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From: Tony Luck <tony.luck@gmail.com>
Date: Fri, 12 Jan 2018 16:15:12 -0800
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Subject: Re: [PATCH v2 00/19] prevent bounds-check bypass via speculative execution
To: Linus Torvalds <torvalds@linux-foundation.org>
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On Thu, Jan 11, 2018 at 5:19 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> Should the array access in entry_SYSCALL_64_fastpath be made to use
> the masking approach?

That one has a bounds check for an inline constant.

     cmpq    $__NR_syscall_max, %rax

so should be safe.

The classic Spectre variant #1 code sequence is:

int array_size;

       if (x < array_size) {
               something with array[x]
       }

which runs into problems because the array_size variable may not
be in cache, and while the CPU core is waiting for the value it
speculates inside the "if" body.

The syscall entry is more like:

#define ARRAY_SIZE 10

     if (x < ARRAY_SIZE) {
          something with array[x]
     }

Here there isn't any reason for speculation. The core has the
value of 'x' in a register and the upper bound encoded into the
"cmp" instruction.  Both are right there, no waiting, no speculation.

-Tony