When using fake-super mode in an rsync receiver, anything that's neither a file nor a directory (e.g. devices, symlinks, etc) is converted into a file, and properties such as original ownership, filetype, and permissions are stored in a specific extended attribute. In the case of a symlink, the contents of the link are stored in a plain file. The original mode of the symlink is normally irrelevant, because (Linux) hosts ignore a symlink's mode and use that of the target instead. But in fake-super mode, the original mode of the link itself (usually 0120777) is used to set the permissions on the receiver's plain-file copy. This results in the copy being world-writable. If this plain file is altered and then transferred back to the origin, the resulting symlink can point to an arbitrary path, which leads to potential security issues. Example: This was first observed in version 3.1.1 on kubuntu, but is still the same in version 3.1.3 as of 28 Jan 2018. See also Storing-ownership-device-nodes-without-root <http://samba.2283325.n4.nabble.com/Storing-ownership-device-nodes-without-root-td2503256.html#a2503261> .Dave. -- Sent from: http://samba.2283325.n4.nabble.com/Samba-rsync-f2500462.html
On 03/02/18 13:20, Dave Gordon via rsync wrote:> When using fake-super mode in an rsync receiver, anything that's neither a > file nor a directory (e.g. devices, symlinks, etc) is converted into a file, > and properties such as original ownership, filetype, and permissions are > stored in a specific extended attribute. > > In the case of a symlink, the contents of the link are stored in a plain > file. The original mode of the symlink is normally irrelevant, because > (Linux) hosts ignore a symlink's mode and use that of the target instead. > But in fake-super mode, the original mode of the link itself (usually > 0120777) is used to set the permissions on the receiver's plain-file copy. > > This results in the copy being world-writable. If this plain file is altered > and then transferred back to the origin, the resulting symlink can point to > an arbitrary path, which leads to potential security issues. > > Example: > > This was first observed in version 3.1.1 on kubuntu, but is still the same > in version 3.1.3 as of 28 Jan 2018. > See also Storing-ownership-device-nodes-without-root > <http://samba.2283325.n4.nabble.com/Storing-ownership-device-nodes-without-root-td2503256.html#a2503261> > > .Dave. > > -- > Sent from: http://samba.2283325.n4.nabble.com/Samba-rsync-f2500462.html >Hmm, the forum seems to have dropped the example (which was in 'raw' tags to preserve the formatting). Oh well, here it is ... Example: As regular user 'dg': dg$ mkdir src dg$ touch src/foo dg$ chmod 444 src/foo dg$ ln -s foo src/bar Then as trusted (but unprivileged) user 'backup': backup$ id -a uid=1000(backup) gid=1000(backup) groups=1000(backup),100(users) backup$ umask 0022 backup$ rsync -av src/ dst-nonsuper/ backup$ rsync -av --fake-super src/ dst-fake-super/ Now as (untrusted) user 'guest': guest$ id -a uid=9999(guest) gid=9999(guest) groups=9999(guest) guest$ echo -n '/etc/shadow' > dst-nonsuper/bar bash: dst-nonsuper/bar: Permission denied guest$ echo -n '/etc/secret' > dst-fake-super/bar # allowed!! And finally, as user 'dg', restore from the fake-super backup: dg$ rsync -av --fake-super -M--super dst-fake-super/ recovered/ dg$ ls -laR */ ./src: total 8 drwxr-xr-x 2 dg users 4096 Feb 3 11:59 . drwxrwxr-x 6 dg users 4096 Feb 3 13:00 .. lrwxrwxrwx 1 dg users 3 Feb 3 11:59 bar -> foo -r--r--r-- 1 dg users 0 Feb 3 11:59 foo ./dst-nonsuper: total 8 drwxr-xr-x 2 backup users 4096 Feb 3 11:59 . drwxrwxr-x 6 dg users 4096 Feb 3 13:00 .. lrwxrwxrwx 1 backup users 3 Feb 3 11:59 bar -> foo -r--r--r-- 1 backup users 0 Feb 3 11:59 foo ./dst-fake-super: total 12 drwxr-xr-x 2 backup backup 4096 Feb 3 11:59 . drwxrwxr-x 6 dg users 4096 Feb 3 13:00 .. -rwxrwxrwx 1 backup backup 11 Feb 3 12:51 bar -rw-r--r-- 1 backup backup 0 Feb 3 11:59 foo ./recovered: total 8 drwxr-xr-x 2 dg users 4096 Feb 3 11:59 . drwxrwxr-x 6 dg users 4096 Feb 3 13:00 .. lrwxrwxrwx 1 dg users 11 Feb 3 12:51 bar -> /etc/shadow -r--r--r-- 1 dg users 0 Feb 3 11:59 foo dg$ getfattr -d -R */ # file: dst-fake-super user.rsync.%stat="40755 0,0 32768:100" # file: dst-fake-super/bar user.rsync.%stat="120777 0,0 32768:100" # file: dst-fake-super/foo user.rsync.%stat="100444 0,0 32768:100" .Dave.
