CVE-2024-46695

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

selinux,smack: dont bypass permissions check in inode_setsecctx hook

Marek Gresko reports that the root user on an NFS client is able to change the security labels on files on an NFS filesystem that is exported with root squashing enabled.

The end of the kerneldoc comment for __vfs_setxattr_noperm() states:

• This function requires the caller to lock the inodes i_mutex before it
• is executed. It also assumes that the caller will make the appropriate
• permission checks.

nfsd_setattr() does do permissions checking via fh_verify() and nfsd_permission(), but those dont do all the same permissions checks that are done by security_inode_setxattr() and its related LSM hooks do.

Since nfsd_setattr() is the only consumer of security_inode_setsecctx(), simplest solution appears to be to replace the call to __vfs_setxattr_noperm() with a call to __vfs_setxattr_locked(). This fixes the above issue and has the added benefit of causing nfsd to recall conflicting delegations on a file when a client tries to change its security label.

Weakness

During installation, installed file permissions are set to allow anyone to modify those files.

Affected Software

Potential Mitigations

• Compartmentalize the system to have “safe” areas where trust boundaries can be unambiguously drawn. Do not allow sensitive data to go outside of the trust boundary and always be careful when interfacing with a compartment outside of the safe area.
• Ensure that appropriate compartmentalization is built into the system design, and the compartmentalization allows for and reinforces privilege separation functionality. Architects and designers should rely on the principle of least privilege to decide the appropriate time to use privileges and the time to drop privileges.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/1.html
• https://cwe.mitre.org/data/definitions/127.html
• https://cwe.mitre.org/data/definitions/81.html

References

• https://git.kernel.org/stable/c/2dbc4b7bac60b02cc6e70d05bf6a7dfd551f9dda
• https://git.kernel.org/stable/c/459584258d47ec3cc6245a82e8a49c9d08eb8b57
• https://git.kernel.org/stable/c/76a0e79bc84f466999fa501fce5bf7a07641b8a7
• https://git.kernel.org/stable/c/eebec98791d0137e455cc006411bb92a54250924
• https://git.kernel.org/stable/c/f71ec019257ba4f7ab198bd948c5902a207bad96
• https://git.kernel.org/stable/c/fe0cd53791119f6287b6532af8ce41576d664930