x86/fpu: Invalidate FPU state correctly on exec()

The thread flag TIF_NEED_FPU_LOAD indicates that the FPU saved state is
valid and should be reloaded when returning to userspace. However, the
kernel will skip doing this if the FPU registers are already valid as
determined by fpregs_state_valid(). The logic embedded there considers
the state valid if two cases are both true:

1: fpu_fpregs_owner_ctx points to the current tasks FPU state
2: the last CPU the registers were live in was the current CPU.

This is usually correct logic. A CPU’s fpu_fpregs_owner_ctx is set to
the current FPU during the fpregs_restore_userregs() operation, so it
indicates that the registers have been restored on this CPU. But this
alone doesn’t preclude that the task hasn’t been rescheduled to a
different CPU, where the registers were modified, and then back to the
current CPU. To verify that this was not the case the logic relies on the
second condition. So the assumption is that if the registers have been
restored, AND they haven’t had the chance to be modified (by being
loaded on another CPU), then they MUST be valid on the current CPU.

Besides the lazy FPU optimizations, the other cases where the FPU
registers might not be valid are when the kernel modifies the FPU register
state or the FPU saved buffer. In this case the operation modifying the
FPU state needs to let the kernel know the correspondence has been
broken. The comment in “arch/x86/kernel/fpu/context.h” has:
/*
...
* If the FPU register state is valid, the kernel can skip restoring the
* FPU state from memory.
*
* Any code that clobbers the FPU registers or updates the in-memory
* FPU state for a task MUST let the rest of the kernel know that the
* FPU registers are no longer valid for this task.
*
* Either one of these invalidation functions is enough. Invalidate
* a resource you control: CPU if using the CPU for something else
* (with preemption disabled), FPU for the current task, or a task that
* is prevented from running by the current task.
*/

However, this is not completely true. When the kernel modifies the
registers or saved FPU state, it can only rely on
__fpu_invalidate_fpregs_state(), which wipes the FPU’s last_cpu
tracking. The exec path instead relies on fpregs_deactivate(), which sets
the CPU’s FPU context to NULL. This was observed to fail to restore the
reset FPU state to the registers when returning to userspace in the
following scenario:

1. A task is executing in userspace on CPU0
- CPU0’s FPU context points to tasks
- fpu->last_cpu=CPU0

2. The task exec()’s

3. While in the kernel the task is preempted
- CPU0 gets a thread executing in the kernel (such that no other
FPU context is activated)
- Scheduler sets task’s fpu->last_cpu=CPU0 when scheduling out

4. Task is migrated to CPU1

5. Continuing the exec(), the task gets to
fpu_flush_thread()->fpu_reset_fpregs()
- Sets CPU1’s fpu context to NULL
- Copies the init state to the task’s FPU buffer
- Sets TIF_NEED_FPU_LOAD on the task

6. The task reschedules back to CPU0 before completing the exec() and
returning to userspace
- During the reschedule, scheduler finds TIF_NEED_FPU_LOAD is set
- Skips saving the registers and updating task’s fpu→last_cpu,
because TIF_NEED_FPU_LOAD is the canonical source.

7. Now CPU0’s FPU context is still pointing to the task’s, and
fpu->last_cpu is still CPU0. So fpregs_state_valid() returns true even
though the reset FPU state has not been restored.

So the root cause is that exec() is doing the wrong kind of invalidate. It
should reset fpu->last_cpu via __fpu_invalidate_fpregs_state(). Further,
fpu__drop() doesn't really seem appropriate as the task (and FPU) are not
going away, they are just getting reset as part of an exec. So switch to
__fpu_invalidate_fpregs_state().

Also, delete the misleading comment that says that either kind of
invalidate will be enough, because it’s not always the case.

Fixes: 33344368cb08 ("x86/fpu: Clean up the fpu__clear() variants")
Reported-by: Lei Wang <lei4.wang@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Lijun Pan <lijun.pan@intel.com>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Acked-by: Lijun Pan <lijun.pan@intel.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230818170305.502891-1-rick.p.edgecombe@intel.com

fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

When switching from kthreads to vhost_tasks two bugs were added:
1. The vhost worker tasks's now show up as processes so scripts doing
ps or ps a would not incorrectly detect the vhost task as another
process. 2. kthreads disabled freeze by setting PF_NOFREEZE, but
vhost tasks's didn't disable or add support for them.

