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

[Russell King <linux@armlinux.org.uk>]

On Tue, Jul 07, 2026 at 07:48:12PM +0800, Qi Xi wrote:
> 
> 
> 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.

No. This information is useful debug for kernel oops.

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