Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read under speculation

[Dan Williams]

On Tue, Feb 6, 2018 at 12:58 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Feb 6, 2018 at 12:49 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>
>> Can you use @cc to make an asm statement that outputs both the masked
>> array index and the "if" condition?  I can never remember the syntax,
>> but something like:
>
> Yes. Although I'd actually suggest just using an "asm goto" if we
> really want to optimize this. Give the "index_mask_nospec()" a third
> argument that is the label to jump to for overflow.
>
> Then you can just decide how to implement it best for any particular
> architecture (and compiler limitation).

At that point we're basically just back to the array_ptr() version
that returned a sanitized pointer to an array element.

        call = array_ptr(sys_call_table, nr & __SYSCALL_MASK, NR_syscalls);
        if (likely(call))
                regs->ax = (*call)(
                        regs->di, regs->si, regs->dx,
                        regs->r10, regs->r8, regs->r9);


     e1e:       ba 4d 01 00 00          mov    $0x14d,%edx
     e23:       48 39 d5                cmp    %rdx,%rbp
     e26:       48 19 d2                sbb    %rdx,%rdx
        call = array_ptr(sys_call_table, nr & __SYSCALL_MASK, NR_syscalls);
     e29:       48 21 d5                and    %rdx,%rbp
     e2c:       48 8d 04 ed 00 00 00    lea    0x0(,%rbp,8),%rax
     e33:       00
        if (likely(call))
     e34:       48 21 d0                and    %rdx,%rax
     e37:       74 1e                   je     e57 <do_syscall_64+0x77>
                regs->ax = (*call)(
     e39:       48 8b 4b 38             mov    0x38(%rbx),%rcx
     e3d:       48 8b 53 60             mov    0x60(%rbx),%rdx
     e41:       48 8b 73 68             mov    0x68(%rbx),%rsi
     e45:       48 8b 7b 70             mov    0x70(%rbx),%rdi
     e49:       4c 8b 4b 40             mov    0x40(%rbx),%r9
     e4d:       4c 8b 43 48             mov    0x48(%rbx),%r8
     e51:       ff 10                   callq  *(%rax)
     e53:       48 89 43 50             mov    %rax,0x50(%rbx)
     e57:       65 48 8b 04 25 00 00    mov    %gs:0x0,%rax