On Tue, Jul 30, 2019 at 10:54:13AM -0700, Isaac J. Manjarres 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.
>
> 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.
>
> Fixes: f5509cc18daa ("mm: Hardened usercopy")
> Co-developed-by: Prasad Sodagudi <[email protected]>
> Signed-off-by: Prasad Sodagudi <[email protected]>
> Signed-off-by: Isaac J. Manjarres <[email protected]>
> Cc: [email protected]
> Reviewed-by: William Kucharski <[email protected]>
> Acked-by: Kees Cook <[email protected]>
Ah, thanks for the reminder! (I got surprised by seeing my Ack in this
email -- next time please use "v2" or "RESEND" to jog my memory.) This
got lost last year; my bad.
Andrew, can you take this or should I send it directly to Linus?
-Kees
> ---
> mm/usercopy.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index 2a09796..98e92486 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -147,7 +147,7 @@ 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
