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

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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: 85ed24dad2 ("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>
(cherry picked from commit b465ace42620970e840c7aeb2c44a6e3b1002fec
 https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git
 for-next/sme-fixes)
Bug: 393087661
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: Id34e2065b9efc4a4eb967367ff76264e701dbd37
This commit is contained in:
Mark Rutland
2025-05-08 14:26:23 +01:00
committed by Will Deacon
parent 63897a249f
commit bed5006f4a
1 changed files with 13 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

15
arch/arm64/kernel/signal.c
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@@ -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;