CVE-2021-47118 — pid: take a reference when initializing `cad_pid`

Affected: Linux

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

pid: take a reference when initializing `cad_pid`

During boot, kernel_init_freeable() initializes `cad_pid` to the init task's struct pid. Later on, we may change `cad_pid` via a sysctl, and when this happens proc_do_cad_pid() will increment the refcount on the new pid via get_pid(), and will decrement the refcount on the old pid via put_pid(). As we never called get_pid() when we initialized `cad_pid`, we decrement a reference we never incremented, can therefore free the init task's struct pid early. As there can be dangling references to the struct pid, we can later encounter a use-after-free (e.g. when delivering signals).

This was spotted when fuzzing v5.13-rc3 with Syzkaller, but seems to have been around since the conversion of `cad_pid` to struct pid in commit 9ec52099e4b8 ("[PATCH] replace cad_pid by a struct pid") from the pre-KASAN stone age of v2.6.19.

Fix this by getting a reference to the init task's struct pid when we assign it to `cad_pid`.

Full KASAN splat below.

================================================================== BUG: KASAN: use-after-free in ns_of_pid include/linux/pid.h:153 [inline] BUG: KASAN: use-after-free in task_active_pid_ns+0xc0/0xc8 kernel/pid.c:509 Read of size 4 at addr ffff23794dda0004 by task syz-executor.0/273

CPU: 1 PID: 273 Comm: syz-executor.0 Not tainted 5.12.0-00001-g9aef892b2d15 #1 Hardware name: linux,dummy-virt (DT) Call t…

USN-7183-1 — linux, linux-lts-xenial vulnerabilities

Affected: Ubuntu:Pro:14.04:LTS: linux-lts-xenial, Ubuntu:Pro:16.04:LTS: linux

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Andy Nguyen discovered that the Bluetooth A2MP implementation in the Linux kernel did not properly initialize memory in some situations. A physically proximate remote attacker could use this to expose sensitive information (kernel memory). (CVE-2020-12352)

Andy Nguyen discovered that the Bluetooth HCI event packet parser in the Linux kernel did not properly handle event advertisements of certain sizes, leading to a heap-based buffer overflow. A physically proximate remote attacker could use this to cause a denial of service (system crash) or possibly execute arbitrary code. (CVE-2020-24490)

Ziming Zhang discovered that the VMware Virtual GPU DRM driver in the Linux kernel contained an integer overflow vulnerability. A local attacker could use this to cause a denial of service (system crash). (CVE-2022-36402)

Zheng Wang discovered a use-after-free in the Renesas Ethernet AVB driver in the Linux kernel during device removal. A privileged attacker could use this to cause a denial of service (system crash). (CVE-2023-35827)

Several security issues were discovered in the Linux kernel. An attacker could possibly use these to compromise the system. This update corrects flaws in the following subsystems: - GPU dr…

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CVE-2021-47118 — pid: take a reference when initializing `cad_pid`

USN-7183-1 — linux, linux-lts-xenial vulnerabilities