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From: Dave Chinner <david@fromorbit.com>
To: Jann Horn <jannh@google.com>
Cc: Richard Henderson <rth@twiddle.net>,
	Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
	Matt Turner <mattst88@gmail.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Theodore Ts'o <tytso@mit.edu>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	linux-alpha@vger.kernel.org, linux-kernel@vger.kernel.org,
	Pavel Machek <pavel@ucw.cz>,
	linux-arch@vger.kernel.org, linux-api@vger.kernel.org
Subject: Re: [PATCH v4 2/3] fs: don't let getdents return bogus names
Date: Mon, 21 Jan 2019 09:17:18 +1100
Message-ID: <20190120221718.GZ4205@dastard> (raw)
In-Reply-To: <20190118161440.220134-2-jannh@google.com>

On Fri, Jan 18, 2019 at 05:14:39PM +0100, Jann Horn wrote:
> When you e.g. run `find` on a directory for which getdents returns
> "filenames" that contain slashes, `find` passes those "filenames" back to
> the kernel, which then interprets them as paths. That could conceivably
> cause userspace to do something bad when accessing something like an
> untrusted USB stick, but I'm not aware of any specific example.
> 
> Instead of returning bogus filenames to userspace, return -EUCLEAN.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
> I ordered this fix before the refactoring one so that it can easily be
> backported.
> 
> changed in v2:
>  - move bogus_dirent_name() out of the #ifdef (kbuild test robot)
> changed in v3:
>  - change calling convention (Al Viro)
>  - comment fix
> changed in v4:
>  - use EFSCORRUPTED instead of EUCLEAN (Dave Chinner)
> 
>  arch/alpha/kernel/osf_sys.c |  4 ++++
>  fs/readdir.c                | 35 +++++++++++++++++++++++++++++++++++
>  include/linux/fs.h          |  2 ++
>  3 files changed, 41 insertions(+)
> 
> diff --git a/arch/alpha/kernel/osf_sys.c b/arch/alpha/kernel/osf_sys.c
> index 792586038808..db1c2144d477 100644
> --- a/arch/alpha/kernel/osf_sys.c
> +++ b/arch/alpha/kernel/osf_sys.c
> @@ -40,6 +40,7 @@
>  #include <linux/vfs.h>
>  #include <linux/rcupdate.h>
>  #include <linux/slab.h>
> +#include <linux/fs.h>
>  
>  #include <asm/fpu.h>
>  #include <asm/io.h>
> @@ -117,6 +118,9 @@ osf_filldir(struct dir_context *ctx, const char *name, int namlen,
>  	unsigned int reclen = ALIGN(NAME_OFFSET + namlen + 1, sizeof(u32));
>  	unsigned int d_ino;
>  
> +	buf->error = check_dirent_name(name, namlen);
> +	if (unlikely(buf->error))
> +		return -EFSCORRUPTED;
>  	buf->error = -EINVAL;	/* only used if we fail */
>  	if (reclen > buf->count)
>  		return -EINVAL;
> diff --git a/fs/readdir.c b/fs/readdir.c
> index 2f6a4534e0df..58088510bb9c 100644
> --- a/fs/readdir.c
> +++ b/fs/readdir.c
> @@ -64,6 +64,26 @@ int iterate_dir(struct file *file, struct dir_context *ctx)
>  }
>  EXPORT_SYMBOL(iterate_dir);
>  
> +/*
> + * Most filesystems don't filter out bogus directory entry names, and userspace
> + * can get very confused by such names. Behave as if a filesystem error had
> + * happened while reading directory entries.
> + */
> +int check_dirent_name(const char *name, int namlen)
> +{
> +	if (namlen == 0) {
> +		pr_err_once("%s: filesystem returned bogus empty name\n",
> +			    __func__);
> +		return -EFSCORRUPTED;
> +	}
> +	if (memchr(name, '/', namlen)) {
> +		pr_err_once("%s: filesystem returned bogus name '%*pEhp' (contains slash)\n",
> +			    __func__, namlen, name);
> +		return -EFSCORRUPTED;
> +	}
> +	return 0;
> +}
> +
>  /*
>   * Traditional linux readdir() handling..
>   *
> @@ -98,6 +118,9 @@ static int fillonedir(struct dir_context *ctx, const char *name, int namlen,
>  
>  	if (buf->result)
>  		return -EINVAL;
> +	buf->result = check_dirent_name(name, namlen);
> +	if (unlikely(buf->result))
> +		return -EFSCORRUPTED;

Why bother returning an error from check_dirent_name() if you just
throw it away? i.e:

	if (!dirent_name_valid(name, namelen))
		return -EFSCORRUPTED;

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

  reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-18 16:14 [PATCH v4 1/3] fs: hoist EFSCORRUPTED definition into uapi header Jann Horn
2019-01-18 16:14 ` [PATCH v4 2/3] fs: don't let getdents return bogus names Jann Horn
2019-01-20 22:17   ` Dave Chinner [this message]
2019-01-18 16:14 ` [PATCH v4 3/3] fs: let filldir_t return bool instead of an error code Jann Horn
2019-01-20 22:40   ` Dave Chinner
2019-01-21 15:49     ` Jann Horn
2019-01-21 22:24       ` Dave Chinner
2019-01-23 15:07         ` Jann Horn
2019-01-31 20:39           ` Darrick J. Wong
2019-01-18 16:23 ` [PATCH v4 1/3] fs: hoist EFSCORRUPTED definition into uapi header Arnd Bergmann
2019-01-20 22:13 ` Dave Chinner
2019-01-21 21:54 ` Theodore Y. Ts'o
2019-01-21 22:13   ` Dave Chinner
2019-01-21 22:14   ` David Sterba
2019-01-21 23:51   ` Darrick J. Wong
2019-01-22  0:38     ` Theodore Y. Ts'o

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