x86/mce: Fix hardware debug register corruption on task migration

In exc_machine_check_user(), local_db_save() and local_db_restore() are
invoked in the outer entry stubs (DEFINE_IDTENTRY_MCE_USER,
DEFINE_FREDENTRY_MCE, and DEFINE_IDTENTRY_RAW), surrounding
exc_machine_check_user().

However, exc_machine_check_user() calls irqentry_exit_to_user_mode(), which
handles pending thread work and may schedule() if TIF_NEED_RESCHED is set. If
the task migrates to another CPU during schedule(), local_db_restore() runs on
the new CPU with the dr7 state saved from the old CPU. This corrupts the new
CPU's DR7 hardware debug register and leaves the old CPU's DR7 disabled.  In
short, local_db_save() and local_db_restore() pair must be run on the same
CPU.

To fix this, move local_db_save() and local_db_restore() inside
exc_machine_check_user() and exc_machine_check_kernel(). In
exc_machine_check_user(), DR7 is saved and restored strictly around
do_machine_check() to avoid schedule() during migration. In
exc_machine_check_kernel(), local_db_save() is called at the entry point to
prevent early memory accesses from triggering nested #DB exceptions, and
restored on all exits.

Fixes: cd840e424f27 ("x86/entry, mce: Disallow #DB during #MC")
Assisted-by: LLM
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: <stable@kernel.org>
Link: https://patch.msgid.link/179005109564.388919.3937970081044095776.stgit@devnote2

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