arm64/ptrace: Don't enable SVE when setting streaming SVE

Systems which implement SME without also implementing SVE are
architecturally valid but were not initially supported by the kernel,
unfortunately we missed one issue in the ptrace code.

The SVE register setting code is shared between SVE and streaming mode
SVE. When we set full SVE register state we currently enable TIF_SVE
unconditionally, in the case where streaming SVE is being configured on a
system that supports vanilla SVE this is not an issue since we always
initialise enough state for both vector lengths but on a system which only
support SME it will result in us attempting to restore the SVE vector
length after having set streaming SVE registers.

Fix this by making the enabling of SVE conditional on setting SVE vector
state. If we set streaming SVE state and SVE was not already enabled this
will result in a SVE access trap on next use of normal SVE, this will cause
us to flush our register state but this is fine since the only way to
trigger a SVE access trap would be to exit streaming mode which will cause
the in register state to be flushed anyway.

Fixes: e12310a0 ("arm64/sme: Implement ptrace support for streaming mode SVE registers")
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230803-arm64-fix-ptrace-ssve-no-sve-v1-1-49df214bfb3e@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/ptrace.c

@@ -932,10 +932,12 @@ static int sve_set_common(struct task_struct *target,
 	/*
 	 * Ensure target->thread.sve_state is up to date with target's
 	 * FPSIMD regs, so that a short copyin leaves trailing
-	 * registers unmodified.  Always enable SVE even if going into
-	 * streaming mode.
+	 * registers unmodified.  Only enable SVE if we are
+	 * configuring normal SVE, a system with streaming SVE may not
+	 * have normal SVE.
 	 */
 	fpsimd_sync_to_sve(target);
+	if (type == ARM64_VEC_SVE)
 		set_tsk_thread_flag(target, TIF_SVE);
 	target->thread.fp_type = FP_STATE_SVE;