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List:       full-disclosure
Subject:    [FD] CVE-2015-7723 - Privilege Escalation Via Symlink Attacks On POSIX Shared Memory With Insecure P
From:       Portcullis Advisories <advisories () portcullis-security ! com>
Date:       2015-10-29 10:27:06
Message-ID: C56B81159AD346478B2C9D1487742F0B467928AD () portcullis-security ! com
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Vulnerability title: Privilege Escalation Via Symlink Attacks On POSIX Shared Memory With \
                Insecure Permissions In AMD fglrx-driver
CVE: CVE-2015-7723
Vendor: AMD
Product: fglrx-driver
Affected version: 14.4.2
Fixed version: 15.7
Reported by: Tim Brown
Details:

It has been identified that the userland portion of the fglrx-driver utilised by Xorg allows \
privilege escalation via symlink attacks on POSIX shared memory with insecure permissions.

On systems 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 libaticaldd.so and fglrx_dri.so) 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-2015-7723/ \


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