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: 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>

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index fdce1b856f4983f18e229721be128d42d426a117..e898948a31b659e4e96295f2c0c41090188cad3d 100644 (file)
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -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;