linux-kernel - Re: [PATCH v6 2/4] x86/syscalls: Specific usage of verify_pre_usermode

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Date:   Tue, 04 Apr 2017 15:34:30 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Thomas Garnier <thgarnie@...gle.com>
CC:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Rik van Riel <riel@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Russell King <linux@...linux.org.uk>, Will.Deacon@...or.com
Subject: Re: [PATCH v6 2/4] x86/syscalls: Specific usage of verify_pre_usermode_state

<will.deacon@....com>,Catalin Marinas <catalin.marinas@....com>,Mark Rutland <mark.rutland@....com>,James Morse <james.morse@....com>,linux-s390 <linux-s390@...r.kernel.org>,LKML <linux-kernel@...r.kernel.org>,Linux API <linux-api@...r.kernel.org>,the arch/x86 maintainers <x86@...nel.org>,linux-arm-kernel@...ts.infradead.org,Kernel Hardening <kernel-hardening@...ts.openwall.com>
From: hpa@...or.com
Message-ID: <C92745AD-C4D9-441E-854C-985512E5FD8F@...or.com>

On April 4, 2017 12:21:48 PM PDT, Thomas Garnier <thgarnie@...gle.com> wrote:
>On Tue, Apr 4, 2017 at 11:27 AM, H. Peter Anvin <hpa@...or.com> wrote:
>> On 04/04/17 10:47, Thomas Garnier wrote:
>>> diff --git a/arch/x86/include/asm/pgtable_64_types.h
>b/arch/x86/include/asm/pgtable_64_types.h
>>> index 516593e66bd6..12fa851c7fa8 100644
>>> --- a/arch/x86/include/asm/pgtable_64_types.h
>>> +++ b/arch/x86/include/asm/pgtable_64_types.h
>>> @@ -78,4 +78,15 @@ typedef struct { pteval_t pte; } pte_t;
>>>
>>>  #define EARLY_DYNAMIC_PAGE_TABLES    64
>>>
>>> +/*
>>> + * User space process size. 47bits minus one guard page.  The guard
>>> + * page is necessary on Intel CPUs: if a SYSCALL instruction is at
>>> + * the highest possible canonical userspace address, then that
>>> + * syscall will enter the kernel with a non-canonical return
>>> + * address, and SYSRET will explode dangerously.  We avoid this
>>> + * particular problem by preventing anything from being mapped
>>> + * at the maximum canonical address.
>>> + */
>>> +#define TASK_SIZE_MAX        ((_AC(1, UL) << 47) - PAGE_SIZE)
>>> +
>>>  #endif /* _ASM_X86_PGTABLE_64_DEFS_H */
>>> diff --git a/arch/x86/include/asm/processor.h
>b/arch/x86/include/asm/processor.h
>>> index 3cada998a402..e80822582d3e 100644
>>> --- a/arch/x86/include/asm/processor.h
>>> +++ b/arch/x86/include/asm/processor.h
>>> @@ -825,17 +825,6 @@ static inline void spin_lock_prefetch(const
>void *x)
>>>  #define KSTK_ESP(task)               (task_pt_regs(task)->sp)
>>>
>>>  #else
>>> -/*
>>> - * User space process size. 47bits minus one guard page.  The guard
>>> - * page is necessary on Intel CPUs: if a SYSCALL instruction is at
>>> - * the highest possible canonical userspace address, then that
>>> - * syscall will enter the kernel with a non-canonical return
>>> - * address, and SYSRET will explode dangerously.  We avoid this
>>> - * particular problem by preventing anything from being mapped
>>> - * at the maximum canonical address.
>>> - */
>>> -#define TASK_SIZE_MAX        ((1UL << 47) - PAGE_SIZE)
>>> -
>>>  /* This decides where the kernel will search for a free chunk of vm
>>>   * space during mmap's.
>>>   */
>>>
>>
>> This should be an entirely separate patch; if nothing else you need
>to
>> explain it in the comments.
>
>I will explain it in the commit message, it should be easier than a
>separate patch.
>
>>
>> Also, you say this is for "x86", but I still don't see any code for
>i386
>> whatsoever.  Have you verified *all* the i386 and i386-compat paths
>to
>> make sure they go via prepare_exit_to_usermode()?  [Cc: Andy]
>
>I did but I will do it again for the next iteration.
>
>>
>> Finally, I can't really believe I'm the only person for whom
>"Specific
>> usage of verity_pre_usermode_state" is completely opaque.
>
>I agree, I will improve it.
>
>>
>>         -hpa
>>

Easier for you, perhaps, but not for everyone else...
-- 
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