In the Linux kernel, the following vulnerability has been resolved:

tracefs: Reset permissions on remount if permissions are options

Theres an inconsistency with the way permissions are handled in tracefs. Because the permissions are generated when accessed, they default to the root inodes permission if they were never set by the user. If the user sets the permissions, then a flag is set and the permissions are saved via the inode (for tracefs files) or an internal attribute field (for eventfs).

But if a remount happens that specify the permissions, all the files that were not changed by the user gets updated, but the ones that were are not. If the user were to remount the file system with a given permission, then all files and directories within that file system should be updated.

This can cause security issues if a files permission was updated but the admin forgot about it. They could incorrectly think that remounting with permissions set would update all files, but miss some.

For example:

cd /sys/kernel/tracing

chgrp 1002 current_tracer

ls -l

[..] -rw-r—– 1 root root 0 May 1 21:25 buffer_size_kb -rw-r—– 1 root root 0 May 1 21:25 buffer_subbuf_size_kb -r–r—– 1 root root 0 May 1 21:25 buffer_total_size_kb -rw-r—– 1 root lkp 0 May 1 21:25 current_tracer -rw-r—– 1 root root 0 May 1 21:25 dynamic_events -r–r—– 1 root root 0 May 1 21:25 dyn_ftrace_total_info -r–r—– 1 root root 0 May 1 21:25 enabled_functions

Where current_tracer now has group lkp.

mount -o remount,gid=1001 .

ls -l

-rw-r—– 1 root tracing 0 May 1 21:25 buffer_size_kb -rw-r—– 1 root tracing 0 May 1 21:25 buffer_subbuf_size_kb -r–r—– 1 root tracing 0 May 1 21:25 buffer_total_size_kb -rw-r—– 1 root lkp 0 May 1 21:25 current_tracer -rw-r—– 1 root tracing 0 May 1 21:25 dynamic_events -r–r—– 1 root tracing 0 May 1 21:25 dyn_ftrace_total_info -r–r—– 1 root tracing 0 May 1 21:25 enabled_functions

Everything changed but the current_tracer.

Add a new link list that keeps track of all the tracefs_inodes which has the permission flags that tell if the file/dir should use the root inodes permission or not. Then on remount, clear all the flags so that the default behavior of using the root inodes permission is done for all files and directories.

Weakness

The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. This allows attackers to bypass intended access restrictions.

Affected Software

Extended Description

Assuming a user with a given identity, authorization is the process of determining whether that user can access a given resource, based on the user’s privileges and any permissions or other access-control specifications that apply to the resource. When access control checks are incorrectly applied, users are able to access data or perform actions that they should not be allowed to perform. This can lead to a wide range of problems, including information exposures, denial of service, and arbitrary code execution.

Potential Mitigations

• Divide the product into anonymous, normal, privileged, and administrative areas. Reduce the attack surface by carefully mapping roles with data and functionality. Use role-based access control (RBAC) [REF-229] to enforce the roles at the appropriate boundaries.
• Note that this approach may not protect against horizontal authorization, i.e., it will not protect a user from attacking others with the same role.
• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
• For example, consider using authorization frameworks such as the JAAS Authorization Framework [REF-233] and the OWASP ESAPI Access Control feature [REF-45].
• For web applications, make sure that the access control mechanism is enforced correctly at the server side on every page. Users should not be able to access any unauthorized functionality or information by simply requesting direct access to that page.
• One way to do this is to ensure that all pages containing sensitive information are not cached, and that all such pages restrict access to requests that are accompanied by an active and authenticated session token associated with a user who has the required permissions to access that page.

References

• https://git.kernel.org/stable/c/414fb08628143203d29ccd0264b5a83fb9523c03
• https://git.kernel.org/stable/c/5f91fc82794d4a6e41cdcd02d00baa377d94ca78
• https://git.kernel.org/stable/c/baa23a8d4360d981a49913841a726edede5cdd54
• https://git.kernel.org/stable/c/414fb08628143203d29ccd0264b5a83fb9523c03
• https://git.kernel.org/stable/c/5f91fc82794d4a6e41cdcd02d00baa377d94ca78
• https://git.kernel.org/stable/c/baa23a8d4360d981a49913841a726edede5cdd54