arm64/fpsimd: ptrace: Mandate SVE payload for streaming-mode state

When a task has PSTATE.SM==1, reads of NT_ARM_SSVE are required to
always present a header with SVE_PT_REGS_SVE, and register data in SVE
format. Reads of NT_ARM_SSVE must never present register data in FPSIMD
format. Within the kernel, we always expect streaming SVE data to be
stored in SVE format.

Currently a user can write to NT_ARM_SSVE with a header presenting
SVE_PT_REGS_FPSIMD rather than SVE_PT_REGS_SVE, placing the task's
FPSIMD/SVE data into an invalid state.

To fix this we can either:

(a) Forbid such writes.
(b) Accept such writes, and immediately convert data into SVE format.

Take the simple option and forbid such writes.

Fixes: e12310a ("arm64/sme: Implement ptrace support for streaming mode SVE registers")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Spickett <david.spickett@arm.com>
Cc: Luis Machado <luis.machado@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-19-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: mrutland-arm

arch/arm64/kernel/ptrace.c

@@ -890,6 +890,7 @@ static int sve_set_common(struct task_struct *target,
 	struct user_sve_header header;
 	unsigned int vq;
 	unsigned long start, end;
+	bool fpsimd;
 
 	/* Header */
 	if (count < sizeof(header))
@@ -899,6 +900,15 @@ static int sve_set_common(struct task_struct *target,
 	if (ret)
 		goto out;
 
+	/*
+	 * Streaming SVE data is always stored and presented in SVE format.
+	 * Require the user to provide SVE formatted data for consistency, and
+	 * to avoid the risk that we configure the task into an invalid state.
+	 */
+	fpsimd = (header.flags & SVE_PT_REGS_MASK) == SVE_PT_REGS_FPSIMD;
+	if (fpsimd && type == ARM64_VEC_SME)
+		return -EINVAL;
+
 	/*
 	 * Apart from SVE_PT_REGS_MASK, all SVE_PT_* flags are consumed by
 	 * vec_set_vector_length(), which will also validate them for us:
@@ -945,7 +955,7 @@ static int sve_set_common(struct task_struct *target,
 	/* Registers: FPSIMD-only case */
 
 	BUILD_BUG_ON(SVE_PT_FPSIMD_OFFSET != sizeof(header));
-	if ((header.flags & SVE_PT_REGS_MASK) == SVE_PT_REGS_FPSIMD) {
+	if (fpsimd) {
 		clear_tsk_thread_flag(target, TIF_SVE);
 		target->thread.fp_type = FP_STATE_FPSIMD;
 		ret = __fpr_set(target, regset, pos, count, kbuf, ubuf,