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Date:   Tue, 15 Feb 2022 09:17:48 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-api@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        Brian Cain <bcain@...eaurora.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>, monstr@...str.eu,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>, dinguyen@...nel.org,
        Stafford Horne <shorne@...il.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Richard Weinberger <richard@....at>, X86 ML <x86@...nel.org>,
        Max Filippov <jcmvbkbc@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        alpha <linux-alpha@...r.kernel.org>,
        linux-snps-arc@...ts.infradead.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-csky@...r.kernel.org, linux-hexagon@...r.kernel.org,
        linux-ia64@...r.kernel.org,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        openrisc@...ts.librecores.org,
        "open list:PARISC ARCHITECTURE" <linux-parisc@...r.kernel.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "open list:S390" <linux-s390@...r.kernel.org>,
        linux-sh@...r.kernel.org,
        "open list:SPARC + UltraSPARC (sparc/sparc64)" 
        <sparclinux@...r.kernel.org>,
        linux-um <linux-um@...ts.infradead.org>,
        "open list:TENSILICA XTENSA PORT (xtensa)" 
        <linux-xtensa@...ux-xtensa.org>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 08/14] arm64: simplify access_ok()

On Mon, 14 Feb 2022 at 17:37, Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.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@...db.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.

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