Date: Wed, 22 Dec 2010 14:06:35 +0000 From: Tim Brown <tmb@...35.com> To: oss-security@...ts.openwall.com Subject: Re: Re: Breaking the links: Exploiting the linker On Wednesday 22 December 2010 11:46:41 Jamie Nguyen wrote: > Tim Brown <timb@...> writes: > > In the interests of a thorough peer review I'd be curious what people > > think of the following paper I've been working on Linux and POSIX > > linkers: > > > > http://www.nth-dimension.org.uk/downloads.php?id=77 > > > > A previous revision has already been reviewed but constructive criticism > > is always useful. There are some sections that I have removed whilst I > > wait on vendors but I'm particularly interested in feedback on pertinent > > references or threats that I may have missed. As per the abstract, the > > aim of the paper wasn't to claim everything as my own but rather to > > document as much about the current state of art as possible. > > > > Tim > > Hi, > > I am somewhat unknowledgeable about the whole linking process, but I was > testing out the execution of a file using ld on a filesystem mounted with > noexec. I followed the example you gave of copying the '/usr/bin/id' > executable to a user writeable directory and removing the executable bit. > > After removing the executable bit, I was still able to execute this on a > normal filesystem using /lib/ld-linux-x86_64.so.2 but on a filesystem > mounted with noexec this method did not work. > > You suggest in the article: > > "...if you're mounting devices with noexec the you should probably ensure > that they [sic] the runtime linker can't be executed either." > > Forgive me if I am being dim, because from what I can see, mounting with > noexec seems to solve the issue of using ld-linux-x86-64.so.2 to execute > non-executable files. You're not being dim. On Linux, mounting the file system with noexec prevents the kernel mmap()ing the pages with execute permissions. Removing the execute bit on a binary doesn't cause the same behaviour. In the paper I was describing the general case. This is something taviso or stealth mentioned to me too so I will update the paper to make this distinction clear. Thanks for the feedback. Tim -- Tim Brown <mailto:tmb@...35.com> Download attachment "signature.asc " of type "application/pgp-signature" (837 bytes)
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