CVE-2021-47566 in Linux
Summary
by MITRE • 05/24/2024
In the Linux kernel, the following vulnerability has been resolved:
proc/vmcore: fix clearing user buffer by properly using clear_user()
To clear a user buffer we cannot simply use memset, we have to use clear_user(). With a virtio-mem device that registers a vmcore_cb and has some logically unplugged memory inside an added Linux memory block, I can easily trigger a BUG by copying the vmcore via "cp":
systemd[1]: Starting Kdump Vmcore Save Service...
kdump[420]: Kdump is using the default log level(3).
kdump[453]: saving to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/
kdump[458]: saving vmcore-dmesg.txt to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/
kdump[465]: saving vmcore-dmesg.txt complete
kdump[467]: saving vmcore
BUG: unable to handle page fault for address: 00007f2374e01000 #PF: supervisor write access in kernel mode #PF: error_code(0x0003) - permissions violation PGD 7a523067 P4D 7a523067 PUD 7a528067 PMD 7a525067 PTE 800000007048f867 Oops: 0003 [#1] PREEMPT SMP NOPTI
CPU: 0 PID: 468 Comm: cp Not tainted 5.15.0+ #6 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-27-g64f37cc530f1-prebuilt.qemu.org 04/01/2014 RIP: 0010:read_from_oldmem.part.0.cold+0x1d/0x86 Code: ff ff ff e8 05 ff fe ff e9 b9 e9 7f ff 48 89 de 48 c7 c7 38 3b 60 82 e8 f1 fe fe ff 83 fd 08 72 3c 49 8d 7d 08 4c 89 e9 89 e8 c7 45 00 00 00 00 00 49 c7 44 05 f8 00 00 00 00 48 83 e7 f81 RSP: 0018:ffffc9000073be08 EFLAGS: 00010212 RAX: 0000000000001000 RBX: 00000000002fd000 RCX: 00007f2374e01000 RDX: 0000000000000001 RSI: 00000000ffffdfff RDI: 00007f2374e01008 RBP: 0000000000001000 R08: 0000000000000000 R09: ffffc9000073bc50 R10: ffffc9000073bc48 R11: ffffffff829461a8 R12: 000000000000f000 R13: 00007f2374e01000 R14: 0000000000000000 R15: ffff88807bd421e8 FS: 00007f2374e12140(0000) GS:ffff88807f000000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f2374e01000 CR3: 000000007a4aa000 CR4: 0000000000350eb0 Call Trace: read_vmcore+0x236/0x2c0 proc_reg_read+0x55/0xa0 vfs_read+0x95/0x190 ksys_read+0x4f/0xc0 do_syscall_64+0x3b/0x90 entry_SYSCALL_64_after_hwframe+0x44/0xae
Some x86-64 CPUs have a CPU feature called "Supervisor Mode Access Prevention (SMAP)", which is used to detect wrong access from the kernel to user buffers like this: SMAP triggers a permissions violation on wrong access. In the x86-64 variant of clear_user(), SMAP is properly handled via clac()+stac().
To fix, properly use clear_user() when we're dealing with a user buffer.
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Analysis
by VulDB Data Team • 09/19/2025
The vulnerability described in CVE-2021-47566 resides within the Linux kernel's handling of vmcore data processing, specifically when dealing with user-space buffers during memory dump operations. This issue manifests in the proc/vmcore subsystem where improper memory clearing operations can lead to kernel page faults and system instability. The root cause stems from the incorrect use of memset instead of the specialized clear_user() function when managing user buffers, particularly in scenarios involving virtio-mem devices with logically unplugged memory regions within added Linux memory blocks.
The technical flaw occurs during the vmcore saving process initiated by kdump services when attempting to copy memory dumps using standard utilities like cp. The system triggers a BUG condition when processing memory regions that contain user-space addresses, specifically failing at address 00007f2374e01000 with a supervisor write access violation. This represents a classic case of improper kernel memory management where kernel code attempts to write to user-space memory regions without proper handling of supervisor mode access prevention mechanisms. The kernel's memory management subsystem fails to properly transition between supervisor and user modes when clearing memory buffers, leading to immediate system crashes.
The operational impact of this vulnerability is significant as it can cause complete system crashes during critical memory dump operations, particularly in environments using kdump for crash analysis. The vulnerability specifically affects systems with x86-64 processors that have Supervisor Mode Access Prevention (SMAP) enabled, which is a security feature designed to prevent kernel code from inadvertently accessing user-space memory. When clear_user() is not properly utilized, the SMAP feature correctly identifies the unauthorized access and triggers a page fault, resulting in system panic and potential data loss. This vulnerability essentially creates a denial-of-service condition during system crash handling, which is particularly problematic in production environments where reliable crash reporting is essential.
The fix for this vulnerability involves properly implementing the clear_user() function instead of using memset when dealing with user buffers in the vmcore processing code. This change ensures that the kernel properly handles the transition between supervisor and user modes, allowing SMAP to function correctly without triggering page faults. The clear_user() function is specifically designed for clearing user-space memory regions and properly manages the SMAP feature by using clac() and stac() instructions to temporarily disable and re-enable supervisor mode access prevention. This vulnerability aligns with CWE-121, which deals with stack-based buffer overflow conditions, and maps to ATT&CK technique T1490, specifically focusing on data destruction through system crash exploitation. Organizations should ensure their kernels are updated to versions containing this fix, particularly those running systems with kdump enabled and virtio-mem devices, to prevent potential system crashes during memory dump operations and maintain system stability during critical failure scenarios.