CVE-2024-26685

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

nilfs2: fix potential bug in end_buffer_async_write

According to a syzbot report, end_buffer_async_write(), which handles the completion of block device writes, may detect abnormal condition of the buffer async_write flag and cause a BUG_ON failure when using nilfs2.

Nilfs2 itself does not use end_buffer_async_write(). But, the async_write flag is now used as a marker by commit 7f42ec394156 (nilfs2: fix issue with race condition of competition between segments for dirty blocks) as a means of resolving double list insertion of dirty blocks in nilfs_lookup_dirty_data_buffers() and nilfs_lookup_node_buffers() and the resulting crash.

This modification is safe as long as it is used for file data and b-tree node blocks where the page caches are independent. However, it was irrelevant and redundant to also introduce async_write for segment summary and super root blocks that share buffers with the backing device. This led to the possibility that the BUG_ON check in end_buffer_async_write would fail as described above, if independent writebacks of the backing device occurred in parallel.

The use of async_write for segment summary buffers has already been removed in a previous change.

Fix this issue by removing the manipulation of the async_write flag for the remaining super root block buffer.

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/2c3bdba00283a6c7a5b19481a59a730f46063803
• https://git.kernel.org/stable/c/5bc09b397cbf1221f8a8aacb1152650c9195b02b
• https://git.kernel.org/stable/c/626daab3811b772086aef1bf8eed3ffe6f523eff
• https://git.kernel.org/stable/c/6589f0f72f8edd1fa11adce4eedbd3615f2e78ab
• https://git.kernel.org/stable/c/8fa90634ec3e9cc50f42dd605eec60f2d146ced8
• https://git.kernel.org/stable/c/c4a09fdac625e64abe478dcf88bfa20406616928
• https://git.kernel.org/stable/c/d31c8721e816eff5ca6573cc487754f357c093cd
• https://git.kernel.org/stable/c/f3e4963566f58726d3265a727116a42b591f6596
• https://git.kernel.org/stable/c/2c3bdba00283a6c7a5b19481a59a730f46063803
• https://git.kernel.org/stable/c/5bc09b397cbf1221f8a8aacb1152650c9195b02b
• https://git.kernel.org/stable/c/626daab3811b772086aef1bf8eed3ffe6f523eff
• https://git.kernel.org/stable/c/6589f0f72f8edd1fa11adce4eedbd3615f2e78ab
• https://git.kernel.org/stable/c/8fa90634ec3e9cc50f42dd605eec60f2d146ced8
• https://git.kernel.org/stable/c/c4a09fdac625e64abe478dcf88bfa20406616928
• https://git.kernel.org/stable/c/d31c8721e816eff5ca6573cc487754f357c093cd
• https://git.kernel.org/stable/c/f3e4963566f58726d3265a727116a42b591f6596
• https://lists.debian.org/debian-lts-announce/2024/06/msg00017.html
• https://lists.debian.org/debian-lts-announce/2024/06/msg00020.html