On 03/02/18 15:52, Dave Gordon via rsync wrote:> On 03/02/18 13:20, Dave Gordon via rsync wrote: >> When using fake-super mode in an rsync receiver, anything that's neither a >> file nor a directory (e.g. devices, symlinks, etc) is converted into a file, >> and properties such as original ownership, filetype, and permissions are >> stored in a specific extended attribute. >> >> In the case of a symlink, the contents of the link are stored in a plain >> file. The original mode of the symlink is normally irrelevant, because >> (Linux) hosts ignore a symlink's mode and use that of the target instead. >> But in fake-super mode, the original mode of the link itself (usually >> 0120777) is used to set the permissions on the receiver's plain-file copy. >> >> This results in the copy being world-writable. If this plain file is altered >> and then transferred back to the origin, the resulting symlink can point to >> an arbitrary path, which leads to potential security issues. >> >> Example: >> >> This was first observed in version 3.1.1 on kubuntu, but is still the same >> in version 3.1.3 as of 28 Jan 2018. >> See also Storing-ownership-device-nodes-without-root >> <http://samba.2283325.n4.nabble.com/Storing-ownership-device-nodes-without-root-td2503256.html#a2503261> >> >> .Dave. >> >> -- >> Sent from: http://samba.2283325.n4.nabble.com/Samba-rsync-f2500462.html >> > > Hmm, the forum seems to have dropped the example (which was in 'raw' > tags to preserve the formatting). Oh well, here it is ... > > Example: > > As regular user 'dg': > dg$ mkdir src > dg$ touch src/foo > dg$ chmod 444 src/foo > dg$ ln -s foo src/bar > > Then as trusted (but unprivileged) user 'backup': > backup$ id -a > uid=1000(backup) gid=1000(backup) groups=1000(backup),100(users) > backup$ umask > 0022 > backup$ rsync -av src/ dst-nonsuper/ > backup$ rsync -av --fake-super src/ dst-fake-super/ > > Now as (untrusted) user 'guest': > guest$ id -a > uid=9999(guest) gid=9999(guest) groups=9999(guest) > guest$ echo -n '/etc/shadow' > dst-nonsuper/bar > bash: dst-nonsuper/bar: Permission denied > guest$ echo -n '/etc/secret' > dst-fake-super/bar # allowed!! > > And finally, as user 'dg', restore from the fake-super backup: > dg$ rsync -av --fake-super -M--super dst-fake-super/ recovered/ > dg$ ls -laR */ > ./src: > total 8 > drwxr-xr-x 2 dg users 4096 Feb 3 11:59 . > drwxrwxr-x 6 dg users 4096 Feb 3 13:00 .. > lrwxrwxrwx 1 dg users 3 Feb 3 11:59 bar -> foo > -r--r--r-- 1 dg users 0 Feb 3 11:59 foo > > ./dst-nonsuper: > total 8 > drwxr-xr-x 2 backup users 4096 Feb 3 11:59 . > drwxrwxr-x 6 dg users 4096 Feb 3 13:00 .. > lrwxrwxrwx 1 backup users 3 Feb 3 11:59 bar -> foo > -r--r--r-- 1 backup users 0 Feb 3 11:59 foo > > ./dst-fake-super: > total 12 > drwxr-xr-x 2 backup backup 4096 Feb 3 11:59 . > drwxrwxr-x 6 dg users 4096 Feb 3 13:00 .. > -rwxrwxrwx 1 backup backup 11 Feb 3 12:51 bar > -rw-r--r-- 1 backup backup 0 Feb 3 11:59 foo > > ./recovered: > total 8 > drwxr-xr-x 2 dg users 4096 Feb 3 11:59 . > drwxrwxr-x 6 dg users 4096 Feb 3 13:00 .. > lrwxrwxrwx 1 dg users 11 Feb 3 12:51 bar -> /etc/secret > -r--r--r-- 1 dg users 0 Feb 3 11:59 foo > > dg$ getfattr -d -R */ > > # file: dst-fake-super > user.rsync.%stat="40755 0,0 32768:100" > > # file: dst-fake-super/bar > user.rsync.%stat="120777 0,0 32768:100" > > # file: dst-fake-super/foo > user.rsync.