[PATCH 6.6 44/72] arm64/fpsimd: signal: Mandate SVE payload for streaming-mode state

Greg Kroah-Hartman
2026-02-04T15:23:08Z

From: Mark Rutland <mark.rutland@arm.com>

[ 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@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: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-4-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
Stable-dep-of: d2907cbe9ea0 ("arm64/fpsimd: signal: Fix restoration of SVE context")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm64/kernel/signal.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)

--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -276,6 +276,7 @@ static int restore_sve_fpsimd_context(st
unsigned int vl, vq;
struct user_fpsimd_state fpsimd;
u16 user_vl, flags;
+ bool sm;

if (user->sve_size < sizeof(*user->sve))
return -EINVAL;
@@ -285,7 +286,8 @@ static int restore_sve_fpsimd_context(st
if (err)
return err;

- if (flags & SVE_SIG_FLAG_SM) {
+ sm = flags & SVE_SIG_FLAG_SM;
+ if (sm) {
if (!system_supports_sme())
return -EINVAL;

@@ -305,7 +307,16 @@ static int restore_sve_fpsimd_context(st
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;