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Message-ID: <CA+8MBb+H0FqciBw9nSO9L0fNQtiRvc_1TREitH9z89YxhtyFAQ@mail.gmail.com>
Date: Fri, 12 Jan 2018 16:15:12 -0800
From: Tony Luck <tony.luck@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Dan Williams <dan.j.williams@...el.com>,
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Subject: Re: [PATCH v2 00/19] prevent bounds-check bypass via speculative execution
On Thu, Jan 11, 2018 at 5:19 PM, Linus Torvalds
<torvalds@...ux-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
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