Re: [kernel-hardening] [PATCH v6 10/11] mm: add a user_virt_to_phys symbol

Mark Rutland
2017-09-14T18:34:06Z

On Thu, Sep 07, 2017 at 11:36:08AM -0600, Tycho Andersen wrote:
> We need someting like this for testing XPFO. Since it's architecture
> specific, putting it in the test code is slightly awkward, so let's make it
> an arch-specific symbol and export it for use in LKDTM.
>
> v6: * add a definition of user_virt_to_phys in the !CONFIG_XPFO case
>
> CC: linux-arm-kernel@lists.infradead.org
> CC: x86@kernel.org
> Signed-off-by: Tycho Andersen <tycho@docker.com>
> Tested-by: Marco Benatto <marco.antonio.780@gmail.com>
> ---
> arch/arm64/mm/xpfo.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++
> arch/x86/mm/xpfo.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> include/linux/xpfo.h | 5 +++++
> 3 files changed, 113 insertions(+)
>
> diff --git a/arch/arm64/mm/xpfo.c b/arch/arm64/mm/xpfo.c
> index 342a9ccb93c1..94a667d94e15 100644
> --- a/arch/arm64/mm/xpfo.c
> +++ b/arch/arm64/mm/xpfo.c
> @@ -74,3 +74,54 @@ void xpfo_dma_map_unmap_area(bool map, const void *addr, size_t size,
>
> xpfo_temp_unmap(addr, size, mapping, sizeof(mapping[0]) * num_pages);
> }
> +
> +/* Convert a user space virtual address to a physical address.
> + * Shamelessly copied from slow_virt_to_phys() and lookup_address() in
> + * arch/x86/mm/pageattr.c
> + */

When can this be called? What prevents concurrent modification of the user page
tables?

i.e. must mmap_sem be held?

> +phys_addr_t user_virt_to_phys(unsigned long addr)

Does this really need to be architecture specific?

Core mm code manages to walk user page tables just fine...

> +{
> + phys_addr_t phys_addr;
> + unsigned long offset;
> + pgd_t *pgd;
> + p4d_t *p4d;
> + pud_t *pud;
> + pmd_t *pmd;
> + pte_t *pte;
> +
> + pgd = pgd_offset(current->mm, addr);
> + if (pgd_none(*pgd))
> + return 0;

Can we please separate the address and return value? e.g. pass the PA by
reference and return an error code.

AFAIK, zero is a valid PA, and even if the tables exist, they might point there
in the presence of an error.

> +
> + p4d = p4d_offset(pgd, addr);
> + if (p4d_none(*p4d))
> + return 0;
> +
> + pud = pud_offset(p4d, addr);
> + if (pud_none(*pud))
> + return 0;
> +
> + if (pud_sect(*pud) || !pud_present(*pud)) {
> + phys_addr = (unsigned long)pud_pfn(*pud) << PAGE_SHIFT;

Was there some problem with:

phys_addr = pmd_page_paddr(*pud);

... and similar for the other levels?

... I'd rather introduce new helpers than more open-coded calculations.

Thanks,
Mark.