回复: [PATCH] mm: Fix possible NULL pointer dereference in __swap_duplicate

[gaoxu <gaoxu2@honor.com>]

> 
> On Wed, 12 Feb 2025 03:13:46 +0000 gaoxu <gaoxu2@honor.com> wrote:
> 
> > swp_swap_info() may return null; it is necessary to check the return
> > value to avoid NULL pointer dereference. The code for other calls to
> > swp_swap_info() includes checks, and __swap_duplicate() should also
> > include checks.
> 
> Actually very few of the swp_swap_info() callers check for a NULL return.
The swapfile.c file contains three instances where the return value of
swp_swap_info() is checked for a NULL return. In other files that call
swp_swap_info(), I have confirmed that there are no such checks.
The description in the patch is inaccurate, and I have made modifications
in patch v2.
> 
> > The reason why swp_swap_info() returns NULL is unclear; it may be due
> > to CPU cache issues or DDR bit flips.
> 
> Quite possibly it's a kernel bug.
> 
> > The probability of this issue is very
> > small, and the stack info we encountered is as follows：
> > Unable to handle kernel NULL pointer dereference at virtual address
> > 0000000000000058
> >
> > ...
> >
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -3521,6 +3521,8 @@ static int __swap_duplicate(swp_entry_t entry,
> unsigned char usage, int nr)
> >  	int err, i;
> >
> >  	si = swp_swap_info(entry);
> > +	if (unlikely(!si))
> > +		return -EINVAL;
> >
> >  	offset = swp_offset(entry);
> >  	VM_WARN_ON(nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER);
> 
> OK, I guess avoiding the crash is good.  But please let's include a WARN so that
> we can perhaps fix the bug, if one is there.
Good. I'll change it as mentioned and send a new patch.
	si = swp_swap_info(entry);
+	if (unlikely(!si)) {
+		WARN(1, KERN_ERR "%s: %s%08lx\n", __func__, Bad_file, entry.val);
+		return -EINVAL;
+	}