Re: [PATCH v17 01/13] x86/mce: Fix hardware debug register corruption on task migration

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 22 Sep 2026 17:27:40 -0700
Borislav Petkov <bp@alien8.de> wrote:

> On Tue, Sep 22, 2026 at 01:24:55PM +0900, Masami Hiramatsu (Google) wrote:
> > 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
> 
> verify_tags: WARNING: Zapping LLM ads tag: ['Antigravity:gemini-3.8-flash'].
> 
> Needs to be:
> 
> Assisted-by: LLM
> 
> Please fix all your patches.

Ah, let me fix it.

> 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > ---
> > Changes in v15:
> >  - Move local_db_save() to the entry of exc_machine_check_kernel() and
> >    restore it on early return from mce_check_crashing_cpu() to prevent
> >    nested #DB on watched variables.
> > Changes in v14:
> >  - Newly added.
> > ---
> >  arch/x86/kernel/cpu/mce/core.c |   27 ++++++++++-----------------
> >  1 file changed, 10 insertions(+), 17 deletions(-)
> 
> Yah, makes sense.
> 
> I'm thinking this needs to be CC:stable and I should take it in now.

OK. thanks for review!

> 
> Peter?
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>