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 <colin.king@xxxxxxxxxxxxx>
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 <colin.king@xxxxxxxxxxxxx>
Signed-off-by: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
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