CVE-2026-23107: arm64/fpsimd: signal: Allocate SSVE storage when restoring ZA

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

arm64/fpsimd: signal: Allocate SSVE storage when restoring ZA

The code to restore a ZA context doesn't attempt to allocate the task's svestate before setting TIFSME. Consequently, restoring a ZA context can place a task into an invalid state where TIFSME is set but the task's svestate is NULL.

In legitimate but uncommon cases where the ZA signal context was NOT created by the kernel in the context of the same task (e.g. if the task is saved/restored with something like CRIU), we have no guarantee that svestate had been allocated previously. In these cases, userspace can enter streaming mode without trapping while svestate is NULL, causing a later NULL pointer dereference when the kernel attempts to store the register state:

| # ./sigreturn-za | Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000 | Mem abort info: | ESR = 0x0000000096000046 | EC = 0x25: DABT (current EL), IL = 32 bits | SET = 0, FnV = 0 | EA = 0, S1PTW = 0 | FSC = 0x06: level 2 translation fault | Data abort info: | ISV = 0, ISS = 0x00000046, ISS2 = 0x00000000 | CM = 0, WnR = 1, TnD = 0, TagAccess = 0 | GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0 | user pgtable: 4k pages, 52-bit VAs, pgdp=0000000101f47c00 | [0000000000000000] pgd=08000001021d8403, p4d=0800000102274403, pud=0800000102275403, pmd=0000000000000000 | Internal error: Oops: 0000000096000046 [#1] SMP | Modules linked in: | CPU: 0 UID: 0 PID: 153 Comm: sigreturn-za Not tainted 6.19.0-rc1 #1 PREEMPT | Hardware name: linux,dummy-virt (DT) | pstate: 214000c9 (nzCv daIF +PAN -UAO -TCO +DIT -SSBS BTYPE=--) | pc : svesavestate+0x4/0xf0 | lr : fpsimdsaveuserstate+0xb0/0x1c0 | sp : ffff80008070bcc0 | x29: ffff80008070bcc0 x28: fff00000c1ca4c40 x27: 63cfa172fb5cf658 | x26: fff00000c1ca5228 x25: 0000000000000000 x24: 0000000000000000 | x23: 0000000000000000 x22: fff00000c1ca4c40 x21: fff00000c1ca4c40 | x20: 0000000000000020 x19: fff00000ff6900f0 x18: 0000000000000000 | x17: fff05e8e0311f000 x16: 0000000000000000 x15: 028fca8f3bdaf21c | x14: 0000000000000212 x13: fff00000c0209f10 x12: 0000000000000020 | x11: 0000000000200b20 x10: 0000000000000000 x9 : fff00000ff69dcc0 | x8 : 00000000000003f2 x7 : 0000000000000001 x6 : fff00000c1ca5b48 | x5 : fff05e8e0311f000 x4 : 0000000008000000 x3 : 0000000000000000 | x2 : 0000000000000001 x1 : fff00000c1ca5970 x0 : 0000000000000440 | Call trace: | svesavestate+0x4/0xf0 (P) | fpsimdthreadswitch+0x48/0x198 | switchto+0x20/0x1c0 | schedule+0x36c/0xce0 | schedule+0x34/0x11c | exittousermodeloop+0x124/0x188 | el0interrupt+0xc8/0xd8 | el0irqhandlercommon+0x18/0x24 | el0t64irqhandler+0x10/0x1c | el0t64irq+0x198/0x19c | Code: 54000040 d51b4408 d65f03c0 d503245f (e5bb5800) | ---[ end trace 0000000000000000 ]---

Fix this by having restorezacontext() ensure that the task's svestate is allocated, matching what we do when taking an SME trap. Any live SVE/SSVE state (which is restored earlier from a separate signal context) must be preserved, and hence this is not zeroed.