Date: Mon, 5 Sep 2011 16:46:47 +0400 From: Vasiliy Kulikov <segoon@...nwall.com> To: kernel-hardening@...ts.openwall.com Subject: Re: [RFC] x86, mm: start mmap allocation for libs from low addresses Solar, On Sun, Sep 04, 2011 at 03:57 +0400, Solar Designer wrote: > On Sat, Sep 03, 2011 at 03:18:49PM +0400, Vasiliy Kulikov wrote: > > On Fri, Sep 02, 2011 at 22:29 +0400, Solar Designer wrote: > > > On Thu, Aug 25, 2011 at 09:19:34PM +0400, Vasiliy Kulikov wrote: > > > > additionally overwrite function arguments, which are located after the > > > > function address on the stack. The attacker's best bet may be to find > > > > an entry point not at function boundary that sets registers and then > > > > proceeds with or branches to the desired library code. The easiest way > > > > to set registers and branch would be a function epilogue - > > > > pop/pop/.../ret - but then there's the difficulty in passing the address > > > > to ret to (we have just one NUL and we've already used it to get to this > > > > code). Similarly, even via such pop's we can't pass an argument that > > > > contains a NUL in it - e.g., the address of "/bin/sh" in libc (it > > > > contains a NUL most significant byte too) or a zero value for root's > > > > uid. > > > > > > The above was partially flawed logic on my part - as written above > > > (without further detail), the pop/pop/.../ret thing doesn't apply > > > because those pop's would read stack right after the just-used return > > > address - that is, the same stack locations that we presumably could not > > > write to in order to pass the arguments in a more straightforward > > > fashion. So this trick would be of no help, and thus its other > > > limitations would be of no relevance. > > > > Why not? > > I am not sure what exactly your "why not" applies to. Hmm, yes, looks like I've lost the thread some time ago :( A good description definitely needs much longer and scrupulous analysis. Probably there is a public paper with a review/analysis/benefits of ASCII-armor that we're able to refer in the patch description? I cannot find any rigorous paper, unfortunately. > > > > If CONFIG_VM86=y, the first megabyte is excluded from the potential > > > > range for mmap allocations as it might be used by vm86 code. If > > > > CONFIG_VM86=n, the allocation begins from the mmap_min_addr. Regardless > > > > of CONFIG_VM86 the base address is randomized with the same entropy size > > > > as mm->mmap_base. > > > > > > OK. Shouldn't CONFIG_VM86 be a sysctl, though? > > > > This is not a hardening setting that was present in -ow, but an existing > > config to disable vm86/vm86_old at the compile time. It was added for > > EMBEDDED. > > Oh, I was not aware of that. > > solar@...t:~/kernel/mainline/linux-3.0.4 $ fgrep -rl CONFIG_VM86 . > ./arch/x86/kernel/Makefile > ./arch/x86/kernel/entry_32.S > ./arch/x86/include/asm/vm86.h > ./arch/x86/include/asm/processor-flags.h > > Looks like there's no Kconfig option for this - perhaps add it with a > separate patch? Since 2.6.x CONFIG_ prefix is not used in Kconfig files: $ grep -w VM86 arch/x86/Kconfig config VM86 bool "Enable VM86 support" if EXPERT Thanks, -- Vasiliy
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