Re: [PATCH 08/14] arm64: simplify access_ok()

[Ard Biesheuvel <ardb@kernel.org>]

On Mon, 14 Feb 2022 at 17:37, Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> arm64 has an inline asm implementation of access_ok() that is derived from
> the 32-bit arm version and optimized for the case that both the limit and
> the size are variable. With set_fs() gone, the limit is always constant,
> and the size usually is as well, so just using the default implementation
> reduces the check into a comparison against a constant that can be
> scheduled by the compiler.
>
> On a defconfig build, this saves over 28KB of .text.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  arch/arm64/include/asm/uaccess.h | 28 +++++-----------------------
>  1 file changed, 5 insertions(+), 23 deletions(-)
>
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index 357f7bd9c981..e8dce0cc5eaa 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -26,6 +26,8 @@
>  #include <asm/memory.h>
>  #include <asm/extable.h>
>
> +static inline int __access_ok(const void __user *ptr, unsigned long size);
> +
>  /*
>   * Test whether a block of memory is a valid user space address.
>   * Returns 1 if the range is valid, 0 otherwise.
> @@ -33,10 +35,8 @@
>   * This is equivalent to the following test:
>   * (u65)addr + (u65)size <= (u65)TASK_SIZE_MAX
>   */
> -static inline unsigned long __access_ok(const void __user *addr, unsigned long size)
> +static inline int access_ok(const void __user *addr, unsigned long size)
>  {
> -       unsigned long ret, limit = TASK_SIZE_MAX - 1;
> -
>         /*
>          * Asynchronous I/O running in a kernel thread does not have the
>          * TIF_TAGGED_ADDR flag of the process owning the mm, so always untag
> @@ -46,27 +46,9 @@ static inline unsigned long __access_ok(const void __user *addr, unsigned long s
>             (current->flags & PF_KTHREAD || test_thread_flag(TIF_TAGGED_ADDR)))
>                 addr = untagged_addr(addr);
>
> -       __chk_user_ptr(addr);
> -       asm volatile(
> -       // A + B <= C + 1 for all A,B,C, in four easy steps:
> -       // 1: X = A + B; X' = X % 2^64
> -       "       adds    %0, %3, %2\n"
> -       // 2: Set C = 0 if X > 2^64, to guarantee X' > C in step 4
> -       "       csel    %1, xzr, %1, hi\n"
> -       // 3: Set X' = ~0 if X >= 2^64. For X == 2^64, this decrements X'
> -       //    to compensate for the carry flag being set in step 4. For
> -       //    X > 2^64, X' merely has to remain nonzero, which it does.
> -       "       csinv   %0, %0, xzr, cc\n"
> -       // 4: For X < 2^64, this gives us X' - C - 1 <= 0, where the -1
> -       //    comes from the carry in being clear. Otherwise, we are
> -       //    testing X' - C == 0, subject to the previous adjustments.
> -       "       sbcs    xzr, %0, %1\n"
> -       "       cset    %0, ls\n"
> -       : "=&r" (ret), "+r" (limit) : "Ir" (size), "0" (addr) : "cc");
> -
> -       return ret;
> +       return likely(__access_ok(addr, size));
>  }
> -#define __access_ok __access_ok
> +#define access_ok access_ok
>
>  #include <asm-generic/access_ok.h>
>
> --
> 2.29.2
>

With set_fs() out of the picture, wouldn't it be sufficient to check
that bit #55 is clear? (the bit that selects between TTBR0 and TTBR1)
That would also remove the need to strip the tag from the address.

Something like

    asm goto("tbnz  %0, #55, %2     \n"
             "tbnz  %1, #55, %2     \n"
             :: "r"(addr), "r"(addr + size - 1) :: notok);
    return 1;
notok:
    return 0;

with an additional sanity check on the size which the compiler could
eliminate for compile-time constant values.