To fix both bugs, this switches the vhost task to be thread in the
process that does the VHOST_SET_OWNER ioctl, and has vhost_worker call
get_signal to support SIGKILL/SIGSTOP and freeze signals. Note that
SIGKILL/STOP support is required because CLONE_THREAD requires
CLONE_SIGHAND which requires those 2 signals to be supported.

This is a modified version of the patch written by Mike Christie
<michael.christie@oracle.com> which was a modified version of patch
originally written by Linus.

Much of what depended upon PF_IO_WORKER now depends on PF_USER_WORKER.
Including ignoring signals, setting up the register state, and having
get_signal return instead of calling do_group_exit.

Tidied up the vhost_task abstraction so that the definition of
vhost_task only needs to be visible inside of vhost_task.c. Making
it easier to review the code and tell what needs to be done where.
As part of this the main loop has been moved from vhost_worker into
vhost_task_fn. vhost_worker now returns true if work was done.

The main loop has been updated to call get_signal which handles
SIGSTOP, freezing, and collects the message that tells the thread to
exit as part of process exit. This collection clears
__fatal_signal_pending. This collection is not guaranteed to
clear signal_pending() so clear that explicitly so the schedule()
sleeps.

For now the vhost thread continues to exist and run work until the
last file descriptor is closed and the release function is called as
part of freeing struct file. To avoid hangs in the coredump
rendezvous and when killing threads in a multi-threaded exec. The
coredump code and de_thread have been modified to ignore vhost threads.

Remvoing the special case for exec appears to require teaching
vhost_dev_flush how to directly complete transactions in case
the vhost thread is no longer running.

Removing the special case for coredump rendezvous requires either the
above fix needed for exec or moving the coredump rendezvous into
get_signal.

Fixes: 6e890c5d5021 ("vhost: use vhost_tasks for worker threads")
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Co-developed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

x86/fpu: Don't set TIF_NEED_FPU_LOAD for PF_IO_WORKER threads

We don't set it on PF_KTHREAD threads as they never return to userspace,
and PF_IO_WORKER threads are identical in that regard. As they keep
running in the kernel until they die, skip setting the FPU flag on them.

More of a cosmetic thing that was found while debugging and
issue and pondering why the FPU flag is set on these threads.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/560c844c-f128-555b-40c6-31baff27537f@kernel.dk

x86/fpu: Update XFD state where required

The IA32_XFD_MSR allows to arm #NM traps for XSTATE components which are
enabled in XCR0. The register has to be restored before the tasks XSTATE is
restored. The life time rules are the same as for FPU state.

XFD is updated on return to userspace only when the FPU state of the task
is not up to date in the registers. It's updated before the XRSTORS so
that eventually enabled dynamic features are restored as well and not
brought into init state.

Also in signal handling for restoring FPU state from user space the
correctness of the XFD state has to be ensured.

Add it to CPU initialization and resume as well.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20211021225527.10184-17-chang.seok.bae@intel.com

x86/fpu: Rework restore_regs_from_fpstate()

xfeatures_mask_fpstate() is no longer valid when dynamically enabled
features come into play.

Rework restore_regs_from_fpstate() so it takes a constant mask which will
then be applied against the maximum feature set so that the restore
operation brings all features which are not in the xsave buffer xfeature
bitmap into init state.

This ensures that if the previous task used a dynamically enabled feature
that the task which restores has all unused components properly initialized.

Cleanup the last user of xfeatures_mask_fpstate() as well and remove it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211014230739.461348278@linutronix.de

x86/fpu: Convert restore_fpregs_from_fpstate() to struct fpstate

Convert restore_fpregs_from_fpstate() and related code to the new
register storage mechanism in preparation for dynamically sized buffers.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211013145322.347395546@linutronix.de

x86/fpu: Move fpregs_restore_userregs() to core

Only used internally in the FPU core code.

While at it, convert to the percpu accessors which verify preemption is
disabled.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011539.686806639@linutronix.de