Re: [PATCH v2 0/2] mm/kasan: dump alloc/free stack for page allocator

[Walter Wu]

On Tue, 2019-09-10 at 13:50 +0300, Andrey Ryabinin wrote:
> 
> On 9/9/19 4:07 PM, Vlastimil Babka wrote:
> > On 9/9/19 10:24 AM, walter-zh.wu@xxxxxxxxxxxx wrote:
> >> From: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> >>
> >> This patch is KASAN report adds the alloc/free stacks for page allocator
> >> in order to help programmer to see memory corruption caused by page.
> >>
> >> By default, KASAN doesn't record alloc and free stack for page allocator.
> >> It is difficult to fix up page use-after-free or dobule-free issue.
> >>
> >> Our patchsets will record the last stack of pages.
> >> It is very helpful for solving the page use-after-free or double-free.
> >>
> >> KASAN report will show the last stack of page, it may be:
> >> a) If page is in-use state, then it prints alloc stack.
> >>     It is useful to fix up page out-of-bound issue.
> > 
> > I still disagree with duplicating most of page_owner functionality for the sake of using a single stack handle for both alloc and free (while page_owner + debug_pagealloc with patches in mmotm uses two handles). It reduces the amount of potentially important debugging information, and I really doubt the u32-per-page savings are significant, given the rest of KASAN overhead.
> > 
> >> BUG: KASAN: slab-out-of-bounds in kmalloc_pagealloc_oob_right+0x88/0x90
> >> Write of size 1 at addr ffffffc0d64ea00a by task cat/115
> >> ...
> >> Allocation stack of page:
> >>   set_page_stack.constprop.1+0x30/0xc8
> >>   kasan_alloc_pages+0x18/0x38
> >>   prep_new_page+0x5c/0x150
> >>   get_page_from_freelist+0xb8c/0x17c8
> >>   __alloc_pages_nodemask+0x1a0/0x11b0
> >>   kmalloc_order+0x28/0x58
> >>   kmalloc_order_trace+0x28/0xe0
> >>   kmalloc_pagealloc_oob_right+0x2c/0x68
> >>
> >> b) If page is freed state, then it prints free stack.
> >>     It is useful to fix up page use-after-free or double-free issue.
> >>
> >> BUG: KASAN: use-after-free in kmalloc_pagealloc_uaf+0x70/0x80
> >> Write of size 1 at addr ffffffc0d651c000 by task cat/115
> >> ...
> >> Free stack of page:
> >>   kasan_free_pages+0x68/0x70
> >>   __free_pages_ok+0x3c0/0x1328
> >>   __free_pages+0x50/0x78
> >>   kfree+0x1c4/0x250
> >>   kmalloc_pagealloc_uaf+0x38/0x80
> >>
> >> This has been discussed, please refer below link.
> >> https://bugzilla.kernel.org/show_bug.cgi?id=203967
> > 
> > That's not a discussion, but a single comment from Dmitry, which btw contains "provide alloc *and* free stacks for it" ("it" refers to page, emphasis mine). It would be nice if he or other KASAN guys could clarify.
> > 
> 
> For slab objects we memorize both alloc and free stacks. You'll never know in advance what information will be usefull
> to fix an issue, so it usually better to provide more information. I don't think we should do anything different for pages.
> 
> Given that we already have the page_owner responsible for providing alloc/free stacks for pages, all that we should in KASAN do is to
> enable the feature by default. Free stack saving should be decoupled from debug_pagealloc into separate option so that it can be enabled 
> by KASAN and/or debug_pagealloc.

Thanks your suggestion.
We will send the patch v3 as described above.