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