RFC: CVE-2017-2619 fix breaks accessing previous versions of directories with snapshots in subdirectories of the share

[Jeremy Allison]

On Fri, Jul 07, 2017 at 02:12:53PM +0200, Ralph Böhme wrote:
> Hi!
> 
> As explained in <https://bugzilla.samba.org/show_bug.cgi?id=12885>:
> 
> With shadow:snapdirseverywhere=true and a snapshot directory that
> 
> * is a subdirectory of a share
> 
> * and that contains a snapshot directory
> 
> we fail the symlink check in the new function non_widelink_open() because
> parent_dirname() cuts off the subdirectory name leaving only the @GMT stanza
> which is then interpreted by the called functions as being relative to the
> parent directory which it isn't.
> 
> The simplest fix as far as I can see is to leverage the fact that (given the
> system defines O_DIRECTORY) we know when we're called for a directory, so we can
> just directly chdir() into the path passed by the caller.
> 
> Can we rely here on O_DIRECTORY? Linux has it, FreeBSD has it, Solaris has it
> and we probably don't care about the rest.
> 
> The subsequent security check done in check_reduced_name() should continue to
> work with this change.
> 
> I've just fire of a private autobuild with the patchset and will follow up with
> the results (fingers crossed :) ).

Great catch Ralph. That's a really minimal fix with no disruption
to the security checks whatsoever !

Words fail me when I try and articulate how much I *HATE* the
shadow_copy2 code (even after I fixed up a lot of it :-).

Jeremy.

> From 48211a8b2d01b22064c16093be0a1be95f9b9ddb Mon Sep 17 00:00:00 2001
> From: Ralph Boehme <slow at samba.org>
> Date: Fri, 7 Jul 2017 12:57:57 +0200
> Subject: [PATCH 1/2] s3/smbd: let non_widelink_open() chdir() to directories
>  directly
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> If the caller passes O_DIRECTORY we just try to chdir() to smb_fname
> directly, not to the parent directory.
> 
> The security check in check_reduced_name() will continue to work, but
> this fixes the case of an open() for a previous version of a
> subdirectory that contains snapshopt.
> 
> Eg:
> 
> [share]
>     path = /shares/test
>     vfs objects = shadow_copy2
>     shadow:snapdir = .snapshots
>     shadow:snapdirseverywhere = yes
> 
> Directory tree with fake snapshots:
> 
> $ tree -a /shares/test/
> /shares/test/
> ├── dir
> │   ├── file
> │   └── .snapshots
> │       └── @GMT-2017.07.04-04.30.12
> │           └── file
> ├── dir2
> │   └── file
> ├── file
> ├── .snapshots
> │   └── @GMT-2001.01.01-00.00.00
> │       ├── dir2
> │       │   └── file
> │       └── file
> └── testfsctl.dat
> 
> ./bin/smbclient -U slow%x //localhost/share -c 'ls @GMT-2017.07.04-04.30.12/dir/*'
> NT_STATUS_OBJECT_NAME_NOT_FOUND listing \@GMT-2017.07.04-04.30.12\dir\*
> 
> Bug: https://bugzilla.samba.org/show_bug.cgi?id=12885
> 
> Signed-off-by: Ralph Boehme <slow at samba.org>
> ---
>  source3/smbd/open.c | 30 +++++++++++++++++++++++++-----
>  1 file changed, 25 insertions(+), 5 deletions(-)
> 
> diff --git a/source3/smbd/open.c b/source3/smbd/open.c
> index 3ccee36..7781a6f 100644
> --- a/source3/smbd/open.c
> +++ b/source3/smbd/open.c
> @@ -550,12 +550,32 @@ static int non_widelink_open(struct connection_struct *conn,
>  	char *parent_dir = NULL;
>  	struct smb_filename parent_dir_fname = {0};
>  	const char *final_component = NULL;
> +	bool is_directory = false;
> +	bool ok;
>  
> -	if (!parent_dirname(talloc_tos(),
> -			smb_fname->base_name,
> -			&parent_dir,
> -			&final_component)) {
> -		goto out;
> +#ifdef O_DIRECTORY
> +	if (flags & O_DIRECTORY) {
> +		is_directory = true;
> +	}
> +#endif
> +
> +	if (is_directory) {
> +		parent_dir = talloc_strdup(talloc_tos(), smb_fname->base_name);
> +		if (parent_dir == NULL) {
> +			saved_errno = errno;
> +			goto out;
> +		}
> +
> +		final_component = ".";
> +	} else {
> +		ok = parent_dirname(talloc_tos(),
> +				    smb_fname->base_name,
> +				    &parent_dir,
> +				    &final_component);
> +		if (!ok) {
> +			saved_errno = errno;
> +			goto out;
> +		}
>  	}
>  
>  	parent_dir_fname = (struct smb_filename) { .base_name = parent_dir };
> -- 
> 2.9.4
> 
> 
> From eedf92df6c26ed29ed5a93cdb34e6da22982b6a3 Mon Sep 17 00:00:00 2001
> From: Ralph Boehme <slow at samba.org>
> Date: Fri, 7 Jul 2017 13:12:19 +0200
> Subject: [PATCH 2/2] selftest: add a test for accessing previous version of
>  directories with snapdirseverywhere
> 
> Bug: https://bugzilla.samba.org/show_bug.cgi?id=12885
> 
> Signed-off-by: Ralph Boehme <slow at samba.org>
> ---
>  source3/script/tests/test_shadow_copy.sh | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/source3/script/tests/test_shadow_copy.sh b/source3/script/tests/test_shadow_copy.sh
> index 783e7f32..eba873f 100755
> --- a/source3/script/tests/test_shadow_copy.sh
> +++ b/source3/script/tests/test_shadow_copy.sh
> @@ -221,6 +221,26 @@ test_fetch_snap_file()
>          -c "get ${SNAPSHOTS[$snapidx]}/$path $WORKDIR/foo"
>  }
>  
> +# Test fetching a previous version of a file
> +test_fetch_snap_dir()
> +{
> +    local share
> +    local path
> +    local snapidx
> +
> +    share=$1
> +    path=$2
> +    snapidx=$3
> +
> +    # This first command is not strictly needed, but it causes the snapshots to
> +    # appear in a network trace which helps debugging...
> +    $SMBCLIENT -U$USERNAME%$PASSWORD "//$SERVER/$share" -I $SERVER_IP \
> +        -c "allinfo $path"
> +
> +    $SMBCLIENT -U$USERNAME%$PASSWORD "//$SERVER/$share" -I $SERVER_IP \
> +        -c "ls ${SNAPSHOTS[$snapidx]}/$path/*"
> +}
> +
>  test_shadow_copy_fixed()
>  {
>      local share     #share to contact
> @@ -329,6 +349,9 @@ test_shadow_copy_everywhere()
>          test_fetch_snap_file $share "bar/lfoo" 3 || \
>          failed=`expr $failed + 1`
>  
> +    testit "list a previous version directory" \
> +        test_fetch_snap_dir $share "bar" 6 || \
> +        failed=`expr $failed + 1`
>  }
>  
>  test_shadow_copy_format()
> -- 
> 2.9.4
>