arm64/fpsimd: ptrace/prctl: Ensure VL changes leave task in a valid state

arm64/fpsimd: ptrace/prctl: Ensure VL changes leave task in a valid state

Currently, vec_set_vector_length() can manipulate a task into an invalid
state as a result of a prctl/ptrace syscall which changes the SVE/SME
vector length, resulting in several problems:

(1) When changing the SVE vector length, if the task initially has
    PSTATE.ZA==1, and sve_alloc() fails to allocate memory, the task
    will be left with PSTATE.ZA==1 and sve_state==NULL. This is not a
    legitimate state, and could result in a subsequent null pointer
    dereference.

(2) When changing the SVE vector length, if the task initially has
    PSTATE.SM==1, the task will be left with PSTATE.SM==1 and
    fp_type==FP_STATE_FPSIMD. Streaming mode state always needs to be
    saved in SVE format, so this is not a legitimate state.

    Attempting to restore this state may cause a task to erroneously
    inherit stale streaming mode predicate registers and FFR contents,
    behaving non-deterministically and potentially leaving information
    from another task.

    While in this state, reads of the NT_ARM_SSVE regset will indicate
    that the registers are not stored in SVE format. For the NT_ARM_SSVE
    regset specifically, debuggers interpret this as meaning that
    PSTATE.SM==0.

(3) When changing the SME vector length, if the task initially has
    PSTATE.SM==1, the lower 128 bits of task's streaming mode vector
    state will be migrated to non-streaming mode, rather than these bits
    being zeroed as is usually the case for changes to PSTATE.SM.

To fix the first issue, we can eagerly allocate the new sve_state and
sme_state before modifying the task. This makes it possible to handle
memory allocation failure without modifying the task state at all, and
removes the need to clear TIF_SVE and TIF_SME.

To fix the second issue, we either need to clear PSTATE.SM or not change
the saved fp_type. Given we're going to eagerly allocate sve_state and
sme_state, the simplest option is to preserve PSTATE.SM and the saves
fp_type, and consistently truncate the SVE state. This ensures that the
task always stays in a valid state, and by virtue of not exiting
streaming mode, this also sidesteps the third issue.

I believe these changes should not be problematic for realistic usage:

* When the SVE/SME vector length is changed via prctl(), syscall entry
  will have cleared PSTATE.SM. Unless the task's state has been
  manipulated via ptrace after entry, the task will have PSTATE.SM==0.

* When the SVE/SME vector length is changed via a write to the
  NT_ARM_SVE or NT_ARM_SSVE regsets, PSTATE.SM will be forced
  immediately after the length change, and new vector state will be
  copied from userspace.

* When the SME vector length is changed via a write to the NT_ARM_ZA
  regset, the (S)SVE state is clobbered today, so anyone who cares about
  the specific state would need to install this after writing to the
  NT_ARM_ZA regset.

As we need to free the old SVE state while TIF_SVE may still be set, we
cannot use sve_free(), and using kfree() directly makes it clear that
the free pairs with the subsequent assignment. As this leaves sve_free()
unused, I've removed the existing sve_free() and renamed __sve_free() to
mirror sme_free().

Fixes: 8bd7f91c03d8 ("arm64/sme: Implement traps and syscall handling for SME")
Fixes: baa8515281b3 ("arm64/fpsimd: Track the saved FPSIMD state type separately to TIF_SVE")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Spickett <david.spickett@arm.com>
Cc: Luis Machado <luis.machado@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-16-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>