CVE-2021-4439 in Linux
Summary
by MITRE • 06/20/2024
In the Linux kernel, the following vulnerability has been resolved:
isdn: cpai: check ctr->cnr to avoid array index out of bound
The cmtp_add_connection() would add a cmtp session to a controller and run a kernel thread to process cmtp.
__module_get(THIS_MODULE); session->task = kthread_run(cmtp_session, session, "kcmtpd_ctr_%d", session->num);
During this process, the kernel thread would call detach_capi_ctr() to detach a register controller. if the controller was not attached yet, detach_capi_ctr() would trigger an array-index-out-bounds bug.
[ 46.866069][ T6479] UBSAN: array-index-out-of-bounds in
drivers/isdn/capi/kcapi.c:483:21 [ 46.867196][ T6479] index -1 is out of range for type 'capi_ctr *[32]'
[ 46.867982][ T6479] CPU: 1 PID: 6479 Comm: kcmtpd_ctr_0 Not tainted
5.15.0-rc2+ #8 [ 46.869002][ T6479] Hardware name: QEMU Standard PC (i440FX + PIIX,
1996), BIOS 1.14.0-2 04/01/2014 [ 46.870107][ T6479] Call Trace:
[ 46.870473][ T6479] dump_stack_lvl+0x57/0x7d
[ 46.870974][ T6479] ubsan_epilogue+0x5/0x40
[ 46.871458][ T6479] __ubsan_handle_out_of_bounds.cold+0x43/0x48
[ 46.872135][ T6479] detach_capi_ctr+0x64/0xc0
[ 46.872639][ T6479] cmtp_session+0x5c8/0x5d0
[ 46.873131][ T6479] ? __init_waitqueue_head+0x60/0x60
[ 46.873712][ T6479] ? cmtp_add_msgpart+0x120/0x120
[ 46.874256][ T6479] kthread+0x147/0x170
[ 46.874709][ T6479] ? set_kthread_struct+0x40/0x40
[ 46.875248][ T6479] ret_from_fork+0x1f/0x30
[ 46.875773][ T6479]
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Analysis
by VulDB Data Team • 09/18/2024
This vulnerability exists within the linux kernel's isdn subsystem specifically in the capi driver component where a classic array index out of bounds condition occurs during controller detachment operations. The flaw manifests when the cmtp_add_connection() function attempts to establish a new connection session and subsequently spawns a kernel thread to process cmtp communications. During this process, the kernel thread invokes detach_capi_ctr() to detach a previously registered controller, but fails to validate whether the controller was actually attached, leading to an invalid array access with a negative index value. The vulnerability is categorized under CWE-129 as an insufficient input validation leading to buffer over-read conditions, and represents a direct violation of the principle of bounds checking in kernel space memory operations.
The technical implementation of this vulnerability stems from the improper handling of controller registration state within the isdn subsystem. When a kernel thread is created via kthread_run() to manage cmtp sessions, it executes the cmtp_session function which eventually calls detach_capi_ctr() without first verifying that the controller was properly registered. The crash occurs at line 483 in drivers/isdn/capi/kcapi.c where an array access attempts to use a negative index value of -1 against a capi_ctr array of size 32, triggering the kernel's undefined behavior sanitizer and resulting in a kernel oops. This particular code path demonstrates a race condition between controller attachment and detachment operations, where the controller number field ctr->cnr contains an invalid value that translates to an out of bounds array access.
The operational impact of this vulnerability is significant as it can lead to system instability and potential denial of service conditions within kernel space operations. An attacker who can trigger the specific sequence of operations leading to this condition could potentially cause the kernel to crash, resulting in system reboot or complete system hang. The vulnerability affects the isdn subsystem specifically in configurations that utilize the capi interface for isdn communications, particularly those involving cmtp (capi multiplexing protocol) connections. The exploitation requires a specific sequence of controller attachment and detachment operations that would typically occur during normal operation, making this a latent vulnerability that could be triggered through legitimate system usage patterns or through crafted inputs that force the kernel into an invalid state.
Mitigation strategies for this vulnerability should focus on implementing proper bounds checking and state validation within the kernel driver code. The fix requires adding validation checks to ensure that controller numbers are valid before array access operations, specifically checking that ctr->cnr is within the valid range before using it as an array index. This approach aligns with the ATT&CK technique T1068 which involves privilege escalation through kernel vulnerabilities, and addresses the underlying security principle of input validation as outlined in the CWE taxonomy. System administrators should ensure that kernel updates containing the fix are applied promptly, and monitoring should be implemented to detect potential exploitation attempts through kernel crash dumps or system stability issues. The vulnerability demonstrates the importance of proper resource management in kernel space and highlights the need for comprehensive testing of edge cases in driver code, particularly around resource cleanup operations.