70 lines
2.2 KiB
Markdown
70 lines
2.2 KiB
Markdown
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## Circumventing OpenBSD 6.0's W^X Protections
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OpenBSD 6.0 and above enforces data-execution prevention (DEP or
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W^X) by default, preventing memory from being mapped as
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simultaneously writeable and executable (i.e., W|X). This causes
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problems for Unicorn, if left in place. If you're seeing
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errors like the following:
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```
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/home/git/unicorn >> ./sample_arm
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Emulate ARM code
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zsh: abort (core dumped) ./sample_arm
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```
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then W^X is likely the culprit. If we run it again with ktrace
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and look at the output with kdump, we see that this is indeed
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the issue:
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```
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82192 sample_arm CALL mmap(0,0x800000,0x7<PROT_READ|PROT_WRITE|PROT_EXEC>,0x1002<MAP_PRIVATE|MAP_ANON>,-1,0)
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82192 sample_arm PSIG SIGABRT SIG_DFL
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82192 sample_arm NAMI "sample_arm.core"
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```
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Right now, we're in the /home filesystem. Let's look at its mount
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options in /etc/fstab:
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```
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1234abcdcafef00d.g /home ffs rw,nodev,nosuid 1 2
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```
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If we edit the options to include ```wxallowed```, appending
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this after nosuid, for example, then we're golden:
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```
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1234abcdcafef00d.g /home ffs rw,nodev,nosuid,wxallowed 1 2
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```
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Note that this *does* diminish the security of your filesystem
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somewhat, and so if you're particularly particular about such
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things, we recommend setting up a dedicated filesystem for
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any activities that require ```(W|X)```, such as unicorn
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development and testing.
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In order for these changes to take effect, you will need to
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reboot.
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_Time passes..._
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Let's try this again. There's no need to recompile unicorn or
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the samples, as (W^X) is strictly a runtime issue.
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First, we double check to see if /home has been mounted with
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wxallowed:
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```
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/home >> mount | grep home
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/dev/sd3g on /home type ffs (local, nodev, nosuid, wxallowed)
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```
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Okay, now let's try running that sample again...
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```
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/home/git/unicorn/samples >> ./sample_arm
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Emulate ARM code
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>>> Tracing basic block at 0x10000, block size = 0x8
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>>> Tracing instruction at 0x10000, instruction size = 0x4
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>>> Emulation done. Below is the CPU context
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>>> R0 = 0x37
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>>> R1 = 0x3456
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==========================
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Emulate THUMB code
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>>> Tracing basic block at 0x10000, block size = 0x2
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>>> Tracing instruction at 0x10000, instruction size = 0x2
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>>> Emulation done. Below is the CPU context
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>>> SP = 0x1228
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```
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works fine.
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