* Reshetova, Elena <elena.reshet...@intel.com> wrote:
> > 4) > > > > But before you tweak the patch, a more fundamental question: > > > > Does the stack offset have to be per *syscall execution* randomized? > > Which threats does this protect against that a simpler per task syscall > > random offset wouldn't protect against? > > We *really* need it per syscall. If you take a look on the recent stack > attacks > [1],[2],[3],[4], they all do some initial probing on syscalls first to > discover stack addresses > or leftover data on the stack (or pre-populate stack with some > attacker-controlled data), > and then in the following syscall execute the actual attack (leak data, use > pre-populated data for execution, etc.). If the offset stays the same during > task life time, it can be easily recovered during this initial probing phase, > and > then nothing changes for the attacker. > > [1] Kernel Exploitation Via Uninitialized Stack, 2011 > https://www.defcon.org/images/defcon-19/dc-19-presentations/Cook/DEFCON-19-Cook-Kernel-Exploitation.pdf > [2] Stackjacking, 2011, > https://jon.oberheide.org/files/stackjacking-infiltrate11.pdf > [3] The Stack is Back, 2012, > https://jon.oberheide.org/files/infiltrate12-thestackisback.pdf > [4] Exploiting Recursion in the Linux Kernel, 2016, > https://googleprojectzero.blogspot.com/2016/06/exploiting-recursion-in-linux-kernel_20.html Yeah, so if there's an information leak from the kernel stack, don't we now effectively store 5 PRNG bits there for every syscall, allowing the systematic probing of the generic PRNG? The kernel can execute millions of syscalls per second, I'm pretty sure there's a statistical attack against: * This is a maximally equidistributed combined Tausworthe generator * based on code from GNU Scientific Library 1.5 (30 Jun 2004) * * lfsr113 version: * * x_n = (s1_n ^ s2_n ^ s3_n ^ s4_n) * * s1_{n+1} = (((s1_n & 4294967294) << 18) ^ (((s1_n << 6) ^ s1_n) >> 13)) * s2_{n+1} = (((s2_n & 4294967288) << 2) ^ (((s2_n << 2) ^ s2_n) >> 27)) * s3_{n+1} = (((s3_n & 4294967280) << 7) ^ (((s3_n << 13) ^ s3_n) >> 21)) * s4_{n+1} = (((s4_n & 4294967168) << 13) ^ (((s4_n << 3) ^ s4_n) >> 12)) * * The period of this generator is about 2^113 (see erratum paper). ... which recovers the real PRNG state much faster than the ~60 seconds seeding interval and allows the prediction of the next stack offset? I.e. I don't see how kernel stack PRNG randomization protects against information leaks from the kernel stack. By putting PRNG information into the kernel stack for *every* system call we add a broad attack surface: any obscure ioctl information leak can now be escalated into an attack against the net_rand_state PRNG, right? > No the above numbers are with CONFIG_PAGE_TABLE_ISOLATION=y for x86_64, > I will test with CONFIG_PAGE_TABLE_ISOLATION turned off from now on > also. Thanks! Ingo
