Comment 4 for bug 1794067

FYI:

---------- Forwarded message ---------
From: Stephen Smalley <email address hidden>
Date: Fr., 21. Sep. 2018 um 19:09 Uhr
Subject: Re: Bug in selinux on ubuntu 16.04 with kernel 4.15.0-34
To: Benjamin Schüle <email address hidden>, <email address hidden>,
Paul Moore <email address hidden>

On 09/21/2018 04:50 AM, Benjamin Schüle wrote:
> Hello,
>
> just found a bug in selinux. It appears on ubuntu 16.04 with kernel
> 4.15, but not with kernel 4.4.
>
> What's going wrong:
> Copy a link with "-a" option while selinux is on.
>
>
> steps to reproduce:
> ~$ mkdir -p a/b
> ~$ ln -s b a/c
> ~$ cp -a a b
> cp: failed to restore the default file creation context: Invalid argument
>
>
> Results of my investigation:
>
> The "cp" of coreutils is calling "setfscreatecon (NULL)" to restore
> the default file creation context (coreutils-8.30/src/copy.c:1771) as
> it is stated in the selinux api
> (/libselinux/include/selinux/selinux.h:71).
>
> As we see in the result of strace below, the kernel returns an -1 on
> try to restore the default file creation context. So, in my opinion,
> is the bug has to be in the selinux_setprocattr method in the
> security/selinux/hooks.c file.
>
>
> Part of "strace cp -a a b"
>
> lgetxattr("a/c", "security.selinux",
> "system_u:object_r:user_home_dir_t:s0", 255) = 37
> readlink("a/c", "b", 2) = 1
> symlink("b", "b/a/c") = 0
> open("/proc/self/task/2136/attr/fscreate", O_RDWR|O_CLOEXEC) = 3
> write(3, NULL, 0) = -1 EINVAL (Invalid argument)
> close(3) = 0
> open("/usr/share/locale/locale.alias", O_RDONLY|O_CLOEXEC) = 3
> fstat(3, {st_mode=S_IFREG|0644, st_size=2995, ...}) = 0
> read(3, "# Locale name alias data base.\n#"..., 4096) = 2995
> read(3, "", 4096) = 0
> close(3)

This appears to be Ubuntu-specific; I can't reproduce upstream. If you
are able to reproduce with an upstream kernel, let us know; otherwise,
file a bug with Ubuntu. A quick look at the Ubuntu kernel git tree
shows the following commit which would explain this regression.

commit 36788bfe15f16b2eba39d0e563ae8027c5072b98
Author: Colin Ian King <email address hidden>
Date: Tue Oct 3 13:12:54 2017 +0100

     UBUNTU: SAUCE: LSM stacking: check for invalid zero sized writes

     BugLink: http://bugs.launchpad.net/bugs/1720779
     BugLink: http://bugs.launchpad.net/bugs/1763062

     Writing zero bytes to /proc/$pid/task/$pid/attr/context via
     security_setprocattr cause an oops in memcpy_erms. Fix this by
     checking for zero size and returning -EINVAL for this invalid
     write size.

     Detected by running stress-ng --procfs 0

     Signed-off-by: Colin Ian King <email address hidden>
     Signed-off-by: Seth Forshee <email address hidden>