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

[Qi Xi]

On 06/07/2026 21:32, Xie Yuanbin wrote:
> On Fri, 26 Jun 2026 15:30:47 +0800, Qi Xi wrote:
> > @@ -181,7 +181,9 @@ __do_user_fault(unsigned long addr, unsigned int fsr, unsigned int sig,
> >   		pr_err("8<--- cut here ---\n");
> >   		pr_err("%s: unhandled page fault (%d) at 0x%08lx, code 0x%03x\n",
> >   		       tsk->comm, sig, addr, fsr);
> > +		mmap_read_lock(tsk->mm);
> >   		show_pte(KERN_ERR, tsk->mm, addr);
> > +		mmap_read_unlock(tsk->mm);
> >   		show_regs(regs);
> >   	}
> >   #endif
> I found that this fix does not completely solve the problem. For a user
> fault, the addr could also be a kernel address. For arm32/x86, the kernel
> address space and user address space share the same pgd page table,
> but the kernel address space's page table is not protected by
> current->mm->mmap_lock.
>
> I have written a use case to construct and verify this point. When A user
> program accesses a kernel address and triggers __do_user_fault(),
> show_pte() will directly print the kernel page table.
>
> So, I suggest that:
> ```c
> 	if (user_mode(regs)) {
> 		struct mm_struct *const pt_mm = addr >= TASK_SIZE ?
> 			&init_mm : current->mm;
>
> 		mmap_read_lock(pt_mm);
> 		show_pte(KERN_ALERT, pt_mm, addr);
> 		mmap_read_unlock(pt_mm);
> 	} else {
> 		// .. keep nothing change
> 		show_pte(KERN_ALERT, current->mm, addr);
> 	}
> ```
>
> I have read this article:
> Link: https://docs.kernel.org/mm/process_addrs.html
> `mmap_read_lock(&init_mm)` should be able to ensure that the kernel
> address's page tables can be traversed. But I'm not quite sure if
> `mmap_read_lock(&current->mm)` provides protection for user-space non-VMA
> addresses?
You're right. And I think the fix is to simply skip show_pte() for 
kernel addresses.
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);
	}

For a user-mode fault on a kernel address, printing kernel page tables via
show_pte() does not help. And The fault address is already printed above.
So it's safe and reasonable to skip it.

Same pattern applies to __do_user_fault().

> Also cc to mm maintainers:
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Cc: Liam R. Howlett <liam@xxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Linus Walleij <linusw@xxxxxxxxxx>