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

[Yosry Ahmed]

On Thu, Feb 13, 2025 at 01:08:54PM +0000, gaoxu wrote:
> > 
> > On Wed, 12 Feb 2025 03:13:46 +0000 gaoxu <gaoxu2@xxxxxxxxx> 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);

WARN() already contains unlikely(). Also, no need to print the function
name it's already in the stack trace.

We should probably just do if (WARN_ON_ONCE(!si)).

> +		return -EINVAL;
> +	}
> 
>