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SubjectRe: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read under speculation
On Tue, Feb 6, 2018 at 12:58 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Tue, Feb 6, 2018 at 12:49 PM, Andy Lutomirski <luto@kernel.org> 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

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