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

[Eugene Syromiatnikov]

On Fri, Oct 05, 2018 at 10:07:46AM -0700, Andy Lutomirski wrote:
> On Fri, Oct 5, 2018 at 10:03 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> >
> > On Fri, 2018-10-05 at 09:28 -0700, Andy Lutomirski wrote:
> > > > On Oct 5, 2018, at 9:13 AM, Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > > >
> > > > > 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
> > > >
> > >
> > > I havenât followed all the details here, but I have a general policy of
> > > objecting to any new use of TASK_SIZE. If you really really need to depend on
> > > 32-bitness in new code, please figure out what exactly you mean by â32-bitâ
> > > and use an explicit check.
> >
> > The explicit check would be:
> >
> > test_thread_flag(TIF_ADDR32) ? IA32_PAGE_OFFSET : TASK_SIZE_MAX
> >
> > which is the same as TASK_SIZE.
> 
> But this is only ever done in response to a syscall, right?  So
> wouldn't in_compat_syscall() be the right check?
> 
> Also, this whole thing makes me extremely nervous.  The MSR only
> contains the start address, not the size, right?  So what prevents
> some goof from causing the CPU to read way past the end of the bitmap
> if the bitmap is short because the kernel thought it was supposed to
> be 32-bit?

That's what I've mentioned initially: every syscall made with int 0x80
is interpreted as compat, even if it was made from long mode.

> I'm inclined to suggest something awful-ish: always allocate the
> bitmap as though it's for a 64-bit process, and just let it be at a
> high address.  And add a syscall or arch_prctl() to manipulate it for
> the benefit of 32-bit programs that can't address it directly.

That's likely the only way to go.