From: Frank Rowand Date: Sat, 1 Oct 2011 18:58:13 -0700 Subject: [PATCH 080/353] ARM: Initialize split page table locks for vector page Origin: https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit?id=be5374988b402d2bc249b848d12564514c117bbf Without this patch, ARM can not use SPLIT_PTLOCK_CPUS if PREEMPT_RT_FULL=y because vectors_user_mapping() creates a VM_ALWAYSDUMP mapping of the vector page (address 0xffff0000), but no ptl->lock has been allocated for the page. An attempt to coredump that page will result in a kernel NULL pointer dereference when follow_page() attempts to lock the page. The call tree to the NULL pointer dereference is: do_notify_resume() get_signal_to_deliver() do_coredump() elf_core_dump() get_dump_page() __get_user_pages() follow_page() pte_offset_map_lock() <----- a #define ... rt_spin_lock() The underlying problem is exposed by mm-shrink-the-page-frame-to-rt-size.patch. Signed-off-by: Frank Rowand Cc: Frank Cc: Peter Zijlstra Link: http://lkml.kernel.org/r/4E87C535.2030907@am.sony.com Signed-off-by: Thomas Gleixner --- arch/arm/kernel/process.c | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c index 82ab015bf42b..8d3c7ce34c24 100644 --- a/arch/arm/kernel/process.c +++ b/arch/arm/kernel/process.c @@ -324,6 +324,30 @@ unsigned long arch_randomize_brk(struct mm_struct *mm) } #ifdef CONFIG_MMU +/* + * CONFIG_SPLIT_PTLOCK_