| Message ID | 20160810222805.GA13733@www.outflux.net (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 08/10/2016 03:28 PM, Kees Cook wrote: > + if (ram_latent_entropy && !PageHighMem(page) && > + page_to_pfn(page) < 0x100000) { > + u64 hash = 0; > + size_t index, end = PAGE_SIZE * nr_pages / sizeof(hash); > + const u64 *data = lowmem_page_address(page); > + > + for (index = 0; index < end; index++) > + hash ^= hash + data[index]; > + add_device_randomness((const void *)&hash, sizeof(hash)); > + } When I was first reading this, I thought it was using the _addresses_ of the freed memory for entropy. But it's actually using the _contents_. The description could probably use a wee bit of sprucing up. It might also be nice to say in the patch description (and the Documentation/) what you expect to be in this memory. It will obviously be zeros for the vast majority of the space, but I do wonder what else ends up in there in practice. Why is it limited to 4GB? Just so it doesn't go and try to XOR the contents of a multi-TB system if it got turned on there? :)
On 11/08/16 08:28, Kees Cook wrote: > From: Emese Revfy <re.emese@gmail.com> > > When "ram_latent_entropy" is passed on the kernel command line, entropy > will be extracted from up to the first 4GB of RAM while the runtime memory > allocator is being initialized. This entropy isn't cryptographically > secure, but does help provide additional unpredictability on otherwise > low-entropy systems. > > Based on work created by the PaX Team. > > Signed-off-by: Emese Revfy <re.emese@gmail.com> > [kees: renamed parameter, dropped relationship with plugin, updated log] > Signed-off-by: Kees Cook <keescook@chromium.org> > --- > This patch has been extracted from the latent_entropy gcc plugin, as > suggested by Linus: https://lkml.org/lkml/2016/8/8/840 > --- > Documentation/kernel-parameters.txt | 5 +++++ > mm/page_alloc.c | 21 +++++++++++++++++++++ > 2 files changed, 26 insertions(+) > > diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt > index 46c030a49186..9d054984370f 100644 > --- a/Documentation/kernel-parameters.txt > +++ b/Documentation/kernel-parameters.txt > @@ -3245,6 +3245,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted. > raid= [HW,RAID] > See Documentation/md.txt. > > + ram_latent_entropy > + Enable a very simple form of latent entropy extraction > + from the first 4GB of memory as the bootmem allocator > + passes the memory pages to the buddy allocator. > + > ramdisk_size= [RAM] Sizes of RAM disks in kilobytes > See Documentation/blockdev/ramdisk.txt. > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index fb975cec3518..1de94f0ff29d 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -64,6 +64,7 @@ > #include <linux/page_owner.h> > #include <linux/kthread.h> > #include <linux/memcontrol.h> > +#include <linux/random.h> > > #include <asm/sections.h> > #include <asm/tlbflush.h> > @@ -1236,6 +1237,15 @@ static void __free_pages_ok(struct page *page, unsigned int order) > local_irq_restore(flags); > } > > +bool __meminitdata ram_latent_entropy; > + > +static int __init setup_ram_latent_entropy(char *str) > +{ > + ram_latent_entropy = true; > + return 0; > +} > +early_param("ram_latent_entropy", setup_ram_latent_entropy); > + > static void __init __free_pages_boot_core(struct page *page, unsigned int order) > { > unsigned int nr_pages = 1 << order; > @@ -1251,6 +1261,17 @@ static void __init __free_pages_boot_core(struct page *page, unsigned int order) > __ClearPageReserved(p); > set_page_count(p, 0); > > + if (ram_latent_entropy && !PageHighMem(page) && > + page_to_pfn(page) < 0x100000) { > + u64 hash = 0; > + size_t index, end = PAGE_SIZE * nr_pages / sizeof(hash); > + const u64 *data = lowmem_page_address(page); > + > + for (index = 0; index < end; index++) > + hash ^= hash + data[index]; Won't the hash be the same across boots? Is this entropy addition for KASLR, since it is so early in boot?q > + add_device_randomness((const void *)&hash, sizeof(hash)); > + } > + > page_zone(page)->managed_pages += nr_pages; > set_page_refcounted(page); > __free_pages(page, order); > Balbir Singh
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index 46c030a49186..9d054984370f 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -3245,6 +3245,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted. raid= [HW,RAID] See Documentation/md.txt. + ram_latent_entropy + Enable a very simple form of latent entropy extraction + from the first 4GB of memory as the bootmem allocator + passes the memory pages to the buddy allocator. + ramdisk_size= [RAM] Sizes of RAM disks in kilobytes See Documentation/blockdev/ramdisk.txt. diff --git a/mm/page_alloc.c b/mm/page_alloc.c index fb975cec3518..1de94f0ff29d 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -64,6 +64,7 @@ #include <linux/page_owner.h> #include <linux/kthread.h> #include <linux/memcontrol.h> +#include <linux/random.h> #include <asm/sections.h> #include <asm/tlbflush.h> @@ -1236,6 +1237,15 @@ static void __free_pages_ok(struct page *page, unsigned int order) local_irq_restore(flags); } +bool __meminitdata ram_latent_entropy; + +static int __init setup_ram_latent_entropy(char *str) +{ + ram_latent_entropy = true; + return 0; +} +early_param("ram_latent_entropy", setup_ram_latent_entropy); + static void __init __free_pages_boot_core(struct page *page, unsigned int order) { unsigned int nr_pages = 1 << order; @@ -1251,6 +1261,17 @@ static void __init __free_pages_boot_core(struct page *page, unsigned int order) __ClearPageReserved(p); set_page_count(p, 0); + if (ram_latent_entropy && !PageHighMem(page) && + page_to_pfn(page) < 0x100000) { + u64 hash = 0; + size_t index, end = PAGE_SIZE * nr_pages / sizeof(hash); + const u64 *data = lowmem_page_address(page); + + for (index = 0; index < end; index++) + hash ^= hash + data[index]; + add_device_randomness((const void *)&hash, sizeof(hash)); + } + page_zone(page)->managed_pages += nr_pages; set_page_refcounted(page); __free_pages(page, order);