CVE-2023-52803 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
SUNRPC: Fix RPC client cleaned up the freed pipefs dentries
RPC client pipefs dentries cleanup is in separated rpc_remove_pipedir() workqueue,which takes care about pipefs superblock locking. In some special scenarios, when kernel frees the pipefs sb of the current client and immediately alloctes a new pipefs sb, rpc_remove_pipedir function would misjudge the existence of pipefs sb which is not the one it used to hold. As a result, the rpc_remove_pipedir would clean the released freed pipefs dentries.
To fix this issue, rpc_remove_pipedir should check whether the current pipefs sb is consistent with the original pipefs sb.
This error can be catched by KASAN: ========================================================= [ 250.497700] BUG: KASAN: slab-use-after-free in dget_parent+0x195/0x200
[ 250.498315] Read of size 4 at addr ffff88800a2ab804 by task kworker/0:18/106503
[ 250.500549] Workqueue: events rpc_free_client_work
[ 250.501001] Call Trace:
[ 250.502880] kasan_report+0xb6/0xf0
[ 250.503209] ? dget_parent+0x195/0x200
[ 250.503561] dget_parent+0x195/0x200
[ 250.503897] ? __pfx_rpc_clntdir_depopulate+0x10/0x10
[ 250.504384] rpc_rmdir_depopulate+0x1b/0x90
[ 250.504781] rpc_remove_client_dir+0xf5/0x150
[ 250.505195] rpc_free_client_work+0xe4/0x230
[ 250.505598] process_one_work+0x8ee/0x13b0
... [ 22.039056] Allocated by task 244:
[ 22.039390] kasan_save_stack+0x22/0x50
[ 22.039758] kasan_set_track+0x25/0x30
[ 22.040109] __kasan_slab_alloc+0x59/0x70
[ 22.040487] kmem_cache_alloc_lru+0xf0/0x240
[ 22.040889] __d_alloc+0x31/0x8e0
[ 22.041207] d_alloc+0x44/0x1f0
[ 22.041514] __rpc_lookup_create_exclusive+0x11c/0x140
[ 22.041987] rpc_mkdir_populate.constprop.0+0x5f/0x110
[ 22.042459] rpc_create_client_dir+0x34/0x150
[ 22.042874] rpc_setup_pipedir_sb+0x102/0x1c0
[ 22.043284] rpc_client_register+0x136/0x4e0
[ 22.043689] rpc_new_client+0x911/0x1020
[ 22.044057] rpc_create_xprt+0xcb/0x370
[ 22.044417] rpc_create+0x36b/0x6c0
... [ 22.049524] Freed by task 0:
[ 22.049803] kasan_save_stack+0x22/0x50
[ 22.050165] kasan_set_track+0x25/0x30
[ 22.050520] kasan_save_free_info+0x2b/0x50
[ 22.050921] __kasan_slab_free+0x10e/0x1a0
[ 22.051306] kmem_cache_free+0xa5/0x390
[ 22.051667] rcu_core+0x62c/0x1930
[ 22.051995] __do_softirq+0x165/0x52a
[ 22.052347]
[ 22.052503] Last potentially related work creation:
[ 22.052952] kasan_save_stack+0x22/0x50
[ 22.053313] __kasan_record_aux_stack+0x8e/0xa0
[ 22.053739] __call_rcu_common.constprop.0+0x6b/0x8b0
[ 22.054209] dentry_free+0xb2/0x140
[ 22.054540] __dentry_kill+0x3be/0x540
[ 22.054900] shrink_dentry_list+0x199/0x510
[ 22.055293] shrink_dcache_parent+0x190/0x240
[ 22.055703] do_one_tree+0x11/0x40
[ 22.056028] shrink_dcache_for_umount+0x61/0x140
[ 22.056461] generic_shutdown_super+0x70/0x590
[ 22.056879] kill_anon_super+0x3a/0x60
[ 22.057234] rpc_kill_sb+0x121/0x200
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Analysis
by VulDB Data Team • 09/24/2025
The vulnerability described in CVE-2023-52803 affects the Linux kernel's rpc client handling mechanism, specifically within the sunrpc subsystem. This issue stems from improper management of pipefs dentries during the cleanup process of rpc clients. The problem manifests when the kernel frees a pipefs superblock associated with an rpc client and immediately allocates a new one. The rpc_remove_pipedir function, which is responsible for cleaning up pipefs dentries, fails to verify that the current superblock matches the original one it was meant to clean. This mismatch leads to the function operating on freed dentries, resulting in use-after-free conditions.
The technical flaw resides in the lack of proper superblock consistency checks within the rpc_remove_pipedir function. When the kernel performs cleanup operations through the rpc_free_client_work workqueue, it assumes that the pipefs superblock it is working with is the same one it originally allocated. However, in certain timing-sensitive scenarios, a new superblock may have been allocated before the cleanup completes, causing the function to manipulate stale references. This behavior is consistent with a classic race condition and memory management error, where the function attempts to access memory that has already been freed. The KASAN (Kernel Address Sanitizer) output clearly demonstrates this issue by tracing the use-after-free error in dget_parent function, which occurs during the dentry cleanup process. The stack trace shows that the freed memory was allocated during rpc_create_client_dir and __rpc_lookup_create_exclusive functions, indicating a direct relationship between the allocation and subsequent freeing of the dentry structures.
The operational impact of this vulnerability is significant, particularly in environments where rpc clients are frequently created and destroyed. The use-after-free condition can lead to system instability, potential privilege escalation, and denial of service attacks. Attackers could potentially exploit this weakness to corrupt kernel memory, leading to arbitrary code execution. The vulnerability is particularly concerning in server environments that heavily utilize rpc services, as it could be leveraged to compromise the integrity of the kernel itself. This type of vulnerability aligns with CWE-416, which describes the use of freed memory condition, and can be mapped to ATT&CK technique T1068, which involves exploiting weaknesses in the system to gain elevated privileges.
The fix implemented addresses the core issue by adding a consistency check in the rpc_remove_pipedir function to ensure that the current pipefs superblock matches the original one that was used for allocation. This validation prevents the function from operating on freed dentries and ensures proper cleanup of only the relevant resources. The mitigation strategy involves updating the kernel to a version that includes this fix, which is typically found in the latest stable kernel releases. System administrators should also monitor for any signs of memory corruption or instability that may indicate exploitation attempts. Additionally, implementing proper kernel hardening measures such as KASAN, KPTI, and other security modules can help detect and prevent similar vulnerabilities from being exploited in the wild. The fix demonstrates a proper understanding of the kernel's memory management subsystem and highlights the importance of maintaining consistency in superblock references during cleanup operations.