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    SubjectRe: [RFC PATCH v4 3/9] x86/cet/ibt: Add IBT legacy code bitmap allocation function
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    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@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;
    >
    > 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

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