Re: [PATCH v3 1/2] ARM: mm: fix use-after-free in __do_user_fault() under CONFIG_DEBUG_USER

[Lorenzo Stoakes <ljs@kernel.org>]

On Tue, Jul 07, 2026 at 09:14:09PM +0800, Xie Yuanbin wrote:
> On Tue, 7 Jul 2026 12:57:45 +0100, Russell King wrote:
> > No. This information is useful debug for kernel oops.
>
> For kernel oops, I think it should be `!user_mode(regs)`, Qi Xi's reply:
>
> On Tue, 7 Jul 2026 19:48:12 +0800, Qi Xi wrote:
> > For do_DataAbort() fallback:
> >
> >	if (user_mode(regs)) {
> >		if (addr < TASK_SIZE) {
> >			mmap_read_lock(current->mm);
> >			show_pte(KERN_ALERT, current->mm, addr);
> >			mmap_read_unlock(current->mm);
> >		}
> >	} else {
> >		show_pte(KERN_ALERT, current->mm, addr);
> >	}
>
> changes nothing to kernel oops. It only skip show_pte() for user-mode
> faults, and the fault addr is a kernel address, which means a user
> program is trying to access a kernel address.
> I think it is reasonable to skip show_pte() in this case?

Well the whole reason you're faulting here might be because a userland process
did that right? The page tables should tell you (presumably on ARM32 :)

And I hate to repeat myself, maybe you didn't read the whole thread but... just
use mmap_write_lock(), this isn't necessary?

What is this trying to achieve?

You're not in a hotpath, why are you bothering to conditionally take/not take
the lock?

Thanks, Lorenzo