From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 95CD412FB1B; Mon, 9 Dec 2024 15:01:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1733756511; cv=none; b=oCHZayK1BdzNwt/Uvm6BabqWxc8V0/FT23QCw/dKpRpRKoMVIenf021jHcrzKT8jq34b8l1+fE3n/2MmL/Zk85f6B4kTX9yFDDaJDhmUXlt6Wi++3IhaqUKtK9alIrMbEVxe8biasuy16+vpPUtujFpFovSXcNFaC6v9Enrurhw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1733756511; c=relaxed/simple; bh=mfvcDe7N+Ar77QJ+nGuU68km7qiz3Sp+f7HMKwLDSWc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=YjWfP36ABNGFB4ETYThXnXBPYGMw8kZQdQovNn48pPP53ozQ/V1zG5WQ3kLLV10HwmifXkudnqioy2TXHkXjkD85POrh4p18qqUHS0fz8HyRNcbT4suXtctsj3m0hnvGc+oP/ymDqrUNvtjpm4FilYr3W+1hCItpNf7ToPi8I0M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JRbzKdkC; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="JRbzKdkC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 88D8DC4CED1; Mon, 9 Dec 2024 15:01:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1733756511; bh=mfvcDe7N+Ar77QJ+nGuU68km7qiz3Sp+f7HMKwLDSWc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=JRbzKdkCTdys7dbWYvTn5AxORvWIMxPcIuw6C0u5pvYeVMmQed91KySsvPd9SHBjD gwNky0/5crEMFA4kxrrSQGvOkS20vqDTc71rqD9AU7gmTgK0VSwKZdxfQur/0W60Pe dkUBP0y2K7Rfobvj0rqJaJ6wCp+uOiAxNvsKZ7N/PWzLr2hT0Gb1n6frEssuTy6w9v xio4QJp6q58uoPWPOGbUMTaOL/w8dTkl8BJZxqBxCvILHfICF6AqMvvPMuM962bB8a Wc9NHywUUfwDP9tRh0GB/s3MI2uNYLyeM2xlg9PEnZ0oSnw3lMXY8AF5tmVIiPdFrh F0wVEJ/3jdXFw== Date: Mon, 9 Dec 2024 16:01:44 +0100 From: Danilo Krummrich To: Alice Ryhl Cc: Greg Kroah-Hartman , Arnd Bergmann , Alexander Viro , Christian Brauner , Jan Kara , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Benno Lossin , Andreas Hindborg , Trevor Gross , Lee Jones , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v2 2/2] rust: miscdevice: access the `struct miscdevice` from fops->open() Message-ID: References: <2024120925-express-unmasked-76b4@gregkh> <2024120954-boring-skeptic-ad16@gregkh> <2024120908-anemic-previous-3db9@gregkh> <2024120939-aide-epidermal-076e@gregkh> <2024120951-botanist-exhale-4845@gregkh> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Mon, Dec 09, 2024 at 02:36:31PM +0100, Alice Ryhl wrote: > On Mon, Dec 9, 2024 at 2:13 PM Greg Kroah-Hartman > wrote: > > > > On Mon, Dec 09, 2024 at 01:53:42PM +0100, Alice Ryhl wrote: > > > On Mon, Dec 9, 2024 at 1:08 PM Greg Kroah-Hartman > > > wrote: > > > > > > > > On Mon, Dec 09, 2024 at 01:00:05PM +0100, Alice Ryhl wrote: > > > > > On Mon, Dec 9, 2024 at 12:53 PM Greg Kroah-Hartman > > > > > wrote: > > > > > > > > > > > > On Mon, Dec 09, 2024 at 12:38:32PM +0100, Alice Ryhl wrote: > > > > > > > On Mon, Dec 9, 2024 at 12:10 PM Greg Kroah-Hartman > > > > > > > wrote: > > > > > > > > > > > > > > > > On Mon, Dec 09, 2024 at 11:50:57AM +0100, Alice Ryhl wrote: > > > > > > > > > On Mon, Dec 9, 2024 at 9:48 AM Greg Kroah-Hartman > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > On Mon, Dec 09, 2024 at 07:27:47AM +0000, Alice Ryhl wrote: > > > > > > > > > > > Providing access to the underlying `struct miscdevice` is useful for > > > > > > > > > > > various reasons. For example, this allows you access the miscdevice's > > > > > > > > > > > internal `struct device` for use with the `dev_*` printing macros. > > > > > > > > > > > > > > > > > > > > > > Note that since the underlying `struct miscdevice` could get freed at > > > > > > > > > > > any point after the fops->open() call, only the open call is given > > > > > > > > > > > access to it. To print from other calls, they should take a refcount on > > > > > > > > > > > the device to keep it