Messages in this thread | | | Subject | Re: [PATCH v3] mm: fix race by making init_zero_pfn() early_initcall | From | Zhou Yanjie <> | Date | Tue, 30 Mar 2021 12:59:27 +0800 |
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Hi Ilya,
On 2021/3/30 下午12:42, Ilya Lipnitskiy wrote: > 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: > [<80340dc8>] kobject_uevent_env+0x7e4/0x7ec > [<8033f8b8>] kset_register+0x68/0x88 > [<803cf824>] bus_register+0xdc/0x34c > [<803cfac8>] subsys_virtual_register+0x34/0x78 > [<8086afb0>] wq_sysfs_init+0x1c/0x4c > [<80001648>] do_one_initcall+0x50/0x1a8 > [<8086503c>] kernel_init_freeable+0x230/0x2c8 > [<8066bca0>] kernel_init+0x10/0x100 > [<80003038>] 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: > [<8015adb4>] add_mm_counter_fast+0xb4/0xc0 > [<80160d58>] handle_mm_fault+0x6e4/0xea0 > [<80158aa4>] __get_user_pages.part.78+0x190/0x37c > [<8015992c>] __get_user_pages_remote+0x128/0x360 > [<801a6d9c>] get_arg_page+0x34/0xa0 > [<801a7394>] copy_string_kernel+0x194/0x2a4 > [<801a880c>] kernel_execve+0x11c/0x298 > [<800420f4>] 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: > [<800285e8>] __mmdrop+0x98/0x1d0 > [<801a6de8>] free_bprm+0x44/0x118 > [<801a86a8>] kernel_execve+0x160/0x1d8 > [<800420f4>] call_usermodehelper_exec_async+0x114/0x194 > [<80003198>] 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. > > Discussion: https://lkml.kernel.org/r/CALCv0x2YqOXEAy2Q=hafjhHCtTHVodChv1qpM=niAXOpqEbt7w@mail.gmail.com > > Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com> > Cc: Hugh Dickins <hughd@google.com> > Cc: "Eric W. Biederman" <ebiederm@xmission.com> > Cc: stable@vger.kernel.org > --- > mm/memory.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-)
Tested-by: 周琰杰 (Zhou Yanjie)<zhouyanjie@wanyeetech.com> # on CU1000-Neo/X1000E and CU1830-Neo/X1830
> diff --git a/mm/memory.c b/mm/memory.c > index 5c3b29d3af66..e66b11ac1659 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -166,7 +166,7 @@ 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); > > void mm_trace_rss_stat(struct mm_struct *mm, int member, long count) > {
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