linux-kernel - Re: [PATCHv3] x86/mm: set x32 syscall bit in SET

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Message-ID: <D1FD1484-E113-4B53-8ED5-E5B34BAECC53@zytor.com>
Date:   Tue, 21 Mar 2017 14:23:51 -0700
From:   hpa@...or.com
To:     Adam Borowski <kilobyte@...band.pl>,
        Dmitry Safonov <dsafonov@...tuozzo.com>
CC:     linux-kernel@...r.kernel.org, 0x7f454c46@...il.com,
        linux-mm@...ck.org, Andrei Vagin <avagin@...il.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        x86@...nel.org, Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCHv3] x86/mm: set x32 syscall bit in SET_PERSONALITY()

On March 21, 2017 2:16:48 PM PDT, Adam Borowski <kilobyte@...band.pl> wrote:
>On Tue, Mar 21, 2017 at 08:47:11PM +0300, Dmitry Safonov wrote:
>> After my changes to mmap(), its code now relies on the bitness of
>> performing syscall. According to that, it chooses the base of
>allocation:
>> mmap_base for 64-bit mmap() and mmap_compat_base for 32-bit syscall.
>> It was done by:
>>   commit 1b028f784e8c ("x86/mm: Introduce mmap_compat_base() for
>> 32-bit mmap()").
>> 
>> The code afterwards relies on in_compat_syscall() returning true for
>> 32-bit syscalls. It's usually so while we're in context of
>application
>> that does 32-bit syscalls. But during exec() it is not valid for x32
>ELF.
>> The reason is that the application hasn't yet done any syscall, so
>x32
>> bit has not being set.
>> That results in -ENOMEM for x32 ELF files as there fired BAD_ADDR()
>> in elf_map(), that is called from do_execve()->load_elf_binary().
>> For i386 ELFs it works as SET_PERSONALITY() sets TS_COMPAT flag.
>> 
>> Set x32 bit before first return to userspace, during setting
>personality
>> at exec(). This way we can rely on in_compat_syscall() during exec().
>> Do also the reverse: drop x32 syscall bit at SET_PERSONALITY for
>64-bits.
>> 
>> Fixes: commit 1b028f784e8c ("x86/mm: Introduce mmap_compat_base() for
>> 32-bit mmap()")
>
>Tested:
>with bash:x32, mksh:amd64, posh:i386, zsh:armhf (binfmt:qemu),
>fork+exec
>works for every parent-child combination.
>
>Contrary to my naive initial reading of your fix, mixing syscalls from
>a
>process of the wrong ABI also works as it did before.  While using a
>glibc
>wrapper will call the right version, x32 processes calling amd64
>syscalls is
>surprisingly common -- this brings seccomp joy.
>
>I've attached a freestanding test case for write() and mmap(); it's
>freestanding asm as most of you don't have an x32 toolchain at hand,
>sorry
>for unfriendly error messages.
>
>So with these two patches:
>x86/tls: Forcibly set the accessed bit in TLS segments
>x86/mm: set x32 syscall bit in SET_PERSONALITY()
>everything appears to be fine.

What userspace is that?  Is this syscall(3) (ab)users or incorrectly ported to x32 software?
-- 
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