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

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.