On Tue, Sep 22, 2026 at 05:27:40PM -0700, Borislav Petkov wrote: > On Tue, Sep 22, 2026 at 01:24:55PM +0900, Masami Hiramatsu (Google) wrote: > > From: Masami Hiramatsu (Google) <[email protected]> > > > > 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. > > > Signed-off-by: Masami Hiramatsu (Google) <[email protected]> > > --- > > 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. > > Peter?
Yep is right. Acked-by: Peter Zijlstra (Intel) <[email protected]>
