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In the Linux kernel, the following vulnerability has been resolved: net: dsa: improve shutdown sequence Alexander Sverdlin presents 2 problems during shutdown with the lan9303 driver. One is specific to lan9303 and the other just happens to reproduce there. The first problem is that lan9303 is unique among DSA drivers in that it calls dev_get_drvdata() at "arbitrary runtime" (not probe, not shutdown, not remove): phy_state_machine() -> ... -> dsa_user_phy_read() -> ds->ops->phy_read() -> lan9303_phy_read() -> chip->ops->phy_read() -> lan9303_mdio_phy_read() -> dev_get_drvdata() But we never stop the phy_state_machine(), so it may continue to run after dsa_switch_shutdown(). Our common pattern in all DSA drivers is to set drvdata to NULL to suppress the remove() method that may come afterwards. But in this case it will result in an NPD. The second problem is that the way in which we set dp->conduit->dsa_ptr = NULL; is concurrent with receive packet processing. dsa_switch_rcv() checks once whether dev->dsa_ptr is NULL, but afterwards, rather than continuing to use that non-NULL value, dev->dsa_ptr is dereferenced again and again without NULL checks: dsa_conduit_find_user() and many other places. In between dereferences, there is no locking to ensure that what was valid once continues to be valid. Both problems have the common aspect that closing the conduit interface solves them. In the first case, dev_close(conduit) triggers the NETDEV_GOING_DOWN event in dsa_user_netdevice_event() which closes user ports as well. dsa_port_disable_rt() calls phylink_stop(), which synchronously stops the phylink state machine, and ds->ops->phy_read() will thus no longer call into the driver after this point. In the second case, dev_close(conduit) should do this, as per Documentation/networking/driver.rst: | Quiescence | ---------- | | After the ndo_stop routine has been called, the hardware must | not receive or transmit any data. All in flight packets must | be aborted. If necessary, poll or wait for completion of | any reset commands. So it should be sufficient to ensure that later, when we zeroize conduit->dsa_ptr, there will be no concurrent dsa_switch_rcv() call on this conduit. The addition of the netif_device_detach() function is to ensure that ioctls, rtnetlinks and ethtool requests on the user ports no longer propagate down to the driver - we're no longer prepared to handle them. The race condition actually did not exist when commit 0650bf52b31f ("net: dsa: be compatible with masters which unregister on shutdown") first introduced dsa_switch_shutdown(). It was created later, when we stopped unregistering the user interfaces from a bad spot, and we just replaced that sequence with a racy zeroization of conduit->dsa_ptr (one which doesn't ensure that the interfaces aren't up).
CVSS 3.1 Base Score 4.7. CVSS Attack Vector: local. CVSS Attack Complexity: high. CVSS Vector: (CVSS:3.1/AV:L/AC:H/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.
The following code checks a file, then updates its contents.
}updateThings();Potentially the file could have been updated between the time of the check and the lstat, especially since the printf has latency.
The following code is from a program installed setuid root. The program performs certain file operations on behalf of non-privileged users, and uses access checks to ensure that it does not use its root privileges to perform operations that should otherwise be unavailable the current user. The program uses the access() system call to check if the person running the program has permission to access the specified file before it opens the file and performs the necessary operations.
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fprintf(stderr,"Unable to open file %s.\n",file);The call to access() behaves as expected, and returns 0 if the user running the program has the necessary permissions to write to the file, and -1 otherwise. However, because both access() and fopen() operate on filenames rather than on file handles, there is no guarantee that the file variable still refers to the same file on disk when it is passed to fopen() that it did when it was passed to access(). If an attacker replaces file after the call to access() with a symbolic link to a different file, the program will use its root privileges to operate on the file even if it is a file that the attacker would otherwise be unable to modify. By tricking the program into performing an operation that would otherwise be impermissible, the attacker has gained elevated privileges. This type of vulnerability is not limited to programs with root privileges. If the application is capable of performing any operation that the attacker would not otherwise be allowed perform, then it is a possible target.
This code prints the contents of a file if a user has permission.
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}//resolve file if its a symbolic link$filename = readlink($filename);return;return false;This code attempts to resolve symbolic links before checking the file and printing its contents. However, an attacker may be able to change the file from a real file to a symbolic link between the calls to is_link() and file_get_contents(), allowing the reading of arbitrary files. Note that this code fails to log the attempted access (CWE-778).
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