Commit b465ace4 authored by Mark Rutland's avatar Mark Rutland Committed by Will Deacon
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arm64/fpsimd: signal: Mandate SVE payload for streaming-mode state 



Non-streaming SVE state may be preserved without an SVE payload, in
which case the SVE context only has a header with VL==0, and all state
can be restored from the FPSIMD context. Streaming SVE state is always
preserved with an SVE payload, where the SVE context header has VL!=0,
and the SVE_SIG_FLAG_SM flag is set.

The kernel never preserves an SVE context where SVE_SIG_FLAG_SM is set
without an SVE payload. However, restore_sve_fpsimd_context() doesn't
forbid restoring such a context, and will handle this case by clearing
PSTATE.SM and restoring the FPSIMD context into non-streaming mode,
which isn't consistent with the SVE_SIG_FLAG_SM flag.

Forbid this case, and mandate an SVE payload when the SVE_SIG_FLAG_SM
flag is set. This avoids an awkward ABI quirk and reduces the risk that
later rework to this code permits configuring a task with PSTATE.SM==1
and fp_type==FP_STATE_FPSIMD.

I've marked this as a fix given that we never intended to support this
case, and we don't want anyone to start relying upon the old behaviour
once we re-enable SME.

Fixes: 85ed24da ("arm64/sme: Implement streaming SVE signal handling")
Signed-off-by: default avatarMark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: default avatarMark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-4-mark.rutland@arm.com


Signed-off-by: default avatarWill Deacon <will@kernel.org>
parent 1bf663a8

arch/arm64/kernel/signal.c

+13 −2
Original line number Diff line number Diff line
@@ -387,6 +387,7 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user) 
	unsigned int vl, vq;

	struct user_fpsimd_state fpsimd;

	u16 user_vl, flags;

	bool sm;



	if (user->sve_size < sizeof(*user->sve))

		return -EINVAL;
@@ -396,7 +397,8 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user) 
	if (err)

		return err;



	if (flags & SVE_SIG_FLAG_SM) {

	sm = flags & SVE_SIG_FLAG_SM;

	if (sm) {

		if (!system_supports_sme())

			return -EINVAL;
@@ -416,7 +418,16 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user) 
	if (user_vl != vl)

		return -EINVAL;



	if (user->sve_size == sizeof(*user->sve)) {

	/*

	 * Non-streaming SVE state may be preserved without an SVE payload, in

	 * which case the SVE context only has a header with VL==0, and all

	 * state can be restored from the FPSIMD context.

	 *

	 * Streaming SVE state is always preserved with an SVE payload. For

	 * consistency and robustness, reject restoring streaming SVE state

	 * without an SVE payload.

	 */

	if (!sm && user->sve_size == sizeof(*user->sve)) {

		clear_thread_flag(TIF_SVE);

		current->thread.svcr &= ~SVCR_SM_MASK;

		current->thread.fp_type = FP_STATE_FPSIMD;