CVE-2026-98745

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

arm64/fpsimd: signal: Forbid non-streaming SVE payload on SME-only systems

When system supports sme() is true but system supports sve() is false, restoring a specifically crafted SVE signal context can result in the task erroneously having non-streaming SVE state. Subsequent attempts to manipulate the task's FPSIMD/SVE/SME state can result in a variety of problems, including fatal EL1 UNDEFs.

In such configurations, the kernel always creates an SVE signal context when delivering a signal, and this can only be in one of two states:

(1) SVE SIG FLAG SM is set, and an SVE payload is present containing streaming mode SVE state. The recorded VL is the task's live streaming VL.

(2) SVE SIG FLAG SM is clear, and an SVE payload is not present. The FPSIMD context contains the non-streaming mode FPSIMD state. The recorded VL is 0.

Currently restore sve fpsimd context() correctly rejects cases where SVE SIG FLAG SM is set and an SVE payload is not present, but fails to reject cases where SVE SIG FLAG SM is clear and an SVE payload is present. Consequently, restore sve fpsimd context() can place the task in a state where it has non-streaming SVE state even when this is not supported by HW.

For example, this can cause a later EL1 UNDEF when the kernel attempts to restore the task's ZCR EL1 value:

| # ./sme-sigcontext-to-sve | Internal error: Oops - Undefined instruction: 0000000002000000 [#1] SMP | Modules linked in: | CPU: 0 UID: 0 PID: 131 Comm: sme-sigcontext- Not tainted 7.3.0-rc1 #1 PREEMPT | Hardware name: linux,dummy-virt (DT) | pstate: 61402009 (nZCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--) | pc : fpsimd restore current state+0x258/0x458 | lr : exit to user mode loop+0xb8/0x188 | sp : ffff80008056be40 | x29: ffff80008056be40 x28: fff00000c1670000 x27: 0000000000000000 | x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000 | x23: ffff80008056bec0 x22: 0000000000000008 x21: 0000000000000040 | x20: 0000000000000081 x19: 0000000008800010 x18: 0000000000000000 | x17: 0000fffffd412570 x16: 0000000000001000 x15: 0000fffffd4123b0 | x14: 0000fffffd412780 x13: 0000fffffd412be8 x12: 0000000047435300 | x11: 0000fffffd412570 x10: 0000000000000000 x9 : 0000000045585401 | x8 : fff00000c18a6c44 x7 : 0000000000000000 x6 : 0000000000000002 | x5 : 0000000000000002 x4 : ffff800080568000 x3 : 0000000000000001 | x2 : 0000000008800010 x1 : fff00000c1670000 x0 : 0000000008800000 | Call trace: | fpsimd restore current state+0x258/0x458 (P) | exit to user mode loop+0xb8/0x188 | el0 svc+0x1cc/0x1d0 | el0t 64 sync handler+0xa0/0xe4 | el0t 64 sync+0x198/0x19c | Code: d5384101 f9400020 53175c03 36b80de0 (d5381202) | ---[ end trace 0000000000000000 ]--- | Kernel panic - not syncing: Oops - Undefined instruction: Fatal exception in interrupt | Kernel Offset: 0x291fb1000000 from 0xffff800080000000 | PHYS OFFSET: 0x40000000 | CPU features: 0x0,00000000,0052802f,ffb88f43,3afcf73f | Memory Limit: none

Rework restore sve fpsimd context() to reject cases where SVE SIG FLAG SM is clear and an SVE payload is not present. As parse user sigframe() rejects SVE signal frames when neither SVE nor SME are supported, it isn't necessary for restore sve fpsimd context() to handle the case where neither are supported.