Messages in this thread | | | Date | Fri, 3 Mar 2017 23:08:57 +0800 | From | Baoquan He <> | Subject | Re: [PATCH v4 1/3] x86: Introduce a new constant KERNEL_MAPPING_SIZE |
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On 03/03/17 at 11:07pm, Baoquan He wrote: > On 03/03/17 at 03:28pm, Borislav Petkov wrote: > > On Fri, Mar 03, 2017 at 09:11:52PM +0800, Baoquan He wrote: > > > And another meaning of defining kernel iamge size and mapping size > > > differently is we can randomize the limited kernel image in the mapping > > > area. If they are the same or kernel image can be very large, the > > > position will be fixed or very few, kernel text KASLR will be > > > meaningless. > > > > This is simply not true: > > > > @@ -408,9 +408,9 @@ static unsigned long find_random_virt_addr(unsigned long minimum, > > /* > > * There are how many CONFIG_PHYSICAL_ALIGN-sized slots > > * that can hold image_size within the range of minimum to > > - * KERNEL_IMAGE_SIZE? > > + * KERNEL_MAPPING_SIZE? > > */ > > - slots = (KERNEL_IMAGE_SIZE - minimum - image_size) / > > + slots = (KERNEL_MAPPING_SIZE - minimum - image_size) / > > CONFIG_PHYSICAL_ALIGN + 1; > > > > *With* kaslr, KERNEL_IMAGE_SIZE = 1G and KERNEL_MAPPING_SIZE = 1G. > > Before your patch KERNEL_IMAGE_SIZE = 1G too with kaslr enabled. > > 512M and 1G is the first case, just an example. Usually kernel image size ~ worst, sorry, typo > is only about 20M, from my laptop. > > Yes, before KERNEL_IMAGE_SIZE is 1G with kaslr enabled. when you > suggested taking a fixed size for the KERNEL_IMAGE_SIZE, but not changed > back and forth with the kaslr set or not, I started to consider this. > > See the 1G, hard-constrainted because of level2_kernel_pgt. In the > future we could put kernel mapping area in another place to remove the > 1G limitation, could be 10G or 512G since virtual address are so > redundent, just an assumption, kernel KASLR can benefit from this > actually, but we can't make upper value of kernel image size also be > that big. That will make linker script checking lose meaning. > > Thanks > Baoquan >
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