CVE-2015-7724 - Privilege Escalation Via Symlink Attacks On POSIX Shared Memory With Insecure Permissions In AMD fglrx-driver

Vulnerability title: Privilege Escalation Via Symlink Attacks On POSIX Shared Memory With Insecure Permissions In AMD fglrx-driver
CVE: CVE-2015-7724
Vendor: AMD
Product: fglrx-driver
Affected version: 15.7
Fixed version: 15.9
Reported by: Tim Brown
Details:

In the process of validating the fix for CVE-2015-7723, it has been identified that the userland portion of the fglrx-driver utilised by Xorg still allows privilege escalation via symlink attacks on POSIX shared memory with insecure permissions.

As with the original vulnerability, with a Linux kernel where fs.protected_symlinks is not set to 1, or where this feature is not available, the following code (present in shared libraries distributed as part of fglrx-driver) calls shm_open() with insecure flags specified which allows the Linux kernel to follow an existent symlink under /dev/shm:

mov $0x1b6,%edx ; $edx (mode) = 0666
mov $0x42,%esi ; $esi (oflag) = O_CREAT | O_RDWR - missing O_EXCL
mov %r13,%rdi
callq 208b68 <shm_open@plt>

The same call to shm_open() requests that the permissions of newly created file are 0666 and as a result, an arbitrary file, owned by root, with permissions of 0666 can be created anywhere on the filesystem

Furthermore, the code then proceeds to force the permissions on the resultant file to 0666 using fchmod(). This can be useful if the symlink target already exists:

mov $0x1b6,%esi ; $esi (mode) = 0666
mov %eax,%edi
callq 209058 <fchmod@plt>

Further details at:

https://www.portcullis-security.com/security-research-and-downloads/security-advisories/cve-2015-7724/

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