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

[Vlastimil Babka <vbabka@suse.cz>]

On 9/9/19 10:24 AM, walter-zh.wu@mediatek.com wrote:
> From: Walter Wu <walter-zh.wu@mediatek.com>
> 
> 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.

> Changes since v1:
> - slim page_owner and move it into kasan
> - enable the feature by default
> 
> Signed-off-by: Walter Wu <walter-zh.wu@mediatek.com>
> ---
>   include/linux/kasan.h |  1 +
>   lib/Kconfig.kasan     |  2 ++
>   mm/kasan/common.c     | 32 ++++++++++++++++++++++++++++++++
>   mm/kasan/kasan.h      |  5 +++++
>   mm/kasan/report.c     | 27 +++++++++++++++++++++++++++
>   5 files changed, 67 insertions(+)