CVE-2021-47549 in Linux
Summary
by MITRE • 05/24/2024
In the Linux kernel, the following vulnerability has been resolved:
sata_fsl: fix UAF in sata_fsl_port_stop when rmmod sata_fsl
When the `rmmod sata_fsl.ko` command is executed in the PPC64 GNU/Linux, a bug is reported: ================================================================== BUG: Unable to handle kernel data access on read at 0x80000800805b502c Oops: Kernel access of bad area, sig: 11 [#1]
NIP [c0000000000388a4] .ioread32+0x4/0x20
LR [80000000000c6034] .sata_fsl_port_stop+0x44/0xe0 [sata_fsl]
Call Trace: .free_irq+0x1c/0x4e0 (unreliable) .ata_host_stop+0x74/0xd0 [libata]
.release_nodes+0x330/0x3f0 .device_release_driver_internal+0x178/0x2c0 .driver_detach+0x64/0xd0 .bus_remove_driver+0x70/0xf0 .driver_unregister+0x38/0x80 .platform_driver_unregister+0x14/0x30 .fsl_sata_driver_exit+0x18/0xa20 [sata_fsl]
.__se_sys_delete_module+0x1ec/0x2d0 .system_call_exception+0xfc/0x1f0 system_call_common+0xf8/0x200 ==================================================================
The triggering of the BUG is shown in the following stack:
driver_detach device_release_driver_internal __device_release_driver drv->remove(dev) --> platform_drv_remove/platform_remove drv->remove(dev) --> sata_fsl_remove iounmap(host_priv->hcr_base); <---- unmap kfree(host_priv); <---- free devres_release_all release_nodes dr->node.release(dev, dr->data) --> ata_host_stop ap->ops->port_stop(ap) --> sata_fsl_port_stop ioread32(hcr_base + HCONTROL) <---- UAF host->ops->host_stop(host)
The iounmap(host_priv->hcr_base) and kfree(host_priv) functions should not be executed in drv->remove. These functions should be executed in host_stop after port_stop. Therefore, we move these functions to the new function sata_fsl_host_stop and bind the new function to host_stop.
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Analysis
by VulDB Data Team • 08/09/2025
The vulnerability described in CVE-2021-47549 represents a use-after-free condition within the Freescale SATA driver implementation of the Linux kernel specifically affecting PowerPC 64-bit systems. This issue arises during the module removal process when the rmmod command is executed against the sata_fsl kernel module. The flaw manifests as a kernel data access violation occurring at address 0x80000800805b502c, resulting in a kernel oops and potential system instability. The underlying problem stems from improper resource management within the driver's cleanup sequence, where memory deallocation occurs before all references to the freed memory have been properly resolved.
The technical execution of this vulnerability follows a well-defined call stack that begins with the platform driver removal process and culminates in the use-after-free scenario. During module removal, the system calls platform_drv_remove which eventually leads to sata_fsl_remove function execution. Within this function, iounmap and kfree operations are performed on the host private data structure, effectively freeing memory that is subsequently accessed during the port_stop operation. The critical flaw occurs when the sata_fsl_port_stop function attempts to read from the hardware control register using ioread32, but the memory address has already been freed by the preceding iounmap and kfree operations. This sequence violates fundamental memory safety principles and creates a classic use-after-free vulnerability as defined by CWE-416.
The operational impact of this vulnerability extends beyond simple system crashes, potentially enabling privilege escalation or denial-of-service conditions within kernel space. Attackers could exploit this condition to cause system instability or potentially gain elevated privileges by manipulating the timing of module removal operations. The vulnerability specifically affects systems running Linux on PowerPC 64-bit architectures where the Freescale SATA driver is utilized, making it relevant to embedded systems and server platforms that employ this hardware configuration. This issue aligns with ATT&CK technique T1068 which covers 'Exploitation for Privilege Escalation' and T1499 which covers 'Endpoint Denial of Service' within the enterprise attack framework.
The fix implemented for this vulnerability involves restructuring the driver's cleanup mechanism by introducing a new sata_fsl_host_stop function that properly coordinates the resource deallocation sequence. This solution ensures that the iounmap and kfree operations occur after all port_stop operations have completed, thereby preventing the use-after-free condition. The change properly separates the host-level cleanup from the port-level cleanup, following established kernel programming practices for resource management. This remediation approach aligns with kernel security best practices and addresses the fundamental architectural flaw in the driver's resource lifecycle management, effectively eliminating the vulnerability while maintaining the driver's functional integrity. The fix demonstrates proper adherence to kernel memory management principles and prevents the dangerous interaction between hardware register access and memory deallocation that characterized the original vulnerability.