Re: [PATCH 2/2] crash_core: fix out-of-bounds access check in crash_exclude_mem_range()

[Yuntao Wang <ytcoode@gmail.com>]

On Tue, 19 Dec 2023 22:22:47 +0800, Baoquan He <bhe@redhat.com> wrote:

> On 12/19/23 at 12:31pm, Yuntao Wang wrote:
> > On Tue, 19 Dec 2023 11:32:02 +0800, Baoquan He <bhe@redhat.com> wrote:
> > > Hi Yuntao,
> > > 
> > > On 12/19/23 at 10:02am, Yuntao Wang wrote:
> > > > On Mon, 18 Dec 2023 09:29:02 -0800, Andrew Morton <akpm@linux-foundation.org> wrote:
> > > > 
> > > > > On Mon, 18 Dec 2023 16:19:15 +0800 Yuntao Wang <ytcoode@gmail.com> wrote:
> > > > > 
> > > > > > mem->nr_ranges represents the current number of elements stored in
> > > > > > the mem->ranges array, and mem->max_nr_ranges represents the maximum number
> > > > > > of elements that the mem->ranges array can hold. Therefore, the correct
> > > > > > array out-of-bounds check should be mem->nr_ranges >= mem->max_nr_ranges.
> > > > > > 
> > > > > 
> > > > > This does not apply after your own "crash_core: fix and simplify the
> > > > > logic of crash_exclude_mem_range()".  What should be done?
> > > > 
> > > > Hi Andrew,
> > > > 
> > > > I actually prefer the "crash_core: fix and simplify the logic of
> > > > crash_exclude_mem_range()" patch as it makes the final code more concise and
> > > > clear, and less prone to errors.
> > > > 
> > > > The current code is too strange, I guess no one can understand why there is
> > > > a break in the for loop when they read this code for the first time.
> > > > 
> > > > Moreover, I think the current code is too fragile, it relies on callers using
> > > > this function correctly to ensure its correctness, rather than being able to
> > > > guarantee the correctness on its own. I even feel that this function is very
> > > > likely to have bugs again as the code evolves.
> > > > 
> > > > However, Baoquan also has his own considerations, he suggests keeping the code
> > > > as it is.
> > > > 
> > > > The link below is our detailed discussion on this issue:
> > > 
> > > There's misunderstanding here.
> > > 
> > > Firstly I said I have concern about the patch, I didn't NACK or reject the patch.
> > > 
> > > [PATCH 3/3] crash_core: fix and simplify the logic of crash_exclude_mem_range()
> > > 
> > > Usually, when people said he/she had concern, you may need to
> > > investigate and resolve it or explain why it's not need be cared about.
> > > 
> > > E.g on above [PATCH 3/3], we can add below code change to stop scanning
> > > when the left ranges are all above the excluded range, assume the passed
> > > in cmem has a ascending order of ranges. Say so because I checked code
> > > and found that crash_exclude_mem_range() is called in arch arm64, ppc,
> > > riscv and x86. Among them, arm64 and ppc create the cmem from memblock,
> > > riscv and x86 create cmem from iomem. All of them should be in ascending
> > > ordr. The below code change based on your patch 3/3 looks safe to me.
> > > What do you think?
> > > 
> > > diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> > > index aab342c2a5ee..39b6c149dc80 100644
> > > --- a/kernel/crash_core.c
> > > +++ b/kernel/crash_core.c
> > > @@ -574,9 +574,12 @@ int crash_exclude_mem_range(struct crash_mem *mem,
> > >  		p_start = mstart;
> > >  		p_end = mend;
> > >  
> > > -		if (p_start > end || p_end < start)
> > > +		if (p_start > end)
> > >  			continue;
> > >  
> > > +		if (p_end < start)
> > > +			break;
> > > +
> > >  		/* Truncate any area outside of range */
> > >  		if (p_start < start)
> > >  			p_start = start;
> > > 
> > > Secondly, I welcome people who are interested kexec/kdump code, and raise
> > > issues or post patches to fix bug, clean up code. I like these patches.
> > > They can help improve kexec/kdump code and solve problem in advance.
> > > I would like to review and make the patches acceptable and merged
> > > inally. And I also hope people can follow the later issue reported by
> > > other people or LKP if their merged patch caused that.
> > > 
> > > Lastly, people are encouraged to help review other people's patch
> > > and give suggestes to improve the code change. If patch author don't
> > > respond for a long while or the work has been suspended for long time, we
> > > can add comment to tell and take over the work to continue.
> > > 
> > > These are my personal understanding and thought about kexec/kdump patch
> > > reviewing and maintance. So cheer up.
> > > 
> > > > 
> > > > https://lore.kernel.org/lkml/20231214163842.129139-3-ytcoode@gmail.com/t/#mfd78a97e16251bcb190b0957a0b6cb4b0a096b54
> > > > 
> > > > The final decision on whether to apply that patch is up to you and Baoquan, if
> > > > you choose to apply that patch, this patch can be ignored. But if you decide not
> > > > to apply that patch, then this patch must be applied, as it fixes a bug in the
> > > > crash_exclude_mem_range() function.
> > > > 
> > > > Sincerely,
> > > > Yuntao
> > 
> > Hi Baoquan,
> > 
> > I must clarify that I was not complaining about you. On the contrary, I am
> > grateful to everyone who takes time to review code for others, because I know
> > it is a lot of work.
> > 
> > I'm relatively new to the Linux community and still learning the various rules
> > of the community. I'm very sorry that I didn't fully grasp your previous intention.
> > 
> > Regarding the method you suggested to add a 'break', I did consider it initially
> > but later decided against it because the memory ranges obtained from iomem may
> > overlap, so I chose a safer way instead.
> 
> In iomem, parent range includes children's range, while
> walk_system_ram_res() traverses ranges not overlapped with each otehr.
> From code in __walk_iomem_res_desc() and find_next_iomem_res(), it
> clearly shows that.
> 
> walk_system_ram_res()
>   -->__walk_iomem_res_desc()
>      -->find_next_iomem_res()
> 

I revisited the relevant code, and yes, you are correct. The memory ranges
obtained from iomem do not overlap.

The reason why I thought these memory ranges would overlap was that I saw
that in the find_next_iomem_res() function, after traversing a parent node,
it starts to traverse its child nodes. If all these nodes meet our
requirements, then the memory ranges they represent will overlap.

However, I overlooked a very important point, which is that after finding a
valid node, the __walk_iomem_res_desc() function will update the start value.
This means that if a parent node is a valid node, all of its child nodes
will be skipped. This ultimately ensures that the memory ranges obtained
from iomem will not overlap.

I will post another patch later, optimizing crash_exclude_mem_range() using
your approach.

> 
> > 
> > Finally, I would like to apologize again if my previous response offended you.
> > That was not my intention.
> 
> No offence felt at all, and no worry about this. In upstream, argument
> is normal, it's fine as long as your intention is making things better,
> not against person. Meantime, let's be kind and friendly to each other,
> we will have a great time.