[PATCH 2/3] arm64: nofpsmid: Clear TIF_FOREIGN_FPSTATE flag for early tasks

[Suzuki K Poulose]

We detect the absence of FP/SIMD after we boot the SMP CPUs, and by then
we have kernel threads running already with TIF_FOREIGN_FPSTATE set which
could be inherited by early userspace applications (e.g, modprobe triggered
from initramfs). This could end up in the applications stuck in
do_nofity_resume() as we never clear the TIF flag, once we now know that
we don't support FP.

Fix this by making sure that we clear the TIF_FOREIGN_FPSTATE flag
for tasks which may have them set, as we would have done in the normal
case, but avoiding touching the hardware state (since we don't support any).

Fixes: 82e0191a1aa11abf ("arm64: Support systems without FP/ASIMD")
Cc: Will Deacon <w...@kernel.org>
Cc: Mark Rutland <mark.rutl...@arm.com>
Cc: Catalin Marinas <catalin.mari...@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poul...@arm.com>
---
 arch/arm64/kernel/fpsimd.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 37d3912cfe06..dfcdd077aeca 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -1128,12 +1128,19 @@ void fpsimd_bind_state_to_cpu(struct user_fpsimd_state 
*st, void *sve_state,
  */
 void fpsimd_restore_current_state(void)
 {
-       if (!system_supports_fpsimd())
-               return;
-
        get_cpu_fpsimd_context();
-
-       if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
+       /*
+        * For the tasks that were created before we detected the absence of
+        * FP/SIMD, the TIF_FOREIGN_FPSTATE could be set via 
fpsimd_thread_switch()
+        * and/or could be inherited from the parent(init_task has this set). 
Even
+        * though userspace has not run yet, this could be inherited by the
+        * processes forked from one of those tasks (e.g, modprobe from 
initramfs).
+        * If the system doesn't support FP/SIMD, we must clear the flag for the
+        * tasks mentioned above, to indicate that the FPSTATE is clean (as we
+        * can't have one) to avoid looping for ever to clear the flag.
+        */
+       if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE) &&
+           system_supports_fpsimd()) {
                task_fpsimd_load();
                fpsimd_bind_task_to_cpu();
        }
@@ -1148,17 +1155,16 @@ void fpsimd_restore_current_state(void)
  */
 void fpsimd_update_current_state(struct user_fpsimd_state const *state)
 {
-       if (!system_supports_fpsimd())
-               return;
-
        get_cpu_fpsimd_context();
 
        current->thread.uw.fpsimd_state = *state;
        if (system_supports_sve() && test_thread_flag(TIF_SVE))
                fpsimd_to_sve(current);
 
-       task_fpsimd_load();
-       fpsimd_bind_task_to_cpu();
+       if (system_supports_fpsimd()) {
+               task_fpsimd_load();
+               fpsimd_bind_task_to_cpu();
+       }
 
        clear_thread_flag(TIF_FOREIGN_FPSTATE);
 
-- 
2.21.0