CVE-2025-39895: sched: Fix sched_numa_find_nth_cpu() if mask offline

In the Linux kernel, the following vulnerability has been resolved:

sched: Fix schednumafindnthcpu() if mask offline

schednumafindnthcpu() uses a bsearch to look for the 'closest' CPU in scheddomainsnumamasks and given cpus mask. However they might not intersect if all CPUs in the cpus mask are offline. bsearch will return NULL in that case, bail out instead of dereferencing a bogus pointer.

The previous behaviour lead to this bug when using maxcpus=4 on an rk3399 (LLLLbb) (i.e. booting with all big CPUs offline):

[ 1.422922] Unable to handle kernel paging request at virtual address ffffff8000000000 [ 1.423635] Mem abort info: [ 1.423889] ESR = 0x0000000096000006 [ 1.424227] EC = 0x25: DABT (current EL), IL = 32 bits [ 1.424715] SET = 0, FnV = 0 [ 1.424995] EA = 0, S1PTW = 0 [ 1.425279] FSC = 0x06: level 2 translation fault [ 1.425735] Data abort info: [ 1.425998] ISV = 0, ISS = 0x00000006, ISS2 = 0x00000000 [ 1.426499] CM = 0, WnR = 0, TnD = 0, TagAccess = 0 [ 1.426952] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0 [ 1.427428] swapper pgtable: 4k pages, 39-bit VAs, pgdp=0000000004a9f000 [ 1.428038] [ffffff8000000000] pgd=18000000f7fff403, p4d=18000000f7fff403, pud=18000000f7fff403, pmd=0000000000000000 [ 1.429014] Internal error: Oops: 0000000096000006 [#1] SMP [ 1.429525] Modules linked in: [ 1.429813] CPU: 3 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.17.0-rc4-dirty #343 PREEMPT [ 1.430559] Hardware name: Pine64 RockPro64 v2.1 (DT) [ 1.431012] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) [ 1.431634] pc : schednumafindnthcpu+0x2a0/0x488 [ 1.432094] lr : schednumafindnthcpu+0x284/0x488 [ 1.432543] sp : ffffffc084e1b960 [ 1.432843] x29: ffffffc084e1b960 x28: ffffff80078a8800 x27: ffffffc0846eb1d0 [ 1.433495] x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000 [ 1.434144] x23: 0000000000000000 x22: fffffffffff7f093 x21: ffffffc081de6378 [ 1.434792] x20: 0000000000000000 x19: 0000000ffff7f093 x18: 00000000ffffffff [ 1.435441] x17: 3030303866666666 x16: 66663d736b73616d x15: ffffffc104e1b5b7 [ 1.436091] x14: 0000000000000000 x13: ffffffc084712860 x12: 0000000000000372 [ 1.436739] x11: 0000000000000126 x10: ffffffc08476a860 x9 : ffffffc084712860 [ 1.437389] x8 : 00000000ffffefff x7 : ffffffc08476a860 x6 : 0000000000000000 [ 1.438036] x5 : 000000000000bff4 x4 : 0000000000000000 x3 : 0000000000000000 [ 1.438683] x2 : 0000000000000000 x1 : ffffffc0846eb000 x0 : ffffff8000407b68 [ 1.439332] Call trace: [ 1.439559] schednumafindnthcpu+0x2a0/0x488 (P) [ 1.440016] smpcallfunctionany+0xc8/0xd0 [ 1.440416] armv8pmuinit+0x58/0x27c [ 1.440770] armv8cortexa72pmuinit+0x20/0x2c [ 1.441199] armpmudeviceprobe+0x1e4/0x5e8 [ 1.441603] armv8pmudeviceprobe+0x1c/0x28 [ 1.442007] platformprobe+0x5c/0xac [ 1.442347] reallyprobe+0xbc/0x298 [ 1.442683] driverprobedevice+0x78/0x12c [ 1.443087] driverprobedevice+0xdc/0x160 [ 1.443475] driverattach+0x94/0x19c [ 1.443833] busforeachdev+0x74/0xd4 [ 1.444190] driverattach+0x24/0x30 [ 1.444525] busadddriver+0xe4/0x208 [ 1.444874] driverregister+0x60/0x128 [ 1.445233] platformdriverregister+0x24/0x30 [ 1.445662] armv8pmudriverinit+0x28/0x4c [ 1.446059] dooneinitcall+0x44/0x25c [ 1.446416] kernelinitfreeable+0x1dc/0x3bc [ 1.446820] kernelinit+0x20/0x1d8 [ 1.447151] retfromfork+0x10/0x20 [ 1.447493] Code: 90022e21 f000e5f5 910de2b5 2a1703e2 (f8767803) [ 1.448040] ---[ end trace 0000000000000000 ]--- [ 1.448483] note: swapper/0[1] exited with preemptcount 1 [ 1.449047] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b [ 1.449741] SMP: stopping secondary CPUs [ 1.450105] Kernel Offset: disabled [ 1.450419] CPU features: 0x000000,00080000,20002001,0400421b [ ---truncated---