CVE-2021-47005 in Linux
Summary
by MITRE • 02/28/2024
In the Linux kernel, the following vulnerability has been resolved:
PCI: endpoint: Fix NULL pointer dereference for ->get_features()
get_features ops of pci_epc_ops may return NULL, causing NULL pointer dereference in pci_epf_test_alloc_space function. Let us add a check for pci_epc_feature pointer in pci_epf_test_bind before we access it to avoid any such NULL pointer dereference and return -ENOTSUPP in case pci_epc_feature is not found.
When the patch is not applied and EPC features is not implemented in the platform driver, we see the following dump due to kernel NULL pointer dereference.
Call trace: pci_epf_test_bind+0xf4/0x388 pci_epf_bind+0x3c/0x80 pci_epc_epf_link+0xa8/0xcc configfs_symlink+0x1a4/0x48c vfs_symlink+0x104/0x184 do_symlinkat+0x80/0xd4 __arm64_sys_symlinkat+0x1c/0x24 el0_svc_common.constprop.3+0xb8/0x170 el0_svc_handler+0x70/0x88 el0_svc+0x8/0x640 Code: d2800581 b9403ab9 f9404ebb 8b394f60 (f9400400) ---[ end trace a438e3c5a24f9df0 ]---
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Analysis
by VulDB Data Team • 12/09/2024
The vulnerability identified as CVE-2021-47005 represents a critical NULL pointer dereference issue within the Linux kernel's PCI endpoint framework, specifically affecting the pci_epf_test_alloc_space function. This flaw occurs when the get_features operations of the pci_epc_ops structure return NULL, creating a dangerous condition where subsequent code attempts to access a null pointer reference. The vulnerability exists in the kernel's PCI endpoint configuration infrastructure, which handles the configuration and management of PCI endpoint functions within the system's hardware abstraction layer. The issue manifests when platform drivers fail to implement EPC (PCIe Endpoint Controller) features properly, leading to kernel crashes and system instability.
The technical implementation of this vulnerability stems from inadequate input validation within the pci_epf_test_bind function, which fails to properly check whether the pci_epc_feature pointer is valid before attempting to access its contents. According to CWE-476, this represents a NULL pointer dereference vulnerability that directly violates proper defensive programming practices. The kernel's PCI endpoint subsystem relies on a well-defined interface where platform drivers must implement specific operations including get_features, but the absence of proper validation allows malicious or faulty drivers to trigger the NULL pointer access. This issue falls under the ATT&CK technique T1068 which involves exploiting local vulnerabilities to gain system-level privileges, as the kernel crash represents an opportunity for privilege escalation or denial of service attacks.
The operational impact of CVE-2021-47005 extends beyond simple system crashes, as it represents a potential vector for denial of service attacks targeting systems that rely heavily on PCI endpoint functionality. When the patch is not applied, the kernel generates a stack trace that clearly shows the execution path leading to the crash, beginning with pci_epf_test_bind and ultimately resulting in a NULL pointer dereference. The crash occurs during the symlink creation process through configfs, indicating that this vulnerability affects not just the PCI endpoint subsystem but also the broader kernel filesystem management capabilities. Systems running affected kernel versions become vulnerable to exploitation by attackers who can trigger the specific sequence of operations that leads to the NULL pointer dereference, potentially causing complete system lockups or forced reboots.
Mitigation strategies for CVE-2021-47005 require immediate kernel updates to apply the patched version that includes proper NULL pointer validation before accessing the pci_epc_feature pointer. The fix implemented in the patch adds a validation check in the pci_epf_test_bind function to ensure that pci_epc_feature is not NULL before attempting to access it, returning -ENOTSUPP in cases where the feature is not found. This approach aligns with the principle of fail-safe design and follows the ATT&CK mitigation technique T1210 which involves updating systems to patch known vulnerabilities. Organizations should also implement monitoring solutions to detect potential exploitation attempts and maintain robust backup and recovery procedures, as the vulnerability's nature makes it particularly dangerous in production environments where system stability and availability are paramount. Additionally, platform driver developers should ensure proper implementation of all required PCI endpoint controller operations to prevent similar issues from occurring in the future.