Re: [RFC PATCH v4 3/9] x86/cet/ibt: Add IBT legacy code bitmap allocation function

[Yu-cheng Yu]

On Wed, 2018-10-03 at 21:57 +0200, Eugene Syromiatnikov wrote:
> On Fri, Sep 21, 2018 at 08:05:47AM -0700, Yu-cheng Yu wrote:
> > Indirect branch tracking provides an optional legacy code bitmap
> > that indicates locations of non-IBT compatible code.  When set,
> > each bit in the bitmap represents a page in the linear address is
> > legacy code.
> > 
> > We allocate the bitmap only when the application requests it.
> > Most applications do not need the bitmap.
> > 
> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > ---
> >  arch/x86/kernel/cet.c | 45 +++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 45 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> > index 6adfe795d692..a65d9745af08 100644
> > --- a/arch/x86/kernel/cet.c
> > +++ b/arch/x86/kernel/cet.c
> > @@ -314,3 +314,48 @@ void cet_disable_ibt(void)
> >  	wrmsrl(MSR_IA32_U_CET, r);
> >  	current->thread.cet.ibt_enabled = 0;
> >  }
> > +
> > +int cet_setup_ibt_bitmap(void)
> > +{
> > +	u64 r;
> > +	unsigned long bitmap;
> > +	unsigned long size;
> > +
> > +	if (!cpu_feature_enabled(X86_FEATURE_IBT))
> > +		return -EOPNOTSUPP;
> > +
> > +	if (!current->thread.cet.ibt_bitmap_addr) {
> > +		/*
> > +		 * Calculate size and put in thread header.
> > +		 * may_expand_vm() needs this information.
> > +		 */
> > +		size = TASK_SIZE / PAGE_SIZE / BITS_PER_BYTE;
> 
> TASK_SIZE_MAX is likely needed here, as an application can easily switch
> between long an 32-bit protected mode.  And then the case of a CPU that
> doesn't support 5LPT.

If we had calculated bitmap size from TASK_SIZE_MAX, all 32-bit apps would have
failed the allocation for bitmap size > TASK_SIZE.  Please see values below,
which is printed from the current code.

Yu-cheng


x64:
TASK_SIZE_MAX	= 0000 7fff ffff f000
TASK_SIZE	= 0000 7fff ffff f000
bitmap size	= 0000 0000 ffff ffff

x32:
TASK_SIZE_MAX	= 0000 7fff ffff f000
TASK_SIZE	= 0000 0000 ffff e000
bitmap size	= 0000 0000 0001 ffff