On Mon, Oct 04, 2021 at 11:42:21PM +0100, Matthew Wilcox (Oracle) wrote:
> If you are copying to an address in the kmap region, you may not copy
> across a page boundary, no matter what the size of the underlying
> allocation. You can't kmap() a slab page because slab pages always
> come from low memory.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
> arch/x86/include/asm/highmem.h | 1 +
> include/linux/highmem-internal.h | 10 ++++++++++
> mm/usercopy.c | 15 +++++++++------
> 3 files changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/highmem.h b/arch/x86/include/asm/highmem.h
> index 032e020853aa..731ee7cc40a5 100644
> --- a/arch/x86/include/asm/highmem.h
> +++ b/arch/x86/include/asm/highmem.h
> @@ -26,6 +26,7 @@
> #include <asm/tlbflush.h>
> #include <asm/paravirt.h>
> #include <asm/fixmap.h>
> +#include <asm/pgtable_areas.h>
>
> /* declarations for highmem.c */
> extern unsigned long highstart_pfn, highend_pfn;
> diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
> index 4aa1031d3e4c..97d6dc836749 100644
> --- a/include/linux/highmem-internal.h
> +++ b/include/linux/highmem-internal.h
> @@ -143,6 +143,11 @@ static inline void totalhigh_pages_add(long count)
> atomic_long_add(count, &_totalhigh_pages);
> }
>
> +static inline bool is_kmap_addr(const void *x)
> +{
> + unsigned long addr = (unsigned long)x;
> + return addr >= PKMAP_ADDR(0) && addr < PKMAP_ADDR(LAST_PKMAP);
> +}
> #else /* CONFIG_HIGHMEM */
>
> static inline struct page *kmap_to_page(void *addr)
> @@ -223,6 +228,11 @@ static inline void __kunmap_atomic(void *addr)
> static inline unsigned int nr_free_highpages(void) { return 0; }
> static inline unsigned long totalhigh_pages(void) { return 0UL; }
>
> +static inline bool is_kmap_addr(const void *x)
> +{
> + return false;
> +}
> +
> #endif /* CONFIG_HIGHMEM */
>
> /*
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index b3de3c4eefba..ac95b22fbbce 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -228,12 +228,15 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
> if (!virt_addr_valid(ptr))
> return;
>
> - /*
> - * When CONFIG_HIGHMEM=y, kmap_to_page() will give either the
> - * highmem page or fallback to virt_to_page(). The following
> - * is effectively a highmem-aware virt_to_head_page().
> - */
> - page = compound_head(kmap_to_page((void *)ptr));
> + if (is_kmap_addr(ptr)) {
> + unsigned long page_end = (unsigned long)ptr | (PAGE_SIZE - 1);
> +
> + if ((unsigned long)ptr + n - 1 > page_end)
> + usercopy_abort("kmap", NULL, to_user, 0, n);
It's likely not worth getting an offset here, but "0" above could be
something like "ptr - PKMAP_ADDR(0)".
Either way:
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Thanks!
-Kees
> + return;
> + }
> +
> + page = virt_to_head_page(ptr);
>
> if (PageSlab(page)) {
> /* Check slab allocator for flags and size. */
> --
> 2.32.0
>
--
Kees Cook
