Re: [PATCH] mm: page_vma_mapped: ensure pmd is loaded with READ_ONCE outside of lock

[Greg KH]

On Tue, Oct 17, 2017 at 12:15:17PM +0100, Will Deacon wrote:
> [Commit a7b100953aa33a5bbdc3e5e7f2241b9c0704606e upstream]
> 
> Loading the pmd without holding the pmd_lock exposes us to races with
> concurrent updaters of the page tables but, worse still, it also allows
> the compiler to cache the pmd value in a register and reuse it later on,
> even if we've performed a READ_ONCE in between and seen a more recent
> value.
> 
> In the case of page_vma_mapped_walk, this leads to the following crash
> when the pmd loaded for the initial pmd_trans_huge check is all zeroes
> and a subsequent valid table entry is loaded by check_pmd.  We then
> proceed into map_pte, but the compiler re-uses the zero entry inside
> pte_offset_map, resulting in a junk pointer being installed in
> pvmw->pte:
> 
>   PC is at check_pte+0x20/0x170
>   LR is at page_vma_mapped_walk+0x2e0/0x540
>   [...]
>   Process doio (pid: 2463, stack limit = 0xffff00000f2e8000)
>   Call trace:
>     check_pte+0x20/0x170
>     page_vma_mapped_walk+0x2e0/0x540
>     page_mkclean_one+0xac/0x278
>     rmap_walk_file+0xf0/0x238
>     rmap_walk+0x64/0xa0
>     page_mkclean+0x90/0xa8
>     clear_page_dirty_for_io+0x84/0x2a8
>     mpage_submit_page+0x34/0x98
>     mpage_process_page_bufs+0x164/0x170
>     mpage_prepare_extent_to_map+0x134/0x2b8
>     ext4_writepages+0x484/0xe30
>     do_writepages+0x44/0xe8
>     __filemap_fdatawrite_range+0xbc/0x110
>     file_write_and_wait_range+0x48/0xd8
>     ext4_sync_file+0x80/0x4b8
>     vfs_fsync_range+0x64/0xc0
>     SyS_msync+0x194/0x1e8
> 
> This patch fixes the problem by ensuring that READ_ONCE is used before
> the initial checks on the pmd, and this value is subsequently used when
> checking whether or not the pmd is present.  pmd_check is removed and
> the pmd_present check is inlined directly.
> 
> Link: http://lkml.kernel.org/r/1507222630-5839-1-git-send-email-will.deacon@xxxxxxx
> Fixes: f27176cfc363 ("mm: convert page_mkclean_one() to use page_vma_mapped_walk()")
> Cc: <stable@xxxxxxxxxxxxxxx> # 4.13
> [will: backport to 4.13.y]
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>

Thanks for the backport!

greg k-h