Re: [PATCH 4.4 20/43] mm/vmalloc: add interfaces to free unmapped page table

[Nathan Chancellor]

On Tue, Mar 27, 2018 at 03:17:00PM -0500, Dan Rue wrote:
> On Tue, Mar 27, 2018 at 06:27:24PM +0200, Greg Kroah-Hartman wrote:
> > 4.4-stable review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Toshi Kani <toshi.kani@xxxxxxx>
> > 
> > commit b6bdb7517c3d3f41f20e5c2948d6bc3f8897394e upstream.
> > 
> > On architectures with CONFIG_HAVE_ARCH_HUGE_VMAP set, ioremap() may
> > create pud/pmd mappings.  A kernel panic was observed on arm64 systems
> > with Cortex-A75 in the following steps as described by Hanjun Guo.
> > 
> >  1. ioremap a 4K size, valid page table will build,
> >  2. iounmap it, pte0 will set to 0;
> >  3. ioremap the same address with 2M size, pgd/pmd is unchanged,
> >     then set the a new value for pmd;
> >  4. pte0 is leaked;
> >  5. CPU may meet exception because the old pmd is still in TLB,
> >     which will lead to kernel panic.
> > 
> > This panic is not reproducible on x86.  INVLPG, called from iounmap,
> > purges all levels of entries associated with purged address on x86.  x86
> > still has memory leak.
> > 
> > The patch changes the ioremap path to free unmapped page table(s) since
> > doing so in the unmap path has the following issues:
> > 
> >  - The iounmap() path is shared with vunmap(). Since vmap() only
> >    supports pte mappings, making vunmap() to free a pte page is an
> >    overhead for regular vmap users as they do not need a pte page freed
> >    up.
> > 
> >  - Checking if all entries in a pte page are cleared in the unmap path
> >    is racy, and serializing this check is expensive.
> > 
> >  - The unmap path calls free_vmap_area_noflush() to do lazy TLB purges.
> >    Clearing a pud/pmd entry before the lazy TLB purges needs extra TLB
> >    purge.
> > 
> > Add two interfaces, pud_free_pmd_page() and pmd_free_pte_page(), which
> > clear a given pud/pmd entry and free up a page for the lower level
> > entries.
> > 
> > This patch implements their stub functions on x86 and arm64, which work
> > as workaround.
> > 
> > [akpm@xxxxxxxxxxxxxxxxxxxx: fix typo in pmd_free_pte_page() stub]
> > Link: http://lkml.kernel.org/r/20180314180155.19492-2-toshi.kani@xxxxxxx
> > Fixes: e61ce6ade404e ("mm: change ioremap to set up huge I/O mappings")
> > Reported-by: Lei Li <lious.lilei@xxxxxxxxxxxxx>
> > Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Wang Xuefeng <wxf.wang@xxxxxxxxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Hanjun Guo <guohanjun@xxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxx>
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Cc: Chintan Pandya <cpandya@xxxxxxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> 
> This patch causes the following build error on 4.4 arm64:
> 
> $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- O=build-arm64 defconfig
> $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- O=build-arm64
> 
>   CC      arch/arm64/mm/mmu.o
> ../arch/arm64/mm/mmu.c:701:5: error: redefinition of âpud_free_pmd_pageâ
>  int pud_free_pmd_page(pud_t *pud)
>      ^~~~~~~~~~~~~~~~~
> In file included from ../arch/arm64/include/asm/pgtable.h:682:0,
>                  from ../include/linux/mm.h:55,
>                  from ../include/linux/mman.h:4,
>                  from ../arch/arm64/mm/mmu.c:25:
> ../include/asm-generic/pgtable.h:777:19: note: previous definition of âpud_free_pmd_pageâ was here
>  static inline int pud_free_pmd_page(pud_t *pud)
>                    ^~~~~~~~~~~~~~~~~
> ../arch/arm64/mm/mmu.c:706:5: error: redefinition of âpmd_free_pte_pageâ
>  int pmd_free_pte_page(pmd_t *pmd)
>      ^~~~~~~~~~~~~~~~~
> In file included from ../arch/arm64/include/asm/pgtable.h:682:0,
>                  from ../include/linux/mm.h:55,
>                  from ../include/linux/mman.h:4,
>                  from ../arch/arm64/mm/mmu.c:25:
> ../include/asm-generic/pgtable.h:781:19: note: previous definition of âpmd_free_pte_pageâ was here
>  static inline int pmd_free_pte_page(pmd_t *pmd)
>                    ^~~~~~~~~~~~~~~~~
> make[2]: *** [../scripts/Makefile.build:270: arch/arm64/mm/mmu.o] Error 1
> make[1]: *** [/home/drue/src/linux/4.4-rc/Makefile:969: arch/arm64/mm] Error 2
> make[1]: Leaving directory '/home/drue/src/linux/4.4-rc/build-arm64'
> make: *** [Makefile:152: sub-make] Error 2
> 
> 

Both of my arm64 devices built fine with this patch... It seems like
the only way to hit that error is if HAVE_ARCH_HUGE_VMAP isn't set,
which seems impossible since it is selected by ARM64...

Someone smarter than I might have more insight but this patch is
unchanged from upstream so I can only assume that this error would
manifest there as well.

Cheers!
Nathan