In the Linux kernel, the following vulnerability has been resolved: x86/alternatives: Exclude text poking against change_page_attr() From time to time, the following BUG can be observed in the x86 alternatives patching code [0]: > k…

Overview

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

x86/alternatives: Exclude text poking against change_page_attr()

From time to time, the following BUG can be observed in the x86 alternatives patching code [0]:

kernel BUG at arch/x86/kernel/alternative.c:2576! Oops: invalid opcode: 0000 [#1] SMP NOPTI CPU: 0 UID: 0 PID: 355 Comm: (udev-worker) Not tainted 7.1.3-1-default #1 PREEMPT(full) openSUSE Tumbleweed 8c1795b03ec64f997e57a8ad38b1161e3b98da64 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS unknown 02/02/2022 RIP: 0010:__text_poke+0x2aa/0x450 Call Trace: <TASK> smp_text_poke_batch_finish+0x2a7/0x320 __static_call_transform+0xb7/0x220 arch_static_call_transform+0x5b/0xb0 __static_call_init+0xe9/0x270 static_call_module_notify+0x11f/0x150 notifier_call_chain+0x61/0xe0 blocking_notifier_call_chain_robust+0x63/0xc0 load_module+0x1c92/0x20c0 init_module_from_file+0xd8/0x140 idempotent_init_module+0x100/0x2f0 __x64_sys_finit_module+0x71/0xe0 do_syscall_64+0xe1/0x610 entry_SYSCALL_64_after_hwframe+0x76/0x7e

which matches the following BUG_ON() in alternative.c:

/*
 * If something went wrong, crash and burn since recovery paths are not
 * implemented.
 */
BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));

This can happen if vmalloc_to_page() fails, for any reason. Such can happen if text poking races with CPA, which can possibly result in the collapsing of page tables (or breaking of PMD hugepages). It is not a problem for most users of vmalloc_to_page() (they solely own the vmalloc'd range) but, when CONFIG_ARCH_HAS_EXECMEM_ROX=y, various modules own a single execmem vmalloc range, and can call set_memory_*() in parallel on it. This can happen to race against __text_poke and cause havoc in vmalloc_to_page().

Fix it by excluding against CPA using the init_mm mmap read lock.

[ dhansen: Fix up SoB ordering. The actual code flow here was: Pedro=>Lorenzo=>Mike=>Me which is reflected in the SoB chain now. I believe Mike simply picked up Lorenzo's update to Pedro's post from the Link ]

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.