Hi William! On Tue, Jul 26, 2016 at 11:22:26AM -0700, william.c.robe...@intel.com wrote: > From: William Roberts <william.c.robe...@intel.com> > > This patch introduces the ability randomize mmap locations where the > address is not requested, for instance when ld is allocating pages for > shared libraries. It chooses to randomize based on the current > personality for ASLR.
Now I see how you found the randomize_range() fix. :-P > Currently, allocations are done sequentially within unmapped address > space gaps. This may happen top down or bottom up depending on scheme. > > For instance these mmap calls produce contiguous mappings: > int size = getpagesize(); > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40026000 > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40027000 > > Note no gap between. > > After patches: > int size = getpagesize(); > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x400b4000 > mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40055000 > > Note gap between. > > Using the test program mentioned here, that allocates fixed sized blocks > till exhaustion: > https://www.linux-mips.org/archives/linux-mips/2011-05/msg00252.html, > no difference was noticed in the number of allocations. Most varied from > run to run, but were always within a few allocations of one another > between patched and un-patched runs. Did you test this with different allocation sizes? > Performance Measurements: > Using strace with -T option and filtering for mmap on the program > ls shows a slowdown of approximate 3.7% I think it would be helpful to show the effect on the resulting object code. > Signed-off-by: William Roberts <william.c.robe...@intel.com> > --- > mm/mmap.c | 24 ++++++++++++++++++++++++ > 1 file changed, 24 insertions(+) > > diff --git a/mm/mmap.c b/mm/mmap.c > index de2c176..7891272 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -43,6 +43,7 @@ > #include <linux/userfaultfd_k.h> > #include <linux/moduleparam.h> > #include <linux/pkeys.h> > +#include <linux/random.h> > > #include <asm/uaccess.h> > #include <asm/cacheflush.h> > @@ -1582,6 +1583,24 @@ unacct_error: > return error; > } > > +/* > + * Generate a random address within a range. This differs from > randomize_addr() by randomizing > + * on len sized chunks. This helps prevent fragmentation of the virtual > memory map. > + */ > +static unsigned long randomize_mmap(unsigned long start, unsigned long end, > unsigned long len) > +{ > + unsigned long slots; > + > + if ((current->personality & ADDR_NO_RANDOMIZE) || !randomize_va_space) > + return 0; Couldn't we avoid checking this every time? Say, by assigning a function pointer during init? > + > + slots = (end - start)/len; > + if (!slots) > + return 0; > + > + return PAGE_ALIGN(start + ((get_random_long() % slots) * len)); > +} > + Personally, I'd prefer this function noop out based on a configuration option. > unsigned long unmapped_area(struct vm_unmapped_area_info *info) > { > /* > @@ -1676,6 +1695,8 @@ found: > if (gap_start < info->low_limit) > gap_start = info->low_limit; > > + gap_start = randomize_mmap(gap_start, gap_end, length) ? : gap_start; > + > /* Adjust gap address to the desired alignment */ > gap_start += (info->align_offset - gap_start) & info->align_mask; > > @@ -1775,6 +1796,9 @@ found: > found_highest: > /* Compute highest gap address at the desired alignment */ > gap_end -= info->length; > + > + gap_end = randomize_mmap(gap_start, gap_end, length) ? : gap_end; > + > gap_end -= (gap_end - info->align_offset) & info->align_mask; > > VM_BUG_ON(gap_end < info->low_limit); I'll have to dig into the mm code more before I can comment intelligently on this. thx, Jason.
