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Re: [PATCH v6 2/4] x86/syscalls: Specific usage of verify_pre_usermode_state

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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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[PATCH v6 2/4] x86/syscalls: Specific usage of verify_pre_usermode_state Thomas Garnier <thgarnie@google.com> - 2017-04-04 19:50 +0200
  Re: [PATCH v6 2/4] x86/syscalls: Specific usage of  verify_pre_usermode_state "H. Peter Anvin" <hpa@zytor.com> - 2017-04-04 20:40 +0200
  Re: [PATCH v6 2/4] x86/syscalls: Specific usage of  verify_pre_usermode_state "H. Peter Anvin" <hpa@zytor.com> - 2017-04-04 20:50 +0200
    Re: [PATCH v6 2/4] x86/syscalls: Specific usage of  verify_pre_usermode_state Borislav Petkov <bp@alien8.de> - 2017-04-04 21:00 +0200
    Re: [PATCH v6 2/4] x86/syscalls: Specific usage of verify_pre_usermode_state Thomas Garnier <thgarnie@google.com> - 2017-04-04 21:30 +0200
      Re: [PATCH v6 2/4] x86/syscalls: Specific usage of verify_pre_usermode_state "H. Peter Anvin" <hpa@zytor.com> - 2017-04-05 01:00 +0200

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