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

[Peter Zijlstra <peterz@infradead.org>]

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.