Re: [PATCH v9 10/11] x86/paravirt: Adapt assembly for PIE support

[Borislav Petkov]

On Wed, Jul 31, 2019 at 02:53:06PM +0200, Peter Zijlstra wrote:
> On Tue, Jul 30, 2019 at 12:12:54PM -0700, Thomas Garnier wrote:
> > if PIE is enabled, switch the paravirt assembly constraints to be
> > compatible. The %c/i constrains generate smaller code so is kept by
> > default.
> > 
> > Position Independent Executable (PIE) support will allow to extend the
> > KASLR randomization range below 0xffffffff80000000.
> > 
> > Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxxxx>
> > Acked-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> >  arch/x86/include/asm/paravirt_types.h | 25 +++++++++++++++++++++----
> >  1 file changed, 21 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
> > index 70b654f3ffe5..fd7dc37d0010 100644
> > --- a/arch/x86/include/asm/paravirt_types.h
> > +++ b/arch/x86/include/asm/paravirt_types.h
> > @@ -338,9 +338,25 @@ extern struct paravirt_patch_template pv_ops;
> >  #define PARAVIRT_PATCH(x)					\
> >  	(offsetof(struct paravirt_patch_template, x) / sizeof(void *))
> >  
> > +#ifdef CONFIG_X86_PIE
> > +#define paravirt_opptr_call "a"
> > +#define paravirt_opptr_type "p"
> > +
> > +/*
> > + * Alternative patching requires a maximum of 7 bytes but the relative call is
> > + * only 6 bytes. If PIE is enabled, add an additional nop to the call
> > + * instruction to ensure patching is possible.
> > + */
> > +#define PARAVIRT_CALL_POST  "nop;"
> 
> I'm confused; where does the 7 come from? The relative call is 6 bytes,

Well, before it, the relative CALL is a CALL reg/mem64, i.e. the target
is mem64. For example:


ffffffff81025c45:       ff 14 25 68 37 02 82    callq  *0xffffffff82023768

That address there is practically pv_ops + offset.

Now, in the opcode bytes you have 0xff opcode, ModRM byte 0x14 and SIB
byte 0x25, and 4 bytes imm32 offset. And this is 7 bytes.

What it becomes is:

ffffffff81025cd0:       ff 15 fa d9 ff 00       callq  *0xffd9fa(%rip)        # ffffffff820236d0 <pv_ops+0x30>
ffffffff81025cd6:       90                      nop

which is a RIP-relative, i.e., opcode 0xff, ModRM byte 0x15 and imm32.
And this is 6 bytes.

And since the paravirt patching doesn't do NOP padding like the
alternatives patching does, you need to pad with a byte.

Thomas, please add the gist of this to the comments because this
incomprehensible machinery better be documented as detailed as possible.

Thx.

-- 
Regards/Gruss,
    Boris.

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