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    SubjectRe: [RFC PATCH v4 3/9] x86/cet/ibt: Add IBT legacy code bitmap allocation function
    On Fri, Sep 21, 2018 at 8:10 AM Yu-cheng Yu <yu-cheng.yu@intel.com> 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@intel.com>
    > ---
    > 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;
    > + current->thread.cet.ibt_bitmap_size = size;
    > + bitmap = do_mmap_locked(0, size, PROT_READ | PROT_WRITE,
    > + MAP_ANONYMOUS | MAP_PRIVATE,
    > + VM_DONTDUMP);
    > +
    > + if (bitmap >= TASK_SIZE) {
    > + current->thread.cet.ibt_bitmap_size = 0;
    > + return -ENOMEM;
    > + }
    > +
    > + current->thread.cet.ibt_bitmap_addr = bitmap;
    > + }
    > +
    > + /*
    > + * Lower bits of MSR_IA32_CET_LEG_IW_EN are for IBT
    > + * settings. Clear lower bits even bitmap is already
    > + * page-aligned.
    > + */
    > + bitmap = current->thread.cet.ibt_bitmap_addr;
    > + bitmap &= PAGE_MASK;
    > +
    > + /*
    > + * Turn on IBT legacy bitmap.
    > + */
    > + rdmsrl(MSR_IA32_U_CET, r);
    > + r |= (MSR_IA32_CET_LEG_IW_EN | bitmap);
    > + wrmsrl(MSR_IA32_U_CET, r);
    > + return 0;

    Why are you writing the MSRs in the case where the bitmap was already allocated?

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