> 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 haven’t 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 “32-bit” and use an explicit check.
Some day I would love to delete TASK_SIZE.
