Re: [RESEND PATCH v1] mm/vmalloc: fix page mapping if vm_area_alloc_pages() with high order fallback to order 0

[Michal Hocko]

On Fri 09-08-24 09:05:05, Barry Song wrote:
> On Fri, Aug 9, 2024 at 12:20 AM Hailong Liu <hailong.liu@xxxxxxxx> wrote:
> >
> > The __vmap_pages_range_noflush() assumes its argument pages** contains
> > pages with the same page shift. However, since commit e9c3cda4d86e
> > ("mm, vmalloc: fix high order __GFP_NOFAIL allocations"), if gfp_flags
> > includes __GFP_NOFAIL with high order in vm_area_alloc_pages()
> > and page allocation failed for high order, the pages** may contain
> > two different page shifts (high order and order-0). This could
> > lead __vmap_pages_range_noflush() to perform incorrect mappings,
> > potentially resulting in memory corruption.
> >
> > Users might encounter this as follows (vmap_allow_huge = true, 2M is for PMD_SIZE):
> > kvmalloc(2M, __GFP_NOFAIL|GFP_X)
> >     __vmalloc_node_range_noprof(vm_flags=VM_ALLOW_HUGE_VMAP)
> >         vm_area_alloc_pages(order=9) ---> order-9 allocation failed and fallback to order-0
> >             vmap_pages_range()
> >                 vmap_pages_range_noflush()
> >                     __vmap_pages_range_noflush(page_shift = 21) ----> wrong mapping happens
> >
> > We can remove the fallback code because if a high-order
> > allocation fails, __vmalloc_node_range_noprof() will retry with
> > order-0. Therefore, it is unnecessary to fallback to order-0
> > here. Therefore, fix this by removing the fallback code.
> >
> > Fixes: e9c3cda4d86e ("mm, vmalloc: fix high order __GFP_NOFAIL allocations")
> > Signed-off-by: Hailong Liu <hailong.liu@xxxxxxxx>
> > Reported-by: Tangquan Zheng <zhengtangquan@xxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > CC: Barry Song <21cnbao@xxxxxxxxx>
> > CC: Baoquan He <bhe@xxxxxxxxxx>
> > CC: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > ---
> 
> Acked-by: Barry Song <baohua@xxxxxxxxxx>
> 
> because we already have a fallback here:
> 
> void *__vmalloc_node_range_noprof :
> 
> fail:
>         if (shift > PAGE_SHIFT) {
>                 shift = PAGE_SHIFT;
>                 align = real_align;
>                 size = real_size;
>                 goto again;
>         }

This really deserves a comment because this is not really clear at all.
The code is also fragile and it would benefit from some re-org.

Thanks for the fix.

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

-- 
Michal Hocko
SUSE Labs