[PATCH 6.12.y 1/2] arm64/fpsimd: signal: Mandate SVE payload for streaming-mode state

[Rahul Sharma]

From: Mark Rutland <mark.rutland@xxxxxxx>

[ Upstream commit b465ace42620970e840c7aeb2c44a6e3b1002fec ]

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: 85ed24dad290 ("arm64/sme: Implement streaming SVE signal handling")
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Marc Zyngier <maz@xxxxxxxxxx>
Cc: Mark Brown <broonie@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Reviewed-by: Mark Brown <broonie@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20250508132644.1395904-4-mark.rutland@xxxxxxx
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Rahul Sharma <black.hawk@xxxxxxx>
---
 arch/arm64/kernel/signal.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index f7b05267a76b..2ff6af928426 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -391,6 +391,7 @@ 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;
@@ -400,7 +401,8 @@ 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;
 
@@ -420,7 +422,16 @@ 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;
-- 
2.34.1