This is a note to let you know that I've just added the patch titled mm: fix race by making init_zero_pfn() early_initcall to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: mm-fix-race-by-making-init_zero_pfn-early_initcall.patch and it can be found in the queue-4.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From e720e7d0e983bf05de80b231bccc39f1487f0f16 Mon Sep 17 00:00:00 2001 From: Ilya Lipnitskiy <ilya.lipnitskiy@xxxxxxxxx> Date: Mon, 29 Mar 2021 21:42:08 -0700 Subject: mm: fix race by making init_zero_pfn() early_initcall MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: Ilya Lipnitskiy <ilya.lipnitskiy@xxxxxxxxx> commit e720e7d0e983bf05de80b231bccc39f1487f0f16 upstream. There are code paths that rely on zero_pfn to be fully initialized before core_initcall. For example, wq_sysfs_init() is a core_initcall function that eventually results in a call to kernel_execve, which causes a page fault with a subsequent mmput. If zero_pfn is not initialized by then it may not get cleaned up properly and result in an error: BUG: Bad rss-counter state mm:(ptrval) type:MM_ANONPAGES val:1 Here is an analysis of the race as seen on a MIPS device. On this particular MT7621 device (Ubiquiti ER-X), zero_pfn is PFN 0 until initialized, at which point it becomes PFN 5120: 1. wq_sysfs_init calls into kobject_uevent_env at core_initcall: kobject_uevent_env+0x7e4/0x7ec kset_register+0x68/0x88 bus_register+0xdc/0x34c subsys_virtual_register+0x34/0x78 wq_sysfs_init+0x1c/0x4c do_one_initcall+0x50/0x1a8 kernel_init_freeable+0x230/0x2c8 kernel_init+0x10/0x100 ret_from_kernel_thread+0x14/0x1c 2. kobject_uevent_env() calls call_usermodehelper_exec() which executes kernel_execve asynchronously. 3. Memory allocations in kernel_execve cause a page fault, bumping the MM reference counter: add_mm_counter_fast+0xb4/0xc0 handle_mm_fault+0x6e4/0xea0 __get_user_pages.part.78+0x190/0x37c __get_user_pages_remote+0x128/0x360 get_arg_page+0x34/0xa0 copy_string_kernel+0x194/0x2a4 kernel_execve+0x11c/0x298 call_usermodehelper_exec_async+0x114/0x194 4. In case zero_pfn has not been initialized yet, zap_pte_range does not decrement the MM_ANONPAGES RSS counter and the BUG message is triggered shortly afterwards when __mmdrop checks the ref counters: __mmdrop+0x98/0x1d0 free_bprm+0x44/0x118 kernel_execve+0x160/0x1d8 call_usermodehelper_exec_async+0x114/0x194 ret_from_kernel_thread+0x14/0x1c To avoid races such as described above, initialize init_zero_pfn at early_initcall level. Depending on the architecture, ZERO_PAGE is either constant or gets initialized even earlier, at paging_init, so there is no issue with initializing zero_pfn earlier. Link: https://lkml.kernel.org/r/CALCv0x2YqOXEAy2Q=hafjhHCtTHVodChv1qpM=niAXOpqEbt7w@xxxxxxxxxxxxxx Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@xxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Tested-by: 周琰杰 (Zhou Yanjie) <zhouyanjie@xxxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/memory.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/mm/memory.c +++ b/mm/memory.c @@ -132,7 +132,7 @@ static int __init init_zero_pfn(void) zero_pfn = page_to_pfn(ZERO_PAGE(0)); return 0; } -core_initcall(init_zero_pfn); +early_initcall(init_zero_pfn); #if defined(SPLIT_RSS_COUNTING) Patches currently in stable-queue which might be from ilya.lipnitskiy@xxxxxxxxx are queue-4.9/mm-fix-race-by-making-init_zero_pfn-early_initcall.patch