CVE-2021-47118 in Linux
Summary
by MITRE • 03/15/2024
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 trace: ns_of_pid include/linux/pid.h:153 [inline]
task_active_pid_ns+0xc0/0xc8 kernel/pid.c:509 do_notify_parent+0x308/0xe60 kernel/signal.c:1950 exit_notify kernel/exit.c:682 [inline]
do_exit+0x2334/0x2bd0 kernel/exit.c:845 do_group_exit+0x108/0x2c8 kernel/exit.c:922 get_signal+0x4e4/0x2a88 kernel/signal.c:2781 do_signal arch/arm64/kernel/signal.c:882 [inline]
do_notify_resume+0x300/0x970 arch/arm64/kernel/signal.c:936 work_pending+0xc/0x2dc
Allocated by task 0: slab_post_alloc_hook+0x50/0x5c0 mm/slab.h:516 slab_alloc_node mm/slub.c:2907 [inline]
slab_alloc mm/slub.c:2915 [inline]
kmem_cache_alloc+0x1f4/0x4c0 mm/slub.c:2920 alloc_pid+0xdc/0xc00 kernel/pid.c:180 copy_process+0x2794/0x5e18 kernel/fork.c:2129 kernel_clone+0x194/0x13c8 kernel/fork.c:2500 kernel_thread+0xd4/0x110 kernel/fork.c:2552 rest_init+0x44/0x4a0 init/main.c:687 arch_call_rest_init+0x1c/0x28 start_kernel+0x520/0x554 init/main.c:1064 0x0
Freed by task 270: slab_free_hook mm/slub.c:1562 [inline]
slab_free_freelist_hook+0x98/0x260 mm/slub.c:1600 slab_free mm/slub.c:3161 [inline]
kmem_cache_free+0x224/0x8e0 mm/slub.c:3177 put_pid.part.4+0xe0/0x1a8 kernel/pid.c:114 put_pid+0x30/0x48 kernel/pid.c:109 proc_do_cad_pid+0x190/0x1b0 kernel/sysctl.c:1401 proc_sys_call_handler+0x338/0x4b0 fs/proc/proc_sysctl.c:591 proc_sys_write+0x34/0x48 fs/proc/proc_sysctl.c:617 call_write_iter include/linux/fs.h:1977 [inline]
new_sync_write+0x3ac/0x510 fs/read_write.c:518 vfs_write fs/read_write.c:605 [inline]
vfs_write+0x9c4/0x1018 fs/read_write.c:585 ksys_write+0x124/0x240 fs/read_write.c:658 __do_sys_write fs/read_write.c:670 [inline]
__se_sys_write fs/read_write.c:667 [inline]
__arm64_sys_write+0x78/0xb0 fs/read_write.c:667 __invoke_syscall arch/arm64/kernel/syscall.c:37 [inline]
invoke_syscall arch/arm64/kernel/syscall.c:49 [inline]
el0_svc_common.constprop.1+0x16c/0x388 arch/arm64/kernel/syscall.c:129 do_el0_svc+0xf8/0x150 arch/arm64/kernel/syscall.c:168 el0_svc+0x28/0x38 arch/arm64/kernel/entry-common.c:416 el0_sync_handler+0x134/0x180 arch/arm64/kernel/entry-common.c:432 el0_sync+0x154/0x180 arch/arm64/kernel/entry.S:701
The buggy address belongs to the object at ffff23794dda0000 which belongs to the cache pid of size 224 The buggy address is located 4 bytes inside of 224-byte region [ff
---truncated---
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Analysis
by VulDB Data Team • 02/27/2025
The vulnerability described in CVE-2021-47118 resides within the Linux kernel's handling of process identifier management, specifically concerning the cad_pid variable that tracks the control access daemon process identifier. This flaw manifests during kernel initialization when kernel_init_freeable() assigns the init task's struct pid to cad_pid without acquiring a reference to it, violating fundamental reference counting principles that govern kernel object lifecycle management. The issue stems from a lack of proper reference counting during initialization, where get_pid() is invoked only when cad_pid is updated via sysctl operations but never during its initial assignment. This inconsistency creates a scenario where the reference count is decremented without a corresponding increment, potentially leading to premature deallocation of the init task's struct pid structure. The vulnerability was identified through systematic fuzzing using Syzkaller against kernel version 5.13-rc3, though it had existed since the transition to struct pid management in kernel version 2.6.19, as indicated by the commit 9ec52099e4b8 that replaced cad_pid with a struct pid.
The technical impact of this vulnerability extends beyond simple memory management issues to encompass potential use-after-free conditions that can compromise system stability and security. When the init task's struct pid is prematurely freed due to incorrect reference counting, any dangling references to this structure can result in memory corruption during subsequent operations such as signal delivery. The KASAN (Kernel Address Sanitizer) output demonstrates a clear use-after-free scenario where ns_of_pid attempts to access a freed memory region, specifically at include/linux/pid.h:153, with the call trace showing that the freed object was allocated during rest_init and subsequently freed by put_pid during a sysctl operation. This type of vulnerability directly maps to CWE-415, which describes double free conditions, and CWE-416, which covers use-after-free errors, both of which are critical in kernel contexts where memory corruption can lead to privilege escalation or system crashes. The vulnerability operates at the intersection of kernel memory management and process control mechanisms, making it particularly dangerous in multi-threaded environments where signal handling and process lifecycle management intersect.
The operational consequences of this vulnerability can be severe, potentially enabling attackers to exploit the use-after-free condition for privilege escalation or system instability. An attacker could manipulate sysctl operations to trigger the cad_pid update path, causing the reference counting to become inconsistent and leading to memory corruption. This vulnerability impacts the kernel's ability to properly manage process identifiers and their associated namespaces, which are fundamental to process isolation and system security. The attack surface is particularly relevant for systems that rely heavily on kernel sysctl interfaces for runtime configuration and process management. The vulnerability's persistence across multiple kernel versions indicates that it represents a fundamental design flaw in the reference counting mechanism rather than a transient issue. From an ATT&CK perspective, this vulnerability maps to T1068 (Local Privilege Escalation) and T1499 (Endpoint Denial of Service) as it could enable an attacker to either escalate privileges through memory corruption or cause system instability through denial of service conditions. The fix implemented addresses this by ensuring that get_pid() is called during the initial assignment of cad_pid to the init task's struct pid, thereby properly incrementing the reference count and preventing premature deallocation.
Mitigation strategies for this vulnerability focus on applying the kernel patch that ensures proper reference counting during cad_pid initialization. System administrators should prioritize updating to kernel versions that include this fix, particularly those released after the vulnerability was addressed in the mainline kernel. The fix involves a minimal code change that adds get_pid() call during initialization, making it a low-risk update. Organizations should also implement monitoring for sysctl operations that modify cad_pid to detect potential exploitation attempts. Security teams should consider this vulnerability when conducting kernel security assessments and should verify that their systems are not vulnerable to similar reference counting issues in other kernel subsystems. The vulnerability highlights the importance of rigorous testing and formal verification of kernel memory management operations, particularly in areas where reference counting is critical for preventing memory corruption. Additionally, system hardening measures such as kernel lockdown and strict sysctl access controls can provide additional defense-in-depth against exploitation attempts targeting this class of vulnerabilities.