6.15-stable review patch. If anyone has any objections, please let me know.
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From: Mark Rutland mark.rutland@arm.com
[ Upstream commit c94f2f326146a34066a0070ed90b8bc656b1842f ]
For backwards compatibility reasons, when a signal return occurs which restores SVE state, the effective lower 128 bits of each of the SVE vector registers are restored from the corresponding FPSIMD vector register in the FPSIMD signal frame, overriding the values in the SVE signal frame. This is intended to be the case regardless of streaming mode.
To make this happen, restore_sve_fpsimd_context() uses fpsimd_update_current_state() to merge the lower 128 bits from the FPSIMD signal frame into the SVE register state. Unfortunately, fpsimd_update_current_state() performs this merging dependent upon TIF_SVE, which is not always correct for streaming SVE register state:
* When restoring non-streaming SVE register state there is no observable problem, as the signal return code configures TIF_SVE and the saved fp_type to match before calling fpsimd_update_current_state(), which observes either:
- TIF_SVE set AND fp_type == FP_STATE_SVE - TIF_SVE clear AND fp_type == FP_STATE_FPSIMD
* On systems which have SME but not SVE, TIF_SVE cannot be set. Thus the merging will never happen for the streaming SVE register state.
* On systems which have SVE and SME, TIF_SVE can be set and cleared independently of PSTATE.SM. Thus the merging may or may not happen for streaming SVE register state.
As TIF_SVE can be cleared non-deterministically during syscalls (including at the start of sigreturn()), the merging may occur non-deterministically from the perspective of userspace.
This logic has been broken since its introduction in commit:
85ed24dad2904f7c ("arm64/sme: Implement streaming SVE signal handling")
... at which point both fpsimd_signal_preserve_current_state() and fpsimd_update_current_state() only checked TIF SVE. When PSTATE.SM==1 and TIF_SVE was clear, signal delivery would place stale FPSIMD state into the FPSIMD signal frame, and signal return would not merge this into the restored register state.
Subsequently, signal delivery was fixed as part of commit:
61da7c8e2a602f66 ("arm64/signal: Don't assume that TIF_SVE means we saved SVE state")
... but signal restore was not given a corresponding fix, and when TIF_SVE was clear, signal restore would still fail to merge the FPSIMD state into the restored SVE register state. The 'Fixes' tag did not indicate that this had been broken since its introduction.
Fix this by merging the FPSIMD state dependent upon the saved fp_type, matching what we (currently) do during signal delivery.
As described above, when backporting this commit, it will also be necessary to backport commit:
61da7c8e2a602f66 ("arm64/signal: Don't assume that TIF_SVE means we saved SVE state")
... and prior to commit:
baa8515281b30861 ("arm64/fpsimd: Track the saved FPSIMD state type separately to TIF_SVE")
... it will be necessary for fpsimd_signal_preserve_current_state() and fpsimd_update_current_state() to consider both TIF_SVE and thread_sm_enabled(¤t->thread), in place of the saved fp_type.
Fixes: 85ed24dad290 ("arm64/sme: Implement streaming SVE signal handling") Signed-off-by: Mark Rutland mark.rutland@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/20250409164010.3480271-10-mark.rutland@arm.com Signed-off-by: Catalin Marinas catalin.marinas@arm.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/arm64/kernel/fpsimd.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c index 7ae01f02e18b7..1edf797d2c710 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -1806,7 +1806,7 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state) get_cpu_fpsimd_context();
current->thread.uw.fpsimd_state = *state; - if (test_thread_flag(TIF_SVE)) + if (current->thread.fp_type == FP_STATE_SVE) fpsimd_to_sve(current);
task_fpsimd_load();