Re: [PATCH V3 3/4] arm64/mm: Inhibit huge-vmap with ptdump
From: David Hildenbrand
Date: Tue May 14 2019 - 05:22:08 EST
On 14.05.19 11:00, Anshuman Khandual wrote:
> From: Mark Rutland <mark.rutland@xxxxxxx>
> The arm64 ptdump code can race with concurrent modification of the
> kernel page tables. At the time this was added, this was sound as:
> * Modifications to leaf entries could result in stale information being
> logged, but would not result in a functional problem.
> * Boot time modifications to non-leaf entries (e.g. freeing of initmem)
> were performed when the ptdump code cannot be invoked.
> * At runtime, modifications to non-leaf entries only occurred in the
> vmalloc region, and these were strictly additive, as intermediate
> entries were never freed.
> However, since commit:
> commit 324420bf91f6 ("arm64: add support for ioremap() block mappings")
> ... it has been possible to create huge mappings in the vmalloc area at
> runtime, and as part of this existing intermediate levels of table my be
> removed and freed.
> It's possible for the ptdump code to race with this, and continue to
> walk tables which have been freed (and potentially poisoned or
> reallocated). As a result of this, the ptdump code may dereference bogus
> addresses, which could be fatal.
> Since huge-vmap is a TLB and memory optimization, we can disable it when
> the runtime ptdump code is in use to avoid this problem.
I wonder if there is another way to protect from such a race happening.
(IOW, a lock). But as you say, it's a debug feature, so this is an easy fix.
Looks good to me (with limited arm64 code insight :) )
> Fixes: 324420bf91f60582 ("arm64: add support for ioremap() block mappings")
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> arch/arm64/mm/mmu.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index ef82312..37a902c 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -955,13 +955,18 @@ void *__init fixmap_remap_fdt(phys_addr_t dt_phys)
> int __init arch_ioremap_pud_supported(void)
> - /* only 4k granule supports level 1 block mappings */
> - return IS_ENABLED(CONFIG_ARM64_4K_PAGES);
> + /*
> + * Only 4k granule supports level 1 block mappings.
> + * SW table walks can't handle removal of intermediate entries.
> + */
> + return IS_ENABLED(CONFIG_ARM64_4K_PAGES) &&
> + !IS_ENABLED(CONFIG_ARM64_PTDUMP_DEBUGFS);
> int __init arch_ioremap_pmd_supported(void)
> - return 1;
> + /* See arch_ioremap_pud_supported() */
> + return !IS_ENABLED(CONFIG_ARM64_PTDUMP_DEBUGFS);
> int pud_set_huge(pud_t *pudp, phys_addr_t phys, pgprot_t prot)
David / dhildenb