%stat="100444 0,0 32768:100" > > .Dave.In an attempt to avoid the above problem, one might try using a --chmod option to ensure that the destination files are not world-writable (here we use a rather silly chmod, just to make it more obvious what is -- and isn't -- affected): <code> backup$ rsync -av src/ dst-nonsuper/ backup$ rsync -av --chmod=D0700,Fu=rx,g=,o=x src/ dst-chmod-501/ backup$ rsync -av --fake-super --chmod=D0700,Fu=rx,g=,o=x src/ dst-fake-chmod-501/ backup$ getfattr -d -R . # file: dst-fake-chmod-501 user.rsync.%stat="40701 0,0 32768:100" # file: dst-fake-chmod-501/bar user.rsync.%stat="120777 0,0 32768:100" # file: dst-fake-chmod-501/foo user.rsync.%stat="100501 0,0 32768:100" backup$ ll -R */ ./src: total 8 drwxr-xr-x 2 dg users 4096 Feb 3 11:59 ./ drwxrwxr-x 6 dg users 4096 Feb 3 17:16 ../ lrwxrwxrwx 1 dg users 3 Feb 3 11:59 bar -> foo -r--r--r-- 1 dg users 0 Feb 3 11:59 foo ./dst-nonsuper: total 8 drwxr-xr-x 2 backup users 4096 Feb 3 11:59 ./ drwxrwxr-x 6 dg users 4096 Feb 3 17:16 ../ lrwxrwxrwx 1 backup users 3 Feb 3 11:59 bar -> foo -r--r--r-- 1 backup users 0 Feb 3 11:59 foo ./dst-chmod-501: total 8 drwx-----x 2 backup users 4096 Feb 3 11:59 ./ drwxrwxr-x 6 dg users 4096 Feb 3 17:16 ../ lrwxrwxrwx 1 backup users 3 Feb 3 11:59 bar -> foo* -r-x-----x 1 backup users 0 Feb 3 11:59 foo* ./dst-fake-chmod-501: total 12 drwx-----x 2 backup backup 4096 Feb 3 11:59 ./ drwxrwxr-x 6 dg users 4096 Feb 3 17:16 ../ -rwxrwxrwx 1 backup backup 3 Feb 3 11:59 bar* -rwx-----x 1 backup backup 0 Feb 3 11:59 foo* </code> In non-fake mode, the permissions on the copies incorporate the --chmod specification, except for the symlink, whose mode is unchanged at 0777 but irrelevant. In fake mode, the tweaks from the --chmod affect the stored value in the xattr, and to some extent the mode of the copy, except that fake-mode copies always include u+rwX, But the symlink is still stored as a world-writable plain file. .Dave.
On Sat, Feb 3, 2018 at 5:20 AM, Dave Gordon via rsync <rsync at lists.samba.org> wrote:> [...fake-super symlink saved as a file...]This results in the copy being world-writable.>Indeed. The file initially gets created as a mode-600 file, but the code later tweaks the permissions to match the symlink, which is (as you note) a bad thing. My first reaction is to change the code in set_stat_xattr() (in xattrs.c) from: if (fst.st_mode != mode) do_chmod(fname, mode); to: if (fst.st_mode != mode && !S_ISLNK(file->mode)) do_chmod(fname, mode); ..wayne.. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.samba.org/pipermail/rsync/attachments/20180204/36fa01ab/attachment.html>
On 05/02/18 05:53, Wayne Davison wrote:> On Sat, Feb 3, 2018 at 5:20 AM, Dave Gordon via rsync > <rsync at lists.samba.org <mailto:rsync at lists.samba.org>> wrote: > > [...fake-super symlink saved as a file...] > > This results in the copy being world-writable. > > Indeed. The file initially gets created as a mode-600 file, but the code > later tweaks the permissions to match the symlink, which is (as you > note) a bad thing. > > My first reaction is to change the code in set_stat_xattr() > (in xattrs.c) from: > > if (fst.st_mode != mode) > do_chmod(fname, mode); > > to: > > if (fst.st_mode != mode && !S_ISLNK(file->mode)) > do_chmod(fname, mode); > > ..wayne..That's certainly an improvement; from the safety point of view, leaving it as 0600 is a lot better than leaving it as 0777. I'm currently investigating a slightly more extensive fix to allow more control over how fake-symlink files end up, also to make fake-super work better with incoming-chmod for the daemon case. .Dave.
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