x86/mm: Fix pti_clone_pgtable() alignment assumption

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: x86/mm: Fix pti_clone_pgtable() alignment assumption Guenter reported dodgy crashes on an i386-nosmp build using GCC-11 that had the form of endless traps until entry stack exhaust and then #DF from the stack guard. It turned out that pti_clone_pgtable() had alignment assumptions on the start address, notably it hard assumes start is PMD aligned. This is true on x86_64, but very much not true on i386. These assumptions can cause the end condition to malfunction, leading to a 'short' clone. Guess what happens when the user mapping has a short copy of the entry text? Use the correct increment form for addr to avoid alignment assumptions.

References

• https://git.kernel.org/stable/c/18da1b27ce16a14a9b636af9232acb4fb24f4c9e
• https://git.kernel.org/stable/c/25a727233a40a9b33370eec9f0cad67d8fd312f8
• https://git.kernel.org/stable/c/d00c9b4bbc442d99e1dafbdfdab848bc1ead73f6
• https://git.kernel.org/stable/c/4d143ae782009b43b4f366402e5c37f59d4e4346
• https://git.kernel.org/stable/c/5c580c1050bcbc15c3e78090859d798dcf8c9763
• https://git.kernel.org/stable/c/ca07aab70dd3b5e7fddb62d7a6ecd7a7d6d0b2ed
• https://git.kernel.org/stable/c/df3eecb5496f87263d171b254ca6e2758ab3c35c
• https://git.kernel.org/stable/c/41e71dbb0e0a0fe214545fe64af031303a08524c