alive. > > > > > > > > > > > > > > > > > > > > The lifespan of the miscdevice is at least from open until close, so > > > > > > > > > > it's safe for at least then (i.e. read/write/ioctl/etc.) > > > > > > > > > > > > > > > > > > How is that enforced? What happens if I call misc_deregister while > > > > > > > > > there are open fds? > > > > > > > > > > > > > > > > You shouldn't be able to do that as the code that would be calling > > > > > > > > misc_deregister() (i.e. in a module unload path) would not work because > > > > > > > > the module reference count is incremented at this point in time due to > > > > > > > > the file operation module reference. > > > > > > > > > > > > > > Oh .. so misc_deregister must only be called when the module is being unloaded? > > > > > > > > > > > > Traditionally yes, that's when it is called. Do you see it happening in > > > > > > any other place in the kernel today? > > > > > > > > > > I had not looked, but I know that Binder allows dynamically creating > > > > > and removing its devices at runtime. It happens to be the case that > > > > > this is only supported when binderfs is used, which is when it doesn't > > > > > use miscdevice, so technically Binder does not call misc_deregister() > > > > > outside of module unload, but following its example it's not hard to > > > > > imagine that such removals could happen. > > > > > > > > That's why those are files and not misc devices :) > > > > > > I grepped for misc_deregister and the first driver I looked at is > > > drivers/misc/bcm-vk which seems to allow dynamic deregistration if the > > > pci device is removed. > > > > Ah, yeah, that's going to get messy and will be a problem if someone has > > the file open then. > > > > > Another tricky path is error cleanup in its probe function. > > > Technically, if probe fails after registering the misc device, there's > > > a brief moment where you could open the miscdevice before it gets > > > removed in the cleanup path, which seems to me that it could lead to > > > UAF? > > > > > > Or is there something I'm missing? > > > > Nope, that too is a window of a problem, luckily you "should" only > > register the misc device after you know the device is safe to use as > > once it is registered, it could be used so it "should" be the last thing > > you do in probe. > > > > So yes, you are right, and we do know about these issues (again see the > > talk I mentioned and some previous ones for many years at plumbers > > conferences by different people.) It's just up to someone to do the > > work to fix them. > > > > If you think we can prevent the race in the rust side, wonderful, I'm > > all for that being a valid fix. > > The current patch prevents the race by only allowing access to the > `struct miscdevice` in fops->open(). That's safe since > `file->f_op->open` runs with `misc_mtx` held. Do we really need the > miscdevice to stay alive for longer? You can already take a refcount > on `this_device` if you want to keep the device alive for longer for > dev_* printing purposes, but it seems like that is the only field you > really need from the `struct miscdevice` past fops->open()? Good point, I also can't really see anything within struct miscdevice that a driver could need other than `this_device`. How would you provide the `device::Device` within the `MiscDevice` trait functions? If we don't guarantee that the `struct miscdevice` is still alive past open() we need to take a reference on `this_device` in open(). I guess the idea would be to let `MiscDeviceRegistration` provide a function to obtain an `ARef`? > > Alice >