On August 8, 2016 9:01:42 PM PDT, Borislav Petkov <b...@suse.de> wrote: >On Mon, Aug 08, 2016 at 11:40:07AM -0700, Thomas Garnier wrote: >> Default implementation expects 6 pages maximum are needed for low >page >> allocations. If KASLR memory randomization is enabled, the worse case >> of e820 layout would require 12 pages (no large pages). It is due to >the >> PUD level randomization and the variable e820 memory layout. > >Can't this number of required pages be computed based on the >randomization offset or somesuch instead of maxing it out by default on >every machine which enables CONFIG_RANDOMIZE_MEMORY? > >> This bug was found while doing extensive testing of KASLR memory >> randomization on different type of hardware. >> >> Signed-off-by: Thomas Garnier <thgar...@google.com> >> --- >> Based on next-20160805 >> --- >> arch/x86/mm/init.c | 8 ++++++++ >> 1 file changed, 8 insertions(+) >> >> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c >> index 6209289..3a27e6a 100644 >> --- a/arch/x86/mm/init.c >> +++ b/arch/x86/mm/init.c >> @@ -130,6 +130,14 @@ void __init early_alloc_pgt_buf(void) >> unsigned long tables = INIT_PGT_BUF_SIZE; >> phys_addr_t base; >> >> + /* >> + * Depending on the machine e860 memory layout and the PUD >alignement. > > e820 alignment > >> + * We may need twice more pages when KASLR memoy randomization is > > memory > >> + * enabled. >> + */ >> + if (IS_ENABLED(CONFIG_RANDOMIZE_MEMORY)) >> + tables *= 2; >> + >> base = __pa(extend_brk(tables, PAGE_SIZE)); >> >> pgt_buf_start = base >> PAGE_SHIFT; >> -- >> 2.8.0.rc3.226.g39d4020 >>
Upping the brk by a few pages is not a big deal. Unused brk gets reclaimed at init release time. -- Sent from my Android device with K-9 Mail. Please excuse brevity and formatting.
