From 01098d893fa8a6edb2b56e178b798e3e6b674f02 Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Wed, 9 Apr 2025 17:40:04 +0100 Subject: [PATCH] arm64/fpsimd: Avoid clobbering kernel FPSIMD state with SMSTOP On system with SME, a thread's kernel FPSIMD state may be erroneously clobbered during a context switch immediately after that state is restored. Systems without SME are unaffected. If the CPU happens to be in streaming SVE mode before a context switch to a thread with kernel FPSIMD state, fpsimd_thread_switch() will restore the kernel FPSIMD state using fpsimd_load_kernel_state() while the CPU is still in streaming SVE mode. When fpsimd_thread_switch() subsequently calls fpsimd_flush_cpu_state(), this will execute an SMSTOP, causing an exit from streaming SVE mode. The exit from streaming SVE mode will cause the hardware to reset a number of FPSIMD/SVE/SME registers, clobbering the FPSIMD state. Fix this by calling fpsimd_flush_cpu_state() before restoring the kernel FPSIMD state. Fixes: e92bee9f861b ("arm64/fpsimd: Avoid erroneous elide of user state reload") Signed-off-by: Mark Rutland Cc: Ard Biesheuvel Cc: Mark Brown Cc: Will Deacon Reviewed-by: Mark Brown Link: https://lore.kernel.org/r/20250409164010.3480271-8-mark.rutland@arm.com Signed-off-by: Catalin Marinas --- 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 b19736f354a7..4a0b0bb3a3fa 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -1550,8 +1550,8 @@ void fpsimd_thread_switch(struct task_struct *next) fpsimd_save_user_state(); if (test_tsk_thread_flag(next, TIF_KERNEL_FPSTATE)) { - fpsimd_load_kernel_state(next); fpsimd_flush_cpu_state(); + fpsimd_load_kernel_state(next); } else { /* * Fix up TIF_FOREIGN_FPSTATE to correctly describe next's -- 2.50.1