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Reading and writing memory of other processes using fasttrap

The /dev/fasttrap device for creating trap nodes in user-space processes for the pid and objc providers has the permissions 666. In contrast to /dev/dtrace, which is also 666, fasttrap lacks a permission check. This allows any process to issue a FASTTRAPIOC_MAKEPROBE or FASTTRAPIOC_GETINSTR ioctl.
An attacker can create probes in any other process for any memory location using FASTTRAPIOC_MAKEPROBE. On activation of these probes, the values at the specified memory location is replaced with a trap instruction (0xCC on x86) and the original value is placed in a shadow memory. Retrieving the original value is possible with the FASTTRAPIOC_GETINSTR ioctl. This allows the attacker to read arbitrary memory of another process by placing multiple traps.

This vulnerability is tracked as CVE-2020-27949, fixed in macOS Big Sur 11.1, Security Update 2020-001 Catalina, Security Update 2020-007 Mojave .

Disclosure Timeline

• 2020-07-07 -> Initial disclosure to Apple Product Security.
• 2020-07-08 <- Investigation confirmation.
• 2020-10-28 -> Request for status update.
• 2020-12-07 <- Notification about upcoming update that addresses issue.
• 2020-12-14 <- Update released.

Requirements

• Attacker can execute code with regular user permissions (non-elevated)
• Probes in the victim process must be enabled (i.e. the victim must be run under DTrace)

Tested system configurations

• SIP is on (no exceptions)
• 10.15.5 on iMac Late 2012
• 10.15.5 on MacBook Pro 2020

Impact

• Attacker can read memory of other processes executed under DTrace without elevated permissions. Even reading memory from processes running as root is possible.
• Attacker can write the trap instruction value (0xCC on x86) to any memory location of other processes executed under DTrace without elevated permissions. Even writing memory to processes running as root is possible. As part of other instructions or jump addresses, the victim's control flow can be altered, allowing escalation of privileges.

Expectations

As a user, I expect that enabling DTrace only allows processes with root permissions to trace, instrument, or modify other processes. DTrace is a valuable tool for administrative tasks offering tools like dtruss, iotop, etc.

Exploit details

Our victim (target) is an executable running as root. The attacker (memaccess) is an executable running with regular user permissions. The tracing script libc_monitor.d records the number of calls to functions in the libsystem_c.dylib shared object (this is just a simple example of an administrative script a machine owner might run).

1. Victim is executed
2. The attacker creates a probe with victim's PID and a memory location they are interested in.
3. The attacker waits for DTrace to trace the victim process. This could happen as part of an administrative operation where the machine owner wants to trace functions in the victim process (e.g. libc_monitor.d).
4. When DTrace starts tracing the victim, the previously created probes will be enabled, thus, replacing the original memory values.
5. The attacker uses FASTTRAPIOC_GETINSTR to read the memory values of the created probes.

Step 4 replaces memory in the victim process with the trap instruction value. Depending on the application, an attacker can use this to write a previously known value to a critical memory location. For example, the attacker could control the seed of an RNG or replace cryptographic keys in a process.

Note: The attacker never requires elevated or root permissions.
Note 2: Self-written DTrace scripts are not required. For example, dapptrace also uses the pid provider to trace any function.

Proposed Fix

Implement privilege checks in /dev/fasttrap like in /dev/dtrace or remove the world-writable permission.

Note: We did not check the released fix.

Instructions

1. Ensure, that DTrace is available and working correctly
2. Run target and follow the instructions on-screen