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SubjectRe: [PATCH 4.9 57/83] mm: prevent get_user_pages() from overflowing page refcount
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On 6/9/19 6:42 PM, Greg Kroah-Hartman wrote:
> From: Linus Torvalds <torvalds@linux-foundation.org>
>
> commit 8fde12ca79aff9b5ba951fce1a2641901b8d8e64 upstream.
>
> If the page refcount wraps around past zero, it will be freed while
> there are still four billion references to it. One of the possible
> avenues for an attacker to try to make this happen is by doing direct IO
> on a page multiple times. This patch makes get_user_pages() refuse to
> take a new page reference if there are already more than two billion
> references to the page.
>
> Reported-by: Jann Horn <jannh@google.com>
> Acked-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> [bwh: Backported to 4.9:
> - Add the "err" variable in follow_hugetlb_page()
> - Adjust context]
> Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
> mm/gup.c | 45 ++++++++++++++++++++++++++++++++++-----------
> mm/hugetlb.c | 16 +++++++++++++++-
> 2 files changed, 49 insertions(+), 12 deletions(-)
>

...

> @@ -1231,6 +1240,20 @@ struct page *get_dump_page(unsigned long
> */
> #ifdef CONFIG_HAVE_GENERIC_RCU_GUP
>
> +/*
> + * Return the compund head page with ref appropriately incremented,
> + * or NULL if that failed.
> + */
> +static inline struct page *try_get_compound_head(struct page *page, int refs)
> +{
> + struct page *head = compound_head(page);
> + if (WARN_ON_ONCE(page_ref_count(head) < 0))
> + return NULL;
> + if (unlikely(!page_cache_add_speculative(head, refs)))
> + return NULL;
> + return head;
> +}
> +
> #ifdef __HAVE_ARCH_PTE_SPECIAL
> static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> int write, struct page **pages, int *nr)
> @@ -1263,9 +1286,9 @@ static int gup_pte_range(pmd_t pmd, unsi
>
> VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
> page = pte_page(pte);
> - head = compound_head(page);
>
> - if (!page_cache_get_speculative(head))
> + head = try_get_compound_head(page, 1);

BTW, several arches in 4.9, including x86, have arch-specific fast gup
implementation, which is not touched by this backport. Didn't check if
Jann's exploit ends up using the fast on non-fast one, though.

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