CVE-2025-37774 in Linux
Summary
by MITRE • 05/01/2025
In the Linux kernel, the following vulnerability has been resolved:
slab: ensure slab->obj_exts is clear in a newly allocated slab page
ktest recently reported crashes while running several buffered io tests with __alloc_tagging_slab_alloc_hook() at the top of the crash call stack. The signature indicates an invalid address dereference with low bits of slab->obj_exts being set. The bits were outside of the range used by page_memcg_data_flags and objext_flags and hence were not masked out by slab_obj_exts() when obtaining the pointer stored in slab->obj_exts. The typical crash log looks like this:
00510 Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010 00510 Mem abort info: 00510 ESR = 0x0000000096000045 00510 EC = 0x25: DABT (current EL), IL = 32 bits 00510 SET = 0, FnV = 0 00510 EA = 0, S1PTW = 0 00510 FSC = 0x05: level 1 translation fault 00510 Data abort info: 00510 ISV = 0, ISS = 0x00000045, ISS2 = 0x00000000 00510 CM = 0, WnR = 1, TnD = 0, TagAccess = 0 00510 GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0 00510 user pgtable: 4k pages, 39-bit VAs, pgdp=0000000104175000 00510 [0000000000000010] pgd=0000000000000000, p4d=0000000000000000, pud=0000000000000000
00510 Internal error: Oops: 0000000096000045 [#1] SMP
00510 Modules linked in: 00510 CPU: 10 UID: 0 PID: 7692 Comm: cat Not tainted 6.15.0-rc1-ktest-g189e17946605 #19327 NONE 00510 Hardware name: linux,dummy-virt (DT) 00510 pstate: 20001005 (nzCv daif -PAN -UAO -TCO -DIT +SSBS BTYPE=--) 00510 pc : __alloc_tagging_slab_alloc_hook+0xe0/0x190 00510 lr : __kmalloc_noprof+0x150/0x310 00510 sp : ffffff80c87df6c0 00510 x29: ffffff80c87df6c0 x28: 000000000013d1ff x27: 000000000013d200 00510 x26: ffffff80c87df9e0 x25: 0000000000000000 x24: 0000000000000001 00510 x23: ffffffc08041953c x22: 000000000000004c x21: ffffff80c0002180 00510 x20: fffffffec3120840 x19: ffffff80c4821000 x18: 0000000000000000 00510 x17: fffffffec3d02f00 x16: fffffffec3d02e00 x15: fffffffec3d00700 00510 x14: fffffffec3d00600 x13: 0000000000000200 x12: 0000000000000006 00510 x11: ffffffc080bb86c0 x10: 0000000000000000 x9 : ffffffc080201e58 00510 x8 : ffffff80c4821060 x7 : 0000000000000000 x6 : 0000000055555556 00510 x5 : 0000000000000001 x4 : 0000000000000010 x3 : 0000000000000060 00510 x2 : 0000000000000000 x1 : ffffffc080f50cf8 x0 : ffffff80d801d000 00510 Call trace: 00510 __alloc_tagging_slab_alloc_hook+0xe0/0x190 (P) 00510 __kmalloc_noprof+0x150/0x310 00510 __bch2_folio_create+0x5c/0xf8 00510 bch2_folio_create+0x2c/0x40 00510 bch2_readahead+0xc0/0x460 00510 read_pages+0x7c/0x230 00510 page_cache_ra_order+0x244/0x3a8 00510 page_cache_async_ra+0x124/0x170 00510 filemap_readahead.isra.0+0x58/0xa0 00510 filemap_get_pages+0x454/0x7b0 00510 filemap_read+0xdc/0x418 00510 bch2_read_iter+0x100/0x1b0 00510 vfs_read+0x214/0x300 00510 ksys_read+0x6c/0x108 00510 __arm64_sys_read+0x20/0x30 00510 invoke_syscall.constprop.0+0x54/0xe8 00510 do_el0_svc+0x44/0xc8 00510 el0_svc+0x18/0x58 00510 el0t_64_sync_handler+0x104/0x130 00510 el0t_64_sync+0x154/0x158 00510 Code: d5384100 f9401c01 b9401aa3 b40002e1 (f8227881) 00510 ---[ end trace 0000000000000000 ]---
00510 Kernel panic - not syncing: Oops: Fatal exception 00510 SMP: stopping secondary CPUs 00510 Kernel Offset: disabled 00510 CPU features: 0x0000,000000e0,00000410,8240500b 00510 Memory Limit: none
Investigation indicates that these bits are already set when we allocate slab page and are not zeroed out after allocation. We are not yet sure why these crashes start happening only recently but regardless of the reason, not initializing a field that gets used later is wrong. Fix it by initializing slab->obj_exts during slab page allocation.
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Analysis
by VulDB Data Team • 03/16/2026
The vulnerability described in CVE-2025-37774 resides within the Linux kernel's slab memory allocator, specifically affecting how the slab->obj_exts field is initialized during slab page allocation. This flaw manifests as a kernel NULL pointer dereference when the system attempts to access memory through an invalid address derived from the obj_exts field, which contains bits outside the expected range used by page_memcg_data_flags and objext_flags. The crash occurs at __alloc_tagging_slab_alloc_hook(), indicating that the kernel's memory allocation path for tagged slab objects has a critical initialization issue where the obj_exts field is not cleared after slab page allocation, leading to undefined behavior when the field is subsequently accessed. The technical root cause is a failure to zero out the slab->obj_exts field during slab page initialization, which results in stale bits being interpreted as valid memory addresses during memory allocation operations.
The operational impact of this vulnerability is severe, as it can lead to kernel panics and system crashes during buffered I/O operations, particularly when the kernel is under memory pressure or during high-frequency allocation patterns. The crash signature consistently shows a data abort at virtual address 0x10, which corresponds to a NULL pointer dereference scenario, indicating that the kernel is attempting to access memory at an invalid offset derived from the uninitialized obj_exts field. This vulnerability affects the stability of systems using the Linux kernel's slab allocator and can compromise system availability during normal I/O workloads, particularly in storage-intensive applications that rely heavily on memory allocation patterns such as the bcache subsystem as demonstrated by the call stack showing bch2_folio_create and related functions. The vulnerability aligns with CWE-754, which describes the weakness of an uninitialized variable, and can be categorized under ATT&CK technique T1490 for Deception, as the uninitialized memory may be exploited to cause system instability.
The fix for this vulnerability requires ensuring that the slab->obj_exts field is properly initialized to zero during slab page allocation, preventing stale bits from being interpreted as valid memory addresses. This initialization must occur during the slab page setup phase, before the page is made available for object allocation, ensuring that all bits in the obj_exts field are cleared and that only valid flag bits are subsequently set by the allocation logic. The solution addresses the fundamental issue of improper memory initialization and prevents the kernel from accessing invalid memory addresses that would otherwise cause system crashes. This remediation aligns with best practices for kernel memory management and follows the principle of least privilege by ensuring that memory fields are properly initialized before use. The fix ensures that the slab allocator maintains integrity by preventing uninitialized memory from being interpreted as valid pointers, which is critical for maintaining system stability and preventing potential escalation to more serious security vulnerabilities. The resolution directly addresses the underlying memory corruption issue and restores proper operation of the kernel's memory allocation subsystem, particularly under buffered I/O workloads where the slab allocator is heavily utilized.