CVE-2024-43882

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

exec: Fix ToCToU between perm check and set-uid/gid usage

When opening a file for exec via do_filp_open(), permission checking is done against the files metadata at that moment, and on success, a file pointer is passed back. Much later in the execve() code path, the file metadata (specifically mode, uid, and gid) is used to determine if/how to set the uid and gid. However, those values may have changed since the permissions check, meaning the execution may gain unintended privileges.

For example, if a file could change permissions from executable and not set-id:

———x 1 root root 16048 Aug 7 13:16 target

to set-id and non-executable:

—S—— 1 root root 16048 Aug 7 13:16 target

it is possible to gain root privileges when execution should have been disallowed.

While this race condition is rare in real-world scenarios, it has been observed (and proven exploitable) when package managers are updating the setuid bits of installed programs. Such files start with being world-executable but then are adjusted to be group-exec with a set-uid bit. For example, chmod o-x,u+s target makes target executable only by uid root and gid cdrom, while also becoming setuid-root:

-rwxr-xr-x 1 root cdrom 16048 Aug 7 13:16 target

becomes:

-rwsr-xr– 1 root cdrom 16048 Aug 7 13:16 target

But racing the chmod means users without group cdrom membership can get the permission to execute target just before the chmod, and when the chmod finishes, the exec reaches brpm_fill_uid(), and performs the setuid to root, violating the expressed authorization of only cdrom group members can setuid to root.

Re-check that we still have execute permissions in case the metadata has changed. It would be better to keep a copy from the perm-check time, but until we can do that refactoring, the least-bad option is to do a full inode_permission() call (under inode lock). It is understood that this is safe against dead-locks, but hardly optimal.

Weakness

The product checks the state of a resource before using that resource, but the resource’s state can change between the check and the use in a way that invalidates the results of the check.

Affected Software

Potential Mitigations

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/27.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/15469d46ba34559bfe7e3de6659115778c624759
• https://git.kernel.org/stable/c/368f6985d46657b8b466a421dddcacd4051f7ada
• https://git.kernel.org/stable/c/90dfbba89ad4f0d9c9744ecbb1adac4aa2ff4f3e
• https://git.kernel.org/stable/c/9b424c5d4130d56312e2a3be17efb0928fec4d64
• https://git.kernel.org/stable/c/d2a2a4714d80d09b0f8eb6438ab4224690b7121e
• https://git.kernel.org/stable/c/d5c3c7e26275a2d83b894d30f7582a42853a958f
• https://git.kernel.org/stable/c/f50733b45d865f91db90919f8311e2127ce5a0cb
• https://git.kernel.org/stable/c/f6cfc6bcfd5e1cf76115b6450516ea4c99897ae1