CVE-2024-26620

In the Linux kernel, the following vulnerability has been resolved:

s390/vfio-ap: always filter entire AP matrix

The vfio_ap_mdev_filter_matrix function is called whenever a new adapter or domain is assigned to the mdev. The purpose of the function is to update the guests AP configuration by filtering the matrix of adapters and domains assigned to the mdev. When an adapter or domain is assigned, only the APQNs associated with the APID of the new adapter or APQI of the new domain are inspected. If an APQN does not reference a queue device bound to the vfio_ap device driver, then its APID will be filtered from the mdevs matrix when updating the guests AP configuration.

Inspecting only the APID of the new adapter or APQI of the new domain will result in passing AP queues through to a guest that are not bound to the vfio_ap device driver under certain circumstances. Consider the following:

guests AP configuration (all also assigned to the mdevs matrix): 14.0004 14.0005 14.0006 16.0004 16.0005 16.0006

unassign domain 4 unbind queue 16.0005 assign domain 4

When domain 4 is re-assigned, since only domain 4 will be inspected, the APQNs that will be examined will be: 14.0004 16.0004

Since both of those APQNs reference queue devices that are bound to the vfio_ap device driver, nothing will get filtered from the mdevs matrix when updating the guests AP configuration. Consequently, queue 16.0005 will get passed through despite not being bound to the driver. This violates the linux device model requirement that a guest shall only be given access to devices bound to the device driver facilitating their pass-through.

To resolve this problem, every adapter and domain assigned to the mdev will be inspected when filtering the mdevs matrix.

Weakness

The product writes data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer.

• Be wary that a language’s interface to native code may still be subject to overflows, even if the language itself is theoretically safe.

• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• Examples include the Safe C String Library (SafeStr) by Messier and Viega [REF-57], and the Strsafe.h library from Microsoft [REF-56]. These libraries provide safer versions of overflow-prone string-handling functions.

• Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking.

• D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail.

• Consider adhering to the following rules when allocating and managing an application’s memory:

• Run or compile the software using features or extensions that randomly arrange the positions of a program’s executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code.

• Examples include Address Space Layout Randomization (ASLR) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64]. Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as “rebasing” (for Windows) and “prelinking” (for Linux) [REF-1332] using randomly generated addresses. ASLR for libraries cannot be used in conjunction with prelink since it would require relocating the libraries at run-time, defeating the whole purpose of prelinking.

• For more information on these techniques see D3-SAOR (Segment Address Offset Randomization) from D3FEND [REF-1335].

• Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX [REF-60] [REF-61]. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment.

• For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND [REF-1336].

References

• https://git.kernel.org/stable/c/850fb7fa8c684a4c6bf0e4b6978f4ddcc5d43d11
• https://git.kernel.org/stable/c/c69d821197611678533fb3eb784fc823b921349a
• https://git.kernel.org/stable/c/cdd134d56138302976685e6c7bc4755450b3880e
• https://git.kernel.org/stable/c/d6b8d034b576f406af920a7bee81606c027b24c6
• https://git.kernel.org/stable/c/850fb7fa8c684a4c6bf0e4b6978f4ddcc5d43d11
• https://git.kernel.org/stable/c/c69d821197611678533fb3eb784fc823b921349a
• https://git.kernel.org/stable/c/cdd134d56138302976685e6c7bc4755450b3880e
• https://git.kernel.org/stable/c/d6b8d034b576f406af920a7bee81606c027b24c6