CVE-2021-47578 in Linux
Summary
by MITRE • 06/19/2024
In the Linux kernel, the following vulnerability has been resolved:
scsi: scsi_debug: Don't call kcalloc() if size arg is zero
If the size arg to kcalloc() is zero, it returns ZERO_SIZE_PTR. Because of that, for a following NULL pointer check to work on the returned pointer, kcalloc() must not be called with the size arg equal to zero. Return early without error before the kcalloc() call if size arg is zero.
BUG: KASAN: null-ptr-deref in memcpy include/linux/fortify-string.h:191 [inline]
BUG: KASAN: null-ptr-deref in sg_copy_buffer+0x138/0x240 lib/scatterlist.c:974 Write of size 4 at addr 0000000000000010 by task syz-executor.1/22789
CPU: 1 PID: 22789 Comm: syz-executor.1 Not tainted 5.15.0-syzk #1 Hardware name: Red Hat KVM, BIOS 1.13.0-2 Call Trace: __dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x89/0xb5 lib/dump_stack.c:106 __kasan_report mm/kasan/report.c:446 [inline]
kasan_report.cold.14+0x112/0x117 mm/kasan/report.c:459 check_region_inline mm/kasan/generic.c:183 [inline]
kasan_check_range+0x1a3/0x210 mm/kasan/generic.c:189 memcpy+0x3b/0x60 mm/kasan/shadow.c:66 memcpy include/linux/fortify-string.h:191 [inline]
sg_copy_buffer+0x138/0x240 lib/scatterlist.c:974 do_dout_fetch drivers/scsi/scsi_debug.c:2954 [inline]
do_dout_fetch drivers/scsi/scsi_debug.c:2946 [inline]
resp_verify+0x49e/0x930 drivers/scsi/scsi_debug.c:4276 schedule_resp+0x4d8/0x1a70 drivers/scsi/scsi_debug.c:5478 scsi_debug_queuecommand+0x8c9/0x1ec0 drivers/scsi/scsi_debug.c:7533 scsi_dispatch_cmd drivers/scsi/scsi_lib.c:1520 [inline]
scsi_queue_rq+0x16b0/0x2d40 drivers/scsi/scsi_lib.c:1699 blk_mq_dispatch_rq_list+0xb9b/0x2700 block/blk-mq.c:1639 __blk_mq_sched_dispatch_requests+0x28f/0x590 block/blk-mq-sched.c:325 blk_mq_sched_dispatch_requests+0x105/0x190 block/blk-mq-sched.c:358 __blk_mq_run_hw_queue+0xe5/0x150 block/blk-mq.c:1761 __blk_mq_delay_run_hw_queue+0x4f8/0x5c0 block/blk-mq.c:1838 blk_mq_run_hw_queue+0x18d/0x350 block/blk-mq.c:1891 blk_mq_sched_insert_request+0x3db/0x4e0 block/blk-mq-sched.c:474 blk_execute_rq_nowait+0x16b/0x1c0 block/blk-exec.c:62 blk_execute_rq+0xdb/0x360 block/blk-exec.c:102 sg_scsi_ioctl drivers/scsi/scsi_ioctl.c:621 [inline]
scsi_ioctl+0x8bb/0x15c0 drivers/scsi/scsi_ioctl.c:930 sg_ioctl_common+0x172d/0x2710 drivers/scsi/sg.c:1112 sg_ioctl+0xa2/0x180 drivers/scsi/sg.c:1165 vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:874 [inline]
__se_sys_ioctl fs/ioctl.c:860 [inline]
__x64_sys_ioctl+0x19d/0x220 fs/ioctl.c:860 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3a/0x80 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x44/0xae
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Analysis
by VulDB Data Team • 10/04/2025
The vulnerability described in CVE-2021-47578 resides within the Linux kernel's SCSI debugging module, specifically in the scsi_debug driver. This flaw manifests as a potential null pointer dereference that occurs when the kcalloc() function is invoked with a zero-sized argument. The kernel's memory allocation function kcalloc() returns ZERO_SIZE_PTR when called with a size of zero, which bypasses normal null pointer checks and can lead to unexpected behavior during memory operations. This issue is particularly concerning in kernel space where such errors can result in system instability or potential exploitation. The vulnerability was identified through kernel address sanitizer (KASAN) reports that trace memory access violations occurring in the memcpy function and scatterlist operations, indicating a critical flaw in how buffer sizes are handled within the SCSI debugging subsystem.
The technical root cause involves improper parameter validation in the scsi_debug driver's implementation where the code does not adequately check if the size argument passed to kcalloc() is zero before proceeding with memory allocation. When size equals zero, kcalloc() returns a special pointer value that is not properly handled by subsequent null checks, leading to a scenario where the program attempts to dereference a null pointer during memory copy operations. This vulnerability was triggered through a specific sequence involving SCSI command processing and scatter-gather buffer management, ultimately manifesting as a kernel crash during a SCSI_IOCTL command execution. The call stack demonstrates the path through multiple kernel subsystems including scatterlist handling, SCSI command queueing, and block I/O operations, showing how the flaw propagates through the kernel's storage subsystem architecture.
The operational impact of this vulnerability extends beyond simple system crashes, as it represents a potential vector for denial-of-service attacks or more serious exploitation depending on the execution context. The vulnerability affects systems running Linux kernel versions that include the scsi_debug driver, particularly those using kernel versions 5.15 and earlier where the fix has not yet been applied. Attackers could potentially exploit this by crafting specific SCSI commands that trigger the zero-sized allocation path, leading to system instability or service disruption. The vulnerability is classified under CWE-476 as a NULL Pointer Dereference, which is a common weakness in kernel security that can be leveraged for privilege escalation or system compromise. The ATT&CK framework would categorize this as a kernel exploitation technique, specifically involving the manipulation of kernel memory management functions to achieve unauthorized access or system control.
Mitigation strategies for CVE-2021-47578 involve applying the upstream kernel patch that ensures early return from functions when a zero-sized allocation is requested, preventing the problematic kcalloc() call. System administrators should update to kernel versions containing the fix, which typically requires kernel version 5.16 or later where the vulnerability has been addressed. Organizations should also implement monitoring for suspicious SCSI command patterns that could indicate exploitation attempts, particularly in environments where the scsi_debug module is enabled. Additional defensive measures include disabling the scsi_debug module in production environments where it is not required, implementing kernel lockdown features, and maintaining strict access controls for SCSI-related operations. Regular kernel updates and security audits are essential to prevent exploitation of similar memory management vulnerabilities that could affect other kernel subsystems. The fix demonstrates proper defensive programming practices by validating input parameters before calling memory allocation functions, which aligns with established security best practices for kernel development.