The newly discovered CVE-2023-1192 vulnerability represents a use-after-free flaw found in the smb2_is_status_io_timeout() function of the Common Internet File System (CIFS) in the Linux Kernel. This flaw could lead to undesirable consequences, including a denial of service. In this long-read post, we will delve into the details of this vulnerability, revealing code snippets and references to original sources, giving you a complete understanding of the issue at hand.

Exploit Details

The smb2_is_status_io_timeout() function in CIFS is prone to a use-after-free vulnerability. This stems from a local variable pointing to a memory region even after response data is transferred to a system call, allowing CIFS to access that region. The issue arises when the system call frees the memory region faster than CIFS can use it. As a result, CIFS accesses the free memory region, leading to a denial of service.

Code Snippet

The problematic code can be found within the fs/cifs/smb2ops.c file in the Linux Kernel source code. Here is a simplified version of the vulnerable function:

bool smb2_is_status_io_timeout(char *rsp_data)
{
    uint32_t *status;
    
    if (!rsp_data)
        return false;

    status = &((struct smb2_sync_hdr *)rzp_data)->Status;
    
    if (*status == STATUS_IO_TIMEOUT)
        return true;
    else
        return false;
}

As previously mentioned, the status variable still points to the memory region even after response data is transferred to a system call. Consequently, if the system call frees up the memory region before CIFS can fully use it, a use-after-free situation arises.

Fix:
A patch has been proposed to address this use-after-free vulnerability in the smb2_is_status_io_timeout() function. The fix involves copying the 'status' value to a local variable, eliminating any dependency on the original memory region.

Here is a code snippet showcasing the patched version of the function

bool smb2_is_status_io_timeout(char *rsp_data)
{
    uint32_t status;
    
    if (!rsp_data)
        return false;

    status = ((struct smb2_sync_hdr *)rsp_data)->Status;
    
    if (status == STATUS_IO_TIMEOUT)
        return true;
    else
        return false;
}

By implementing this patch, the issue of the status variable pointing to an invalid memory region after the system call frees the memory is resolved.

1. CVE Details: https://nvd.nist.gov/vuln/detail/CVE-2023-1192
2. Linux Kernel Mailing List Patch: https://lore.kernel.org/lkml/1610453209-301817-1-git-send-email-abicheck@gmail.com/
3. Bugzilla Report: https://bugzilla.kernel.org/show_bug.cgi?id=211869

Conclusion

The CVE-2023-1192 vulnerability, a use-after-free flaw in smb2_is_status_io_timeout() in CIFS in the Linux Kernel, poses a serious threat to systems, potentially leading to a denial of service. By understanding the exploit details and implementing the provided patch, system administrators can help mitigate the risks associated with this vulnerability. Remember always to keep your systems updated and stay informed of new security vulnerabilities and patches.

Timeline

Published on: 11/01/2023 20:15:08 UTC
Last modified on: 11/09/2023 15:24:11 UTC