CVE-2024-27036

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

cifs: Fix writeback data corruption

cifs writeback doesnt correctly handle the case where cifs_extend_writeback() hits a point where it is considering an additional folio, but this would overrun the wsize - at which point it drops out of the xarray scanning loop and calls xas_pause(). The problem is that xas_pause() advances the loop counter - thereby skipping that page.

What needs to happen is for xas_reset() to be called any time we decide we dont want to process the page were looking at, but rather send the request we are building and start a new one.

Fix this by copying and adapting the netfslib writepages code as a temporary measure, with cifs writeback intending to be offloaded to netfslib in the near future.

This also fixes the issue with the use of filemap_get_folios_tag() causing retry of a bunch of pages which the extender already dealt with.

This can be tested by creating, say, a 64K file somewhere not on cifs (otherwise copy-offload may get underfoot), mounting a cifs share with a wsize of 64000, copying the file to it and then comparing the original file and the copy:

    dd if=/dev/urandom of=/tmp/64K bs=64k count=1
    mount //192.168.6.1/test /mnt -o user=...,pass=...,wsize=64000
    cp /tmp/64K /mnt/64K
    cmp /tmp/64K /mnt/64K

Without the fix, the cmp fails at position 64000 (or shortly thereafter).

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/65f2ced695982ccd516196d0a9447d85dbe2eed5
• https://git.kernel.org/stable/c/844b4e132f57f1333dc79feaa035075a096762e4
• https://git.kernel.org/stable/c/e45deec35bf7f1f4f992a707b2d04a8c162f2240
• https://git.kernel.org/stable/c/f3dc1bdb6b0b0693562c7c54a6c28bafa608ba3c
• https://git.kernel.org/stable/c/65f2ced695982ccd516196d0a9447d85dbe2eed5
• https://git.kernel.org/stable/c/844b4e132f57f1333dc79feaa035075a096762e4
• https://git.kernel.org/stable/c/e45deec35bf7f1f4f992a707b2d04a8c162f2240
• https://git.kernel.org/stable/c/f3dc1bdb6b0b0693562c7c54a6c28bafa608ba3c