CVE-2024-53142

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

initramfs: avoid filename buffer overrun

The initramfs filename field is defined in Documentation/driver-api/early-userspace/buffer-format.rst as:

37 cpio_file := ALGN(4) + cpio_header + filename + 0 + ALGN(4) + data … 55 ============= ================== ========================= 56 Field name Field size Meaning 57 ============= ================== ========================= … 70 c_namesize 8 bytes Length of filename, including final 0

When extracting an initramfs cpio archive, the kernels do_name() path handler assumes a zero-terminated path at @collected, passing it directly to filp_open() / init_mkdir() / init_mknod().

If a specially crafted cpio entry carries a non-zero-terminated filename and is followed by uninitialized memory, then a file may be created with trailing characters that represent the uninitialized memory. The ability to create an initramfs entry would imply already having full control of the system, so the buffer overrun shouldnt be considered a security vulnerability.

Append the output of the following bash script to an existing initramfs and observe any created /initramfs_test_fname_overrunAA* path. E.g. ./reproducer.sh | gzip » /myinitramfs

Its easiest to observe non-zero uninitialized memory when the output is gzipped, as itll overflow the heap allocated @out_buf in __gunzip(), rather than the initrd_start+initrd_size block.

—- reproducer.sh —- nilchar=A # change to 0 to properly zero terminate / pad magic=070701 ino=1 mode=$(( 0100777 )) uid=0 gid=0 nlink=1 mtime=1 filesize=0 devmajor=0 devminor=1 rdevmajor=0 rdevminor=0 csum=0 fname=initramfs_test_fname_overrun namelen=$(( ${#fname} + 1 )) # plus one to account for terminator

printf %s%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%s $magic $ino $mode $uid $gid $nlink $mtime $filesize $devmajor $devminor $rdevmajor $rdevminor $namelen $csum $fname

termpadlen=$(( 1 + ((4 - ((110 + $namelen) & 3)) % 4) )) printf %.s${nilchar} $(seq 1 $termpadlen) —- reproducer.sh —-

Symlink filename fields handled in do_symlink() wont overrun past the data segment, due to the explicit zero-termination of the symlink target.

Fix filename buffer overrun by aborting the initramfs FSM if any cpio entry doesnt carry a zero-terminator at the expected (name_len - 1) offset.

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/1a423bbbeaf9e3e20c4686501efd9b661fe834db
• https://git.kernel.org/stable/c/49d01e736c3045319e030d1e75fb983011abaca7
• https://git.kernel.org/stable/c/6983b8ac787b3add5571cda563574932a59a99bb
• https://git.kernel.org/stable/c/bb7ac96670ab1d8d681015f9d66e45dad579af4d
• https://git.kernel.org/stable/c/c509b1acbd867d9e09580fe059a924cb5825afb1
• https://git.kernel.org/stable/c/d3df9f26cff97beaa5643e551031795d5d5cddbe
• https://git.kernel.org/stable/c/e017671f534dd3f568db9e47b0583e853d2da9b5
• https://git.kernel.org/stable/c/f892ddcf9f645380c358e73653cb0900f6bc9eb8
• https://git.kernel.org/stable/c/fb83b093f75806333b6f4ae29b158d2e0e3ec971