linux-kernel - Re: [fuse-devel] [PATCH v2] fuse: Fix IOC_[GS]ET{FLAGS, VERSION} argument size brokenness.

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Date:	Mon, 6 Jan 2014 18:50:20 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	fuse-devel@...ts.sourceforge.net,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Richard Hansen <rhansen@...nsen.org>
Subject: Re: [fuse-devel] [PATCH v2] fuse: Fix IOC_[GS]ET{FLAGS, VERSION}
 argument size brokenness.

On Fri, Dec 20, 2013 at 03:35:34PM -0800, Darrick J. Wong wrote:
> The IOC_[GS]ETFLAGS and IOC_[GS]ETVERSION ioctls, despite being
> defined to take a "long" parameter, actually take "int" parameters.
> FUSE unfortunately assumed that the ioctl definitions never lie, and

FUSE doesn't really assume anything.  It just says, that ioctl's that *properly*
use the _IO* macros will work.  Everything else will not.

It's unfortunate that IOC_SET/GETFLAGS is something that may actually make sense
on a fuse filesystem...

Two ways out of this:

 A) Add quirks to fuse
 B) Add new, fixed ioctls and use those from chattr if available

I'd very much prefer B.

Thanks,
Miklos

> transfers a long's worth of data in and out of userspace, which causes
> stack smashing in chattr, and other bugs elsewhere.
> 
> So, special-case this in FUSE so that we don't crash userland.
> 
> v2: Do the same for the IOC_[GS]ETVERSION ioctls, as Richard Hansen
>     points out.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> ---
>  fs/fuse/file.c |   16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 7e70506..f8766ab 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -2385,6 +2385,22 @@ long fuse_do_ioctl(struct file *file, unsigned int cmd, unsigned long arg,
>  		iov->iov_base = (void __user *)arg;
>  		iov->iov_len = _IOC_SIZE(cmd);
>  
> +		/*
> +		 * The IOC_[GS]ETFLAGS and IOC_[GS]ETVERSION ioctls take int
> +		 * parameters even though the ioctl definition specifies long.
> +		 * Userland has been expecting int for ages (and chattr
> +		 * segfaults on FUSE filesystems), so special case that here.
> +		 * The IOC32 variants were declared with int, so they don't
> +		 * need this correction.
> +		 */
> +		switch (cmd) {
> +		case FS_IOC_GETFLAGS:
> +		case FS_IOC_SETFLAGS:
> +		case FS_IOC_GETVERSION:
> +		case FS_IOC_SETVERSION:
> +			iov->iov_len = sizeof(int);
> +		}
> +
>  		if (_IOC_DIR(cmd) & _IOC_WRITE) {
>  			in_iov = iov;
>  			in_iovs = 1;
> 
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