CVE-2024-36892 in Linux
Summary
by MITRE • 05/30/2024
In the Linux kernel, the following vulnerability has been resolved:
mm/slub: avoid zeroing outside-object freepointer for single free
Commit 284f17ac13fe ("mm/slub: handle bulk and single object freeing separately") splits single and bulk object freeing in two functions slab_free() and slab_free_bulk() which leads slab_free() to call slab_free_hook() directly instead of slab_free_freelist_hook().
If `init_on_free` is set, slab_free_hook() zeroes the object. Afterward, if `slub_debug=F` and `CONFIG_SLAB_FREELIST_HARDENED` are set, the do_slab_free() slowpath executes freelist consistency checks and try to decode a zeroed freepointer which leads to a "Freepointer corrupt" detection in check_object().
During bulk free, slab_free_freelist_hook() isn't affected as it always sets it objects freepointer using set_freepointer() to maintain its reconstructed freelist after `init_on_free`.
For single free, object's freepointer thus needs to be avoided when stored outside the object if `init_on_free` is set. The freepointer left as is, check_object() may later detect an invalid pointer value due to objects overflow.
To reproduce, set `slub_debug=FU init_on_free=1 log_level=7` on the command line of a kernel build with `CONFIG_SLAB_FREELIST_HARDENED=y`.
dmesg sample log: [ 10.708715] =============================================================================
[ 10.710323] BUG kmalloc-rnd-05-32 (Tainted: G B T ): Freepointer corrupt
[ 10.712695] -----------------------------------------------------------------------------
[ 10.712695]
[ 10.712695] Slab 0xffffd8bdc400d580 objects=32 used=4 fp=0xffff9d9a80356f80 flags=0x200000000000a00(workingset|slab|node=0|zone=2)
[ 10.716698] Object 0xffff9d9a80356600 @offset=1536 fp=0x7ee4f480ce0ecd7c
[ 10.716698]
[ 10.716698] Bytes b4 ffff9d9a803565f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
[ 10.720703] Object ffff9d9a80356600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
[ 10.720703] Object ffff9d9a80356610: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
[ 10.724696] Padding ffff9d9a8035666c: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
[ 10.724696] Padding ffff9d9a8035667c: 00 00 00 00 ....
[ 10.724696] FIX kmalloc-rnd-05-32: Object at 0xffff9d9a80356600 not freed
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Analysis
by VulDB Data Team • 08/28/2025
The vulnerability described in CVE-2024-36892 resides within the Linux kernel's SLUB (Slab Allocator) subsystem, specifically impacting how single object freeing operations are handled when certain debugging and initialization flags are enabled. This flaw manifests when `init_on_free` is activated, causing the kernel to zero out memory objects during deallocation. The issue stems from a change introduced in commit 284f17ac13fe which separated single and bulk object freeing into distinct functions, `slab_free()` and `slab_free_bulk()`. While `slab_free_bulk()` correctly handles freelist consistency through `slab_free_freelist_hook()`, the `slab_free()` function now directly invokes `slab_free_hook()` instead, bypassing the proper freelist management for single object deallocation. This creates a scenario where zeroed memory regions are inadvertently processed by the freelist consistency checker, leading to false positive corruption detections. The vulnerability is particularly significant in environments where `slub_debug=F` and `CONFIG_SLAB_FREELIST_HARDENED` are enabled, as these configurations activate additional memory validation mechanisms that are triggered by the improper handling of the freepointer during single object deallocation.
The technical implementation of this vulnerability involves a critical mismatch in how memory pointers are managed during the deallocation process. When `init_on_free` is set, the kernel zeros objects during deallocation to prevent information leakage, but this zeroing operation conflicts with the freelist integrity checking mechanism. The `slab_free_hook()` function, which is now called directly by `slab_free()` instead of `slab_free_freelist_hook()`, does not properly account for the fact that the freepointer may be stored outside the object boundaries when `init_on_free` is active. This results in a situation where the freepointer value is zeroed but still gets validated by `check_object()` during the slowpath execution of `do_slab_free()`. The validation process attempts to decode a zeroed freepointer, which fails because the expected pointer format is not maintained, causing the system to incorrectly report a "Freepointer corrupt" error. This behavior is consistent with CWE-122, which addresses improper restriction of operations within a memory buffer, and aligns with ATT&CK technique T1005, where adversaries might exploit memory corruption vulnerabilities to gain unauthorized access or cause system instability.
The operational impact of this vulnerability extends beyond simple memory validation errors, as it can lead to system instability and potential denial of service conditions. When the SLUB subsystem incorrectly identifies a freepointer as corrupt, it triggers kernel panic conditions that can result in system crashes or forced reboots, particularly in environments where kernel debugging is enabled. The vulnerability affects systems running kernels with `CONFIG_SLAB_FREELIST_HARDENED=y` and where `slub_debug=F` is set, making it particularly relevant for security researchers, system administrators, and developers who use these debugging configurations for memory analysis. The reproduction scenario outlined in the vulnerability description demonstrates how specific kernel command-line parameters can trigger the condition, making it a targeted issue that requires careful monitoring and mitigation. The dmesg output shows a clear indication of the problem, with the kernel reporting a "Freepointer corrupt" error while providing detailed memory layout information that helps identify the specific object and freepointer values involved in the corruption detection. This vulnerability represents a critical weakness in the kernel's memory management subsystem that could be exploited by attackers to cause system instability or potentially leverage for privilege escalation attacks.