Date: Mon, 18 Jul 2011 23:48:03 +0400 From: Vasiliy Kulikov <segoon@...nwall.com> To: kernel-hardening@...ts.openwall.com Subject: GNU_STACK policy problem Hi, I've written kernel part of PT_GNU_STACK NX enforcement for binaries and trampolines emulation (the latter is taken from PaX as-is). It Just Works for both 32 and 64 bits. But there is a major problem - it also needs support from libc to handle DSO's GNU_STACK. As for now, GNU_STACK of the binary is handled by the kernel and GNU_STACK of the DSO is handled by glibc. So, if there is some DSO with GNU_STACK=X, which is loaded for an executable with GNU_STACK=NX, the stack will be made =X by glibc in the runtime. This case is fully handled by the userspace, there is nothing kernel can enforce. (Unless hook mprotect and identify it is related to the stack, which is ugly. Btw, stacks for all threads should be altered in multi-threaded app.). As I've implemented it as sysctl kernel.execstack_mode, there is a possibility to patch libc to look at this sysctl while loading a DSO (1). However, it will not work if either procfs is not mounted (it is fully legitimate case for simple DSO loading, e.g. for early boot tasks) or the task is chrooted into a very restricted tree (or /proc is restricted by some LSM). I don't see a simple generic way to handle it as the kernel has 2 policies for GNU_STACK - if it is X and if it is completely missing. So, libc should handle these 2 situations of DSO's according to the kernel policy. Some fs-free way to pass these 2 bits from kernel to userspace is needed. One solution I see is somehow passing this information (GNU_STACK handling policy for 2 cases) at the task creation time. Something like VDSO symbol (2) or auxv entry (3). VDSO case is complicated as IIRC now it is global to the kernel and is not per pid namespace (I recall a problem in OpenVZ kernel when kernel version symbol was globally removed to fool some pid namespaces expecting new kernel). auxv is complicated as it is ELF ABI changing :-( Fundamentally different way (4) - in the kernel implement trampolines emulation only. The policy of handling GNU_STACK is fully userspace thing. Store the variable in some /etc/ config file (any existing file?). As libc already relies on /etc/ld.so* files presence, it shouldn't be a problem (unlike procfs). But given we're trying to harden mainline kernel/libc, the glibc maintainer, Ulrich Drepper, should accept this change. It is very unlikely, given he "supports glibc only for the mainline kernel" and, AFAIU, he is fully satisfied with current GNU_STACK situation. Fallback "solution" - "recommend" distros to force GNU_STACK=NX for all binaries and libraries via execstack(1) and implement trampolines emulation in the kernel (with sysctl) (5). This is bad in sense there is *some* way to execute code on the stack (however, I might overestimate the significance as the code is very restricted and can be executed by ret2libc anyway). Or "enhance" GNU_STACK to have additional value (6) - whether trampolines should be emulated or not (this will be set by execstack). It might not be accepted by glibc as it is a gcc-specific hack. I'm not sure what to do in this situation - all solutions above are far from ideal. I'd be glad to hear any comments. Thanks, -- Vasiliy
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