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An issue was discovered in Xen through 4.13.x, allowing Intel guest OS users to gain privileges or cause a denial of service because of non-atomic modification of a live EPT PTE. When mapping guest EPT (nested paging) tables, Xen would in some circumstances use a series of non-atomic bitfield writes. Depending on the compiler version and optimisation flags, Xen might expose a dangerous partially written PTE to the hardware, which an attacker might be able to race to exploit. A guest administrator or perhaps even an unprivileged guest user might be able to cause denial of service, data corruption, or privilege escalation. Only systems using Intel CPUs are vulnerable. Systems using AMD CPUs, and Arm systems, are not vulnerable. Only systems using nested paging (hap, aka nested paging, aka in this case Intel EPT) are vulnerable. Only HVM and PVH guests can exploit the vulnerability. The presence and scope of the vulnerability depends on the precise optimisations performed by the compiler used to build Xen. If the compiler generates (a) a single 64-bit write, or (b) a series of read-modify-write operations in the same order as the source code, the hypervisor is not vulnerable. For example, in one test build using GCC 8.3 with normal settings, the compiler generated multiple (unlocked) read-modify-write operations in source-code order, which did not constitute a vulnerability. We have not been able to survey compilers; consequently we cannot say which compiler(s) might produce vulnerable code (with which code-generation options). The source code clearly violates the C rules, and thus should be considered vulnerable.
CVSS 3.1 Base Score 7.8. CVSS Attack Vector: local. CVSS Attack Complexity: high. CVSS Vector: (CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H).
CVSS 2.0 Base Score 4.4. CVSS Attack Vector: local. CVSS Attack Complexity: medium. CVSS Vector: (AV:L/AC:M/Au:N/C:P/I:P/A:P).
This code temporarily raises the program's privileges to allow creation of a new user folder.
return True
return False#avoid CWE-22 and CWE-78lowerPrivileges()return FalseWhile the program only raises its privilege level to create the folder and immediately lowers it again, if the call to os.mkdir() throws an exception, the call to lowerPrivileges() will not occur. As a result, the program is indefinitely operating in a raised privilege state, possibly allowing further exploitation to occur.
Evidence of privilege change:
seteuid(getuid());/* do some stuff */
}
// privileged code goes here, for example:// nothing to returnThis code intends to allow only Administrators to print debug information about a system.
}ADMIN,USER,GUEST
}
}
break;break;System.out.println("You must be logged in to perform this command");While the intention was to only allow Administrators to print the debug information, the code as written only excludes those the with the role of "GUEST". Someone with the role of "ADMIN" or "USER" will be allowed access, which goes against the original intent. An attacker may be able to use this debug information to craft an attack on the system.
This code allows someone with the role of "ADMIN" or "OPERATOR" to reset a user's password. The role of "OPERATOR" is intended to have less privileges than an "ADMIN", but still be able to help users with small issues such as forgotten passwords.
}ADMIN,OPERATOR,USER,GUEST
}
}
}break;break;break;System.out.println("You must be logged in to perform this command");This code does not check the role of the user whose password is being reset. It is possible for an Operator to gain Admin privileges by resetting the password of an Admin account and taking control of that account.
This code could be used in an e-commerce application that supports transfers between accounts. It takes the total amount of the transfer, sends it to the new account, and deducts the amount from the original account.
NotifyUser("New balance: $newbalance");FatalError("Bad Transfer Amount");FatalError("Insufficient Funds");A race condition could occur between the calls to GetBalanceFromDatabase() and SendNewBalanceToDatabase().
Suppose the balance is initially 100.00. An attack could be constructed as follows:
PROGRAM-2 sends a request to update the database, setting the balance to 99.00At this stage, the attacker should have a balance of 19.00 (due to 81.00 worth of transfers), but the balance is 99.00, as recorded in the database.
To prevent this weakness, the programmer has several options, including using a lock to prevent multiple simultaneous requests to the web application, or using a synchronization mechanism that includes all the code between GetBalanceFromDatabase() and SendNewBalanceToDatabase().
The following function attempts to acquire a lock in order to perform operations on a shared resource.
}
pthread_mutex_unlock(mutex);/* access shared resource */However, the code does not check the value returned by pthread_mutex_lock() for errors. If pthread_mutex_lock() cannot acquire the mutex for any reason, the function may introduce a race condition into the program and result in undefined behavior.
In order to avoid data races, correctly written programs must check the result of thread synchronization functions and appropriately handle all errors, either by attempting to recover from them or reporting it to higher levels.
}
return pthread_mutex_unlock(mutex);return result;/* access shared resource */Suppose a processor's Memory Management Unit (MMU) has 5 other shadow MMUs to distribute its workload for its various cores. Each MMU has the start address and end address of "accessible" memory. Any time this accessible range changes (as per the processor's boot status), the main MMU sends an update message to all the shadow MMUs.
Suppose the interconnect fabric does not prioritize such "update" packets over other general traffic packets. This introduces a race condition. If an attacker can flood the target with enough messages so that some of those attack packets reach the target before the new access ranges gets updated, then the attacker can leverage this scenario.
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