On Tue, Nov 3, 2015 at 2:39 PM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote: > On Tue, Nov 03, 2015 at 11:19:44AM -0800, Kees Cook wrote: >> On Tue, Nov 3, 2015 at 10:10 AM, Daniel Cashman <dcashman@xxxxxxxxxxx> wrote: >> > From: dcashman <dcashman@xxxxxxxxxx> >> > >> > arm: arch_mmap_rnd() uses a hard-code value of 8 to generate the >> > random offset for the mmap base address. This value represents a >> > compromise between increased ASLR effectiveness and avoiding >> > address-space fragmentation. Replace it with a Kconfig option, which >> > is sensibly bounded, so that platform developers may choose where to >> > place this compromise. Keep 8 as the minimum acceptable value. >> > >> > Signed-off-by: Daniel Cashman <dcashman@xxxxxxxxxx> >> >> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx> >> >> Russell, if you don't see any problems here, it might make sense not >> to put this through the ARM patch tracker since it depends on the 1/2, >> and I think x86 and arm64 (and possibly other arch) changes are coming >> too. > > Yes, it looks sane, though I do wonder whether there should also be > a Kconfig option to allow archtectures to specify the default, instead > of the default always being the minimum randomisation. I can see scope > to safely pushing our mmap randomness default to 12, especially on 3GB > setups, as we already have 11 bits of randomness on the sigpage and if > enabled, 13 bits on the heap. My thinking is that the there shouldn't be a reason to ever have a minimum that was below the default. I have no objection with it, but it seems needless. Frankly minimum is "0", really, so I don't think it makes much sense to have default != arch minimum. I actually view "arch minimum" as "known good", so if we are happy with raising the "known good" value, that should be the new minimum. -Kees -- Kees Cook Chrome OS Security -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>