Re: [PATCH] mm/usercopy: Use memory range to be accessed for wraparound check

[Kees Cook <keescook@chromium.org>]

On Tue, Nov 13, 2018 at 6:51 PM, Isaac J. Manjarres
<isaacm@codeaurora.org> wrote:
> Currently, when checking to see if accessing n bytes starting at
> address "ptr" will cause a wraparound in the memory addresses,
> the check in check_bogus_address() adds an extra byte, which is
> incorrect, as the range of addresses that will be accessed is
> [ptr, ptr + (n - 1)].
>
> This can lead to incorrectly detecting a wraparound in the
> memory address, when trying to read 4 KB from memory that is
> mapped to the the last possible page in the virtual address
> space, when in fact, accessing that range of memory would not
> cause a wraparound to occur.

I'm kind of surprised anything is using the -4K memory range -- this
is ERR_PTR() area and I'd expect there to be an explicit unallocated
memory hole here.

>
> Use the memory range that will actually be accessed when
> considering if accessing a certain amount of bytes will cause
> the memory address to wrap around.
>
> Change-Id: I2563a5988e41122727ede17180f365e999b953e6
> Fixes: f5509cc18daa ("mm: Hardened usercopy")
> Co-Developed-by: Prasad Sodagudi <psodagud@codeaurora.org>
> Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>
> Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
> Cc: stable@vger.kernel.org

Regardless, I'll take it in my tree if akpm doesn't grab it first. :)

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  mm/usercopy.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index 852eb4e..0293645 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -151,7 +151,7 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
>                                        bool to_user)
>  {
>         /* Reject if object wraps past end of memory. */
> -       if (ptr + n < ptr)
> +       if (ptr + (n - 1) < ptr)
>                 usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
>
>         /* Reject if NULL or ZERO-allocation. */
> --
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
>



-- 
Kees Cook