CVE-2024-42102

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

Revert mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again

Patch series mm: Avoid possible overflows in dirty throttling.

Dirty throttling logic assumes dirty limits in page units fit into 32-bits. This patch series makes sure this is true (see patch 2/2 for more details).

This patch (of 2):

This reverts commit 9319b647902cbd5cc884ac08a8a6d54ce111fc78.

The commit is broken in several ways. Firstly, the removed (u64) cast from the multiplication will introduce a multiplication overflow on 32-bit archs if wb_thresh * bg_thresh >= 1«32 (which is actually common - the default settings with 4GB of RAM will trigger this). Secondly, the div64_u64() is unnecessarily expensive on 32-bit archs. We have div64_ul() in case we want to be safe & cheap. Thirdly, if dirty thresholds are larger than 1«32 pages, then dirty balancing is going to blow up in many other spectacular ways anyway so trying to fix one possible overflow is just moot.

Weakness

The product divides a value by zero.

Affected Software

References

• https://git.kernel.org/stable/c/000099d71648504fb9c7a4616f92c2b70c3e44ec
• https://git.kernel.org/stable/c/145faa3d03688cbb7bbaaecbd84c01539852942c
• https://git.kernel.org/stable/c/23a28f5f3f6ca1e4184bd0e9631cd0944cf1c807
• https://git.kernel.org/stable/c/253f9ea7e8e53a5176bd80ceb174907b10724c1a
• https://git.kernel.org/stable/c/2820005edae13b140f2d54267d1bd6bb23915f59
• https://git.kernel.org/stable/c/30139c702048f1097342a31302cbd3d478f50c63
• https://git.kernel.org/stable/c/cbbe17a324437c0ff99881a3ee453da45b228a00
• https://git.kernel.org/stable/c/f6620df12cb6bdcad671d269debbb23573502f9d
• https://git.kernel.org/stable/c/000099d71648504fb9c7a4616f92c2b70c3e44ec
• https://git.kernel.org/stable/c/145faa3d03688cbb7bbaaecbd84c01539852942c
• https://git.kernel.org/stable/c/23a28f5f3f6ca1e4184bd0e9631cd0944cf1c807
• https://git.kernel.org/stable/c/253f9ea7e8e53a5176bd80ceb174907b10724c1a
• https://git.kernel.org/stable/c/2820005edae13b140f2d54267d1bd6bb23915f59
• https://git.kernel.org/stable/c/30139c702048f1097342a31302cbd3d478f50c63
• https://git.kernel.org/stable/c/cbbe17a324437c0ff99881a3ee453da45b228a00
• https://git.kernel.org/stable/c/f6620df12cb6bdcad671d269debbb23573502f9d
• https://lists.debian.org/debian-lts-announce/2025/01/msg00001.html