CVE-2024-26720: mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again

CVE-2024-26720: mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again

[Greg Kroah-Hartman]

Description
===========

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

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

(struct dirty_throttle_control *)->thresh is an unsigned long, but is
passed as the u32 divisor argument to div_u64().  On architectures where
unsigned long is 64 bytes, the argument will be implicitly truncated.

Use div64_u64() instead of div_u64() so that the value used in the "is
this a safe division" check is the same as the divisor.

Also, remove redundant cast of the numerator to u64, as that should happen
implicitly.

This would be difficult to exploit in memcg domain, given the ratio-based
arithmetic domain_drity_limits() uses, but is much easier in global
writeback domain with a BDI_CAP_STRICTLIMIT-backing device, using e.g. 
vm.dirty_bytes=(1<<32)*PAGE_SIZE so that dtc->thresh == (1<<32)

The Linux kernel CVE team has assigned CVE-2024-26720 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 3.16 with commit f6789593d5ce and fixed in 4.19.307 with commit c593d26fb5d5
	Issue introduced in 3.16 with commit f6789593d5ce and fixed in 5.4.269 with commit 1f12e4b3284d
	Issue introduced in 3.16 with commit f6789593d5ce and fixed in 5.10.210 with commit 81e7d2530d45
	Issue introduced in 3.16 with commit f6789593d5ce and fixed in 5.15.149 with commit 5099871b3703
	Issue introduced in 3.16 with commit f6789593d5ce and fixed in 6.1.79 with commit 16b1025eaa8f
	Issue introduced in 3.16 with commit f6789593d5ce and fixed in 6.6.18 with commit ec18ec230301
	Issue introduced in 3.16 with commit f6789593d5ce and fixed in 6.7.6 with commit 65977bed167a
	Issue introduced in 3.16 with commit f6789593d5ce and fixed in 6.8 with commit 9319b647902c
	Issue introduced in 3.12.27 with commit c5fec566bef6
	Issue introduced in 3.14.16 with commit c05d1fe6c19f
	Issue introduced in 3.15.9 with commit 3f7d325fbbe1

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-26720
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	mm/page-writeback.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/c593d26fb5d577ef31b6e49a31e08ae3ebc1bc1e
	https://git.kernel.org/stable/c/1f12e4b3284d6c863f272eb2de0d4248ed211cf4
	https://git.kernel.org/stable/c/81e7d2530d458548b90a5c5e76b77ad5e5d1c0df
	https://git.kernel.org/stable/c/5099871b370335809c0fd1abad74d9c7c205d43f
	https://git.kernel.org/stable/c/16b1025eaa8fc223ab4273ece20d1c3a4211a95d
	https://git.kernel.org/stable/c/ec18ec230301583395576915d274b407743d8f6c
	https://git.kernel.org/stable/c/65977bed167a92e87085e757fffa5798f7314c9f
	https://git.kernel.org/stable/c/9319b647902cbd5cc884ac08a8a6d54ce111fc78