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In the Linux kernel, the following vulnerability has been resolved: acct: perform last write from workqueue In [1] it was reported that the acct(2) system call can be used to trigger NULL deref in cases where it is set to write to a file that triggers an internal lookup. This can e.g., happen when pointing acc(2) to /sys/power/resume. At the point the where the write to this file happens the calling task has already exited and called exit_fs(). A lookup will thus trigger a NULL-deref when accessing current->fs. Reorganize the code so that the the final write happens from the workqueue but with the caller's credentials. This preserves the (strange) permission model and has almost no regression risk. This api should stop to exist though.
CVSS 3.1 Base Score 5.5. CVSS Attack Vector: local. CVSS Attack Complexity: low. CVSS Vector: (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H).
While there are no complete fixes aside from conscientious programming, the following steps will go a long way to ensure that NULL pointer dereferences do not occur.
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/* make use of pointer1 *//* ... */If you are working with a multithreaded or otherwise asynchronous environment, ensure that proper locking APIs are used to lock before the if statement; and unlock when it has finished.
This example takes an IP address from a user, verifies that it is well formed and then looks up the hostname and copies it into a buffer.
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strcpy(hostname, hp->h_name);/*routine that ensures user_supplied_addr is in the right format for conversion */If an attacker provides an address that appears to be well-formed, but the address does not resolve to a hostname, then the call to gethostbyaddr() will return NULL. Since the code does not check the return value from gethostbyaddr (CWE-252), a NULL pointer dereference would then occur in the call to strcpy().
Note that this example is also vulnerable to a buffer overflow (see CWE-119).
In the following code, the programmer assumes that the system always has a property named "cmd" defined. If an attacker can control the program's environment so that "cmd" is not defined, the program throws a NULL pointer exception when it attempts to call the trim() method.
cmd = cmd.trim();This application has registered to handle a URL when sent an intent:
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}int length = URL.length();...The application assumes the URL will always be included in the intent. When the URL is not present, the call to getStringExtra() will return null, thus causing a null pointer exception when length() is called.
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