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

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Aug 09, 2024 at 11:33:06AM +0200, Michal Hocko wrote:
> On Fri 09-08-24 09:05:05, Barry Song wrote:
> > On Fri, Aug 9, 2024 at 12:20 AM Hailong Liu <hailong.liu@oppo.com> 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@oppo.com>
> > > Reported-by: Tangquan Zheng <zhengtangquan@oppo.com>
> > > Cc: <stable@vger.kernel.org>
> > > CC: Barry Song <21cnbao@gmail.com>
> > > CC: Baoquan He <bhe@redhat.com>
> > > CC: Matthew Wilcox <willy@infradead.org>
> > > ---
> > 
> > Acked-by: Barry Song <baohua@kernel.org>
> > 
> > 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@suse.com>
> 
I agree. This is only clear for people who know the code. A "fallback"
to order-0 should be commented.

--
Uladzislau Rezki