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Date: Tue, 4 Apr 2017 11:27:07 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Thomas Garnier <thgarnie@...gle.com>,
        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 <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>, James Morse <james.morse@....com>
Cc: linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        kernel-hardening@...ts.openwall.com, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v6 2/4] x86/syscalls: Specific usage of
 verify_pre_usermode_state

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.

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]

Finally, I can't really believe I'm the only person for whom "Specific
usage of verity_pre_usermode_state" is completely opaque.

	-hpa

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