MIPS: pgalloc: fix memory leak caused by pgd_free()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: MIPS: pgalloc: fix memory leak caused by pgd_free() pgd page is freed by generic implementation pgd_free() since commit f9cb654cb550 ("asm-generic: pgalloc: provide generic pgd_free()"), however, there are scenarios that the system uses more than one page as the pgd table, in such cases the generic implementation pgd_free() won't be applicable anymore. For example, when PAGE_SIZE_4KB is enabled and MIPS_VA_BITS_48 is not enabled in a 64bit system, the macro "PGD_ORDER" will be set as "1", which will cause allocating two pages as the pgd table. Well, at the same time, the generic implementation pgd_free() just free one pgd page, which will result in the memory leak. The memory leak can be easily detected by executing shell command: "while true; do ls > /dev/null; grep MemFree /proc/meminfo; done"

References

• https://git.kernel.org/stable/c/fa3d44424579972cc7c4fac3d9cf227798ebdfa0
• https://git.kernel.org/stable/c/d29cda15cab086d82d692de016f7249545d4b6b4
• https://git.kernel.org/stable/c/5a8501d34b261906e4c76ec9da679f2cb4d309ed
• https://git.kernel.org/stable/c/1bf0d78c8cc3cf615a6e7bf33ada70b73592f0a1
• https://git.kernel.org/stable/c/2bc5bab9a763d520937e4f3fe8df51c6a1eceb97