linux-kernel - Re: [PATCH 08/14] arm64: simplify access

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAMj1kXGkG0KMD2rnKAJc3V7X9LP1grbcHTNYMnj_q4GiYfG2pQ@mail.gmail.com>
Date:   Tue, 15 Feb 2022 10:21:16 +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 <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>,
        Michal Simek <monstr@...str.eu>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Dinh Nguyen <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>,
        "open list:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@...ts.infradead.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-csky@...r.kernel.org,
        "open list:QUALCOMM HEXAGON..." <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 <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 list <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 Tue, 15 Feb 2022 at 10:13, Arnd Bergmann <arnd@...nel.org> wrote:
>
> On Tue, Feb 15, 2022 at 9:17 AM Ard Biesheuvel <ardb@...nel.org> wrote:
> > On Mon, 14 Feb 2022 at 17:37, Arnd Bergmann <arnd@...nel.org> wrote:
> > > From: Arnd Bergmann <arnd@...db.de>
> > >
> >
> > 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.
>
> That should work, but I don't see it as a clear enough advantage to
> have a custom implementation. For the constant-size case, it probably
> isn't better than a compiler-scheduled comparison against a
> constant limit, but it does hurt maintainability when the next person
> wants to change the behavior of access_ok() globally.
>

arm64 also has this leading up to the range check, and I think we'd no
longer need it:

    if (IS_ENABLED(CONFIG_ARM64_TAGGED_ADDR_ABI) &&
        (current->flags & PF_KTHREAD || test_thread_flag(TIF_TAGGED_ADDR)))
            addr = untagged_addr(addr);

> If we want to get into micro-optimizing uaccess, I think a better target
> would be a CONFIG_CC_HAS_ASM_GOTO_OUTPUT version
> of __get_user()/__put_user as we have on x86 and powerpc.
>
>          Arnd