On Thu, Jan 18, 2018 at 5:18 AM, Will Deacon [off-list ref] wrote:

Hi Dan, Linus,

On Thu, Jan 11, 2018 at 05:41:08PM -0800, Dan Williams wrote:

quoted

On Thu, Jan 11, 2018 at 5:19 PM, Linus Torvalds
[off-list ref] wrote:

quoted

On Thu, Jan 11, 2018 at 4:46 PM, Dan Williams [off-list ref] wrote:

quoted

This series incorporates Mark Rutland's latest ARM changes and adds
the x86 specific implementation of 'ifence_array_ptr'. That ifence
based approach is provided as an opt-in fallback, but the default
mitigation, '__array_ptr', uses a 'mask' approach that removes
conditional branches instructions, and otherwise aims to redirect
speculation to use a NULL pointer rather than a user controlled value.

Do you have any performance numbers and perhaps example code
generation? Is this noticeable? Are there any microbenchmarks showing
the difference between lfence use and the masking model?

I don't have performance numbers, but here's a sample code generation
from __fcheck_files, where the 'and; lea; and' sequence is portion of
array_ptr() after the mask generation with 'sbb'.

        fdp = array_ptr(fdt->fd, fd, fdt->max_fds);
     8e7:       8b 02                   mov    (%rdx),%eax
     8e9:       48 39 c7                cmp    %rax,%rdi
     8ec:       48 19 c9                sbb    %rcx,%rcx
     8ef:       48 8b 42 08             mov    0x8(%rdx),%rax
     8f3:       48 89 fe                mov    %rdi,%rsi
     8f6:       48 21 ce                and    %rcx,%rsi
     8f9:       48 8d 04 f0             lea    (%rax,%rsi,8),%rax
     8fd:       48 21 c8                and    %rcx,%rax

quoted

Having both seems good for testing, but wouldn't we want to pick one in the end?

I was thinking we'd keep it as a 'just in case' sort of thing, at
least until the 'probably safe' assumption of the 'mask' approach has
more time to settle out.

From the arm64 side, the only concern I have (and this actually applies to
our CSDB sequence as well) is the calculation of the array size by the
caller. As Linus mentioned at the end of [1], if the determination of the
size argument is based on a conditional branch, then masking doesn't help
because you bound within the wrong range under speculation.

We ran into this when trying to use masking to protect our uaccess routines
where the conditional bound is either KERNEL_DS or USER_DS. It's possible
that a prior conditional set_fs(KERNEL_DS) could defeat the masking and so
we'd need to throw some heavy barriers in set_fs to make it robust.

At least in the conditional mask case near set_fs() usage the approach
we are taking is to use a barrier. I.e. the following guidance from
Linus:

"Basically, the rule is trivial: find all 'stac' users, and use address
masking if those users already integrate the limit check, and lfence
they don't."

...which translates to narrow the pointer for get_user() and use a
barrier  for __get_user().