ6 iwIsKGQ6XSmGQO6Wpz3p1zXUnFSY7Zk= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Kent Overstreet To: Jens Axboe Cc: Jann Horn , linux-bcachefs@vger.kernel.org, kernel list , Pavel Begunkov , io-uring Subject: Re: bcachefs: suspicious mm pointer in struct dio_write Message-ID: References: <69510752-d6f9-4cf1-b93d-dcd249d911ef@kernel.dk> Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT On Wed, Nov 27, 2024 at 01:01:31PM -0700, Jens Axboe wrote: > On 11/27/24 12:43 PM, Jann Horn wrote: > > On Wed, Nov 27, 2024 at 7:09?PM Jens Axboe wrote: > >> On 11/27/24 9:57 AM, Jann Horn wrote: > >>> Hi! > >>> > >>> In fs/bcachefs/fs-io-direct.c, "struct dio_write" contains a pointer > >>> to an mm_struct. This pointer is grabbed in bch2_direct_write() > >>> (without any kind of refcount increment), and used in > >>> bch2_dio_write_continue() for kthread_use_mm()/kthread_unuse_mm() > >>> which are used to enable userspace memory access from kthread context. > >>> I believe kthread_use_mm()/kthread_unuse_mm() require that the caller > >>> guarantees that the MM hasn't gone through exit_mmap() yet (normally > >>> by holding an mmget() reference). > >>> > >>> If we reach this codepath via io_uring, do we have a guarantee that > >>> the mm_struct that called bch2_direct_write() is still alive and > >>> hasn't yet gone through exit_mmap() when it is accessed from > >>> bch2_dio_write_continue()? > >>> > >>> I don't know the async direct I/O codepath particularly well, so I > >>> cc'ed the uring maintainers, who probably know this better than me. > >> > >> I _think_ this is fine as-is, even if it does look dubious and bcachefs > >> arguably should grab an mm ref for this just for safety to avoid future > >> problems. The reason is that bcachefs doesn't set FMODE_NOWAIT, which > >> means that on the io_uring side it cannot do non-blocking issue of > >> requests. This is slower as it always punts to an io-wq thread, which > >> shares the same mm. Hence if the request is alive, there's always a > >> thread with the same mm alive as well. > >> > >> Now if FMODE_NOWAIT was set, then the original task could exit. I'd need > >> to dig a bit deeper to verify that would always be safe and there's not > >> a of time today with a few days off in the US looming, so I'll defer > >> that to next week. It certainly would be fine with an mm ref grabbed. > > > > Ah, thanks for looking into it! I missed this implication of not > > setting FMODE_NOWAIT. > > > > Anyway, what you said sounds like it would be cleaner for bcachefs to > > grab its own extra reference, maybe by initially grabbing an mm > > reference with mmgrab() in bch2_direct_write(), and then use > > mmget_not_zero() in bch2_dio_write_continue() to ensure the MM is > > stable. > > Yep I think that would definitely make it more sturdy, and also less > headscratchy in terms of being able to verify it's actually safe. > > > What do other file systems do for this? I think they normally grab > > page references so that they don't need the MM anymore when > > asynchronously fulfilling the request, right? Like in > > iomap_dio_bio_iter(), which uses bio_iov_iter_get_pages() to grab > > references to the pages corresponding to the userspace regions in > > dio->submit.iter? > > Not aware of anything else doing it like this, where it's punted to a > kthread and then the mm used from there. The upfront page > getting/mapping is the common approach, like you described. Which does > seem like a much better choice, rather than needing to rely on the mm in > a kworker. More common, but not necessarily better: the "pin everything up front" approach had the disadvantage that - well, you pinned everything up front: if userspace requests a single multi-gigabyte IO, that is likely not what you want. The old dio code didn't pin everything up front for this reason (but IIRC wasn't fully asynchronous, and also had a silly baked in 64 page limit).