Re: [PATCH v16 01/13] x86/mce: Fix hardware debug register corruption on task migration

[sashiko-bot@kernel.org]

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> 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: Antigravity:gemini-3.8-flash
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

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