Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

Andy Lutomirski
2019-10-28T18:03:00Z

On Sun, Oct 27, 2019 at 3:17 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> From: Mike Rapoport <rppt@linux.ibm.com>
>
> The mappings created with MAP_EXCLUSIVE are visible only in the context of
> the owning process and can be used by applications to store secret
> information that will not be visible not only to other processes but to the
> kernel as well.
>
> The pages in these mappings are removed from the kernel direct map and
> marked with PG_user_exclusive flag. When the exclusive area is unmapped,
> the pages are mapped back into the direct map.
>
> The MAP_EXCLUSIVE flag implies MAP_POPULATE and MAP_LOCKED.
>
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
> arch/x86/mm/fault.c | 14 ++++++++++
> fs/proc/task_mmu.c | 1 +
> include/linux/mm.h | 9 +++++++
> include/linux/page-flags.h | 7 +++++
> include/linux/page_excl.h | 49 ++++++++++++++++++++++++++++++++++
> include/trace/events/mmflags.h | 9 ++++++-
> include/uapi/asm-generic/mman-common.h | 1 +
> kernel/fork.c | 3 ++-
> mm/Kconfig | 3 +++
> mm/gup.c | 8 ++++++
> mm/memory.c | 3 +++
> mm/mmap.c | 16 +++++++++++
> mm/page_alloc.c | 5 ++++
> 13 files changed, 126 insertions(+), 2 deletions(-)
> create mode 100644 include/linux/page_excl.h
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 9ceacd1..8f73a75 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -17,6 +17,7 @@
> #include <linux/context_tracking.h> /* exception_enter(), ... */
> #include <linux/uaccess.h> /* faulthandler_disabled() */
> #include <linux/efi.h> /* efi_recover_from_page_fault()*/
> +#include <linux/page_excl.h> /* page_is_user_exclusive() */
> #include <linux/mm_types.h>
>
> #include <asm/cpufeature.h> /* boot_cpu_has, ... */
> @@ -1218,6 +1219,13 @@ static int fault_in_kernel_space(unsigned long address)
> return address >= TASK_SIZE_MAX;
> }
>
> +static bool fault_in_user_exclusive_page(unsigned long address)
> +{
> + struct page *page = virt_to_page(address);
> +
> + return page_is_user_exclusive(page);
> +}
> +
> /*
> * Called for all faults where 'address' is part of the kernel address
> * space. Might get called for faults that originate from *code* that
> @@ -1261,6 +1269,12 @@ do_kern_addr_fault(struct pt_regs *regs, unsigned long hw_error_code,
> if (spurious_kernel_fault(hw_error_code, address))
> return;
>
> + /* FIXME: warn and handle gracefully */
> + if (unlikely(fault_in_user_exclusive_page(address))) {
> + pr_err("page fault in user exclusive page at %lx", address);
> + force_sig_fault(SIGSEGV, SEGV_MAPERR, (void __user *)address);
> + }

Sending a signal here is not a reasonable thing to do in response to
an unexpected kernel fault. You need to OOPS. Printing a nice
message would be nice.

--Andy