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

[Eugene Syromiatnikov]

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.

> +		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) {

Shouldn't bitmap be unmapped here?

> +			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;

In a hypothetical situation of bitmap & PAGE_MASK < bitmap that would lead
to bitmap pointing to unmapped memory. A check that bitmap is sane would
probably be better.

> +
> +	/*
> +	 * 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;
> +}
> -- 
> 2.17.1
>