CVE-2025-37774

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.