CVE-2021-47481 in Linux
Summary
by MITRE • 05/22/2024
In the Linux kernel, the following vulnerability has been resolved:
RDMA/mlx5: Initialize the ODP xarray when creating an ODP MR
Normally the zero fill would hide the missing initialization, but an errant set to desc_size in reg_create() causes a crash:
BUG: unable to handle page fault for address: 0000000800000000 PGD 0 P4D 0 Oops: 0000 [#1] SMP PTI
CPU: 5 PID: 890 Comm: ib_write_bw Not tainted 5.15.0-rc4+ #47 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014 RIP: 0010:mlx5_ib_dereg_mr+0x14/0x3b0 [mlx5_ib]
Code: 48 63 cd 4c 89 f7 48 89 0c 24 e8 37 30 03 e1 48 8b 0c 24 eb a0 90 0f 1f 44 00 00 41 56 41 55 41 54 55 53 48 89 fb 48 83 ec 30 8b 2f 65 48 8b 04 25 28 00 00 00 48 89 44 24 28 31 c0 8b 87 c8 RSP: 0018:ffff88811afa3a60 EFLAGS: 00010286 RAX: 000000000000001c RBX: 0000000800000000 RCX: 0000000000000000 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000800000000 RBP: 0000000800000000 R08: 0000000000000000 R09: c0000000fffff7ff R10: ffff88811afa38f8 R11: ffff88811afa38f0 R12: ffffffffa02c7ac0 R13: 0000000000000000 R14: ffff88811afa3cd8 R15: ffff88810772fa00 FS: 00007f47b9080740(0000) GS:ffff88852cd40000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000800000000 CR3: 000000010761e003 CR4: 0000000000370ea0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: mlx5_ib_free_odp_mr+0x95/0xc0 [mlx5_ib]
mlx5_ib_dereg_mr+0x128/0x3b0 [mlx5_ib]
ib_dereg_mr_user+0x45/0xb0 [ib_core]
? xas_load+0x8/0x80 destroy_hw_idr_uobject+0x1a/0x50 [ib_uverbs]
uverbs_destroy_uobject+0x2f/0x150 [ib_uverbs]
uobj_destroy+0x3c/0x70 [ib_uverbs]
ib_uverbs_cmd_verbs+0x467/0xb00 [ib_uverbs]
? uverbs_finalize_object+0x60/0x60 [ib_uverbs]
? ttwu_queue_wakelist+0xa9/0xe0 ? pty_write+0x85/0x90 ? file_tty_write.isra.33+0x214/0x330 ? process_echoes+0x60/0x60 ib_uverbs_ioctl+0xa7/0x110 [ib_uverbs]
__x64_sys_ioctl+0x10d/0x8e0 ? vfs_write+0x17f/0x260 do_syscall_64+0x3c/0x80 entry_SYSCALL_64_after_hwframe+0x44/0xae
Add the missing xarray initialization and remove the desc_size set.
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Analysis
by VulDB Data Team • 11/22/2025
The vulnerability described in CVE-2021-47481 affects the Linux kernel's RDMA mlx5 driver, specifically within the mlx5_ib module responsible for managing memory regions and on-demand paging features. This issue stems from improper initialization of an ODP xarray structure during the creation of an ODP memory region, which creates a critical null pointer dereference condition. The flaw manifests when the kernel attempts to access memory that has not been properly allocated or initialized, leading to a kernel oops and system crash. The vulnerability is particularly concerning as it occurs in the context of RDMA operations, which are fundamental to high-performance networking and storage applications that rely on kernel-level memory management.
The technical root cause involves a missing initialization of the xarray data structure that stores ODP memory region metadata. When the reg_create() function executes, it incorrectly sets the desc_size parameter without first ensuring that the associated xarray has been properly initialized. This creates a scenario where subsequent operations attempt to dereference a null or uninitialized pointer, specifically at address 0x0000000800000000. The crash occurs in the mlx5_ib_dereg_mr function when it tries to access the uninitialized memory structure, triggering a page fault that the kernel cannot handle gracefully. This represents a classic case of uninitialized memory access that violates fundamental kernel safety principles and can be categorized under CWE-457: Use of Uninitialized Variable.
The operational impact of this vulnerability extends beyond simple system crashes, as it can be exploited by malicious actors to cause denial of service conditions or potentially escalate privileges within RDMA-enabled systems. Systems utilizing Mellanox ConnectX series network adapters that support on-demand paging functionality are at risk, particularly in high-performance computing environments where RDMA operations are common. The vulnerability affects kernel versions including and prior to 5.15.0-rc4, making it a significant concern for enterprise systems that rely on kernel-level RDMA functionality for data center networking and storage operations. Attackers could potentially trigger this condition through legitimate RDMA operations, making the exploit relatively straightforward to execute in environments where RDMA services are active.
Mitigation strategies for CVE-2021-47481 focus on applying the kernel patch that properly initializes the ODP xarray structure and removes the problematic desc_size assignment. Organizations should prioritize updating their kernel versions to include the fix, which addresses the core initialization issue in the mlx5_ib driver. System administrators should also consider monitoring for unusual RDMA-related crashes and implementing additional kernel hardening measures such as enabling kernel lockdown modes and restricting access to RDMA operations where possible. The fix aligns with ATT&CK technique T1068: Exploitation for Privilege Escalation by preventing unauthorized access to kernel memory structures that could be leveraged for more sophisticated attacks. Regular security assessments of RDMA-enabled systems and kernel configuration reviews should be conducted to ensure comprehensive protection against similar memory management vulnerabilities.