Red Hat Bugzilla – Bug 853085
ld.so can be used to bypass audit watches
Last modified: 2014-02-05 07:07:03 EST
Description of problem: The audit system is a security requirement in many settings. There are rules that can watch for the execution of certain programs by hooking execve system call inside the kernel. But it turns out that we can't really succeed because of ld.so bypassing execve. [root@x2 ~]# auditctl -w /usr/bin/ls -p x -k ls [root@x2 ~]# /lib64/ld-2.15.so /usr/bin/ls /var/run/log/ journal [root@x2 ~]# ausearch --start recent -k ls ---- time->Mon Aug 20 10:17:10 2012 type=CONFIG_CHANGE msg=audit(1345472230.249:329): auid=4325 ses=2 subj=unconfined_u:unconfined_r:auditctl_t:s0 op="add rule" key="ls" list=4 res=1 ---- time->Mon Aug 20 10:18:05 2012 type=CONFIG_CHANGE msg=audit(1345472285.476:330): auid=4325 ses=2 subj=unconfined_u:unconfined_r:auditctl_t:s0 op="remove rule" key="ls" list=4 res=1 [root@x2 ~]# It would have been expected that a SYSCALL event would have been logged and another record detailing the passed arguments. Why is the event missed? Execve is never called: [root@x2 ~]# strace /lib64/ld-2.15.so /usr/bin/ls /var/run/log/ execve("/lib64/ld-2.15.so", ["/lib64/ld-2.15.so", "/usr/bin/ls", "/var/run/log/"], [/* 32 vars */]) = 0 brk(0) = 0x7f16ccb7b000 open("/usr/bin/ls", O_RDONLY|O_CLOEXEC) = 3 read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\2\0>\0\1\0\0\0lK@\0\0\0\0\0"..., 832) = 832 fstat(3, {st_mode=S_IFREG|0755, st_size=110032, ...}) = 0 mmap(0x400000, 102400, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED| MAP_DENYWRITE, 3, 0) = 0x400000 mmap(0x619000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED| MAP_DENYWRITE, 3, 0x19000) = 0x619000 mmap(0x61b000, 672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x61b000 close(3) = 0 And it keeps going. But the above shows that it reads the executable into memory rather than calling execve. Somehow, we need to get an event on watched files.
You'll need to add a special case for ld.so so that whenever it is executed by a user, you look for the arguments to ld.so to decide what was really executed. That reduces the problem to that of identifying the dynamic linker. The dynamic linker will typically not have the INTERP in the ELF header. Also, it will be the only one that has the _start symbol defined. However, both are true for statically linked binaries as well, so you may have to modify the message you log so that it informs the user that a program has been executed with a different linker. That or log all static binaries (look for the INTERP) differently, with the commandline arguments. Either way, I don't think there's much that can be done in the dynamic linker itself.
Or depending on the level of accuracy needed, when an audit watch is set up for exec-ing a particular executable an audit is automatically set up if the same executable is mmap'd. Obviously in the latter case just because it's mmap'd doesn't mean must be exec'd, it just means it may be exec'd. But I'm a bit curious, what's to stop someone from bypassing the audit by copying the executable to another location and executing it that way. Or what's to stop someone from mmaping some pages and using read to fill the pages, then transferring control. There's probably other ways to get around the audits. But I agree with SIddhesh, I don't really see there's something here for glibc.
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