Re: [PATCH v3 07/10] x86/ibt: Add paranoid FineIBT mode

Peter Zijlstra
2025-02-19T20:08:05Z

On Wed, Feb 19, 2025 at 05:31:39PM +0000, Andrew Cooper wrote:
> On 19/02/2025 4:21 pm, Peter Zijlstra wrote:
> > --- a/arch/x86/include/asm/cfi.h
> > +++ b/arch/x86/include/asm/cfi.h
> > @@ -1116,6 +1129,52 @@ extern u8 fineibt_caller_end[];
> >
> > #define fineibt_caller_jmp (fineibt_caller_size - 2)
> >
> > +/*
> > + * Since FineIBT does hash validation on the callee side it is prone to
> > + * circumvention attacks where a 'naked' ENDBR instruction exists that
> > + * is not part of the fineibt_preamble sequence.
> > + *
> > + * Notably the x86 entry points must be ENDBR and equally cannot be
> > + * fineibt_preamble.
> > + *
> > + * The fineibt_paranoid caller sequence adds additional caller side
> > + * hash validation. This stops such circumvetion attacks dead, but at the cost
> > + * of adding a load.
> > + *
> > + * <fineibt_paranoid_start>:
> > + * 0: 41 ba 78 56 34 12 mov $0x12345678, %r10d
> > + * 6: 45 3b 53 f7 cmp -0x9(%r11), %r10d
> > + * a: 4d 8d 5b <f0> lea -0x10(%r11), %r11
> > + * e: 75 fd jne d <fineibt_paranoid_start+0xd>
> > + * 10: 41 ff d3 call *%r11
> > + * 13: 90 nop
> > + *
> > + * Notably LEA does not modify flags and can be reordered with the CMP,
> > + * avoiding a dependency. Again, using a non-taken (backwards) branch
> > + * for the failure case, abusing LEA's immediate 0xf0 as LOCK prefix for the
> > + * JCC.d8, causing #UD.
> > + */
>
> I don't know what to say.  This is equal parts horrifying and beautiful.
>
> > +asm( ".pushsection .rodata \n"
> > + "fineibt_paranoid_start: \n"
> > + " movl $0x12345678, %r10d \n"
> > + " cmpl -9(%r11), %r10d \n"
> > + " lea -0x10(%r11), %r11 \n"
> > + " jne fineibt_paranoid_start+0xd \n"
>
> Maybe `jne . - 3` ?
>
> Or perhaps `1: jne 1b - 1` ?
>
> Both seem marginally less fragile than tying the reference to
> fineibt_paranoid_start.

Right, so I initially had '. - 3' (and '. - 7' in
fineibt_preamble_start), but I ended up going with this form because
that's what you end up with if you run it through an assembler and
disassembler.

So I figured this form was easier to compare vs disassembly, which is
what most people will see if they ever look at this.

Anyway, I'm happy to go '. - 3' again if that's preferred.