On Thu, May 9, 2019 at 1:00 AM Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx> wrote: > This early added entropy is also going to be used for stack canary. At > the time it's created there's not be much entropy (before > boot_init_stack_canary(), there's only add_latent_entropy() and > command_line). > On arm64, there is a single canary for all tasks. If RNG is weak or > the seed can be read, it might be easier to figure out the canary. With newer compilers[1] there will be a per-task canary on arm64[2], which will improve this situation, but many architectures lack a per-task canary, unfortunately. I've also recently rearranged the RNG initialization[3] which should also help with better entropy mixing. But each of these are kind of band-aids against not having sufficient initial entropy, which leaves the canary potentially exposed. -Kees [1] https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=359c1bf35e3109d2f3882980b47a5eae46123259 [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0a1213fa7432778b71a1c0166bf56660a3aab030 [3] https://git.kernel.org/pub/scm/linux/kernel/git/tytso/random.git/commit/?h=dev&id=d55535232c3dbde9a523a9d10d68670f5fe5dec3 -- Kees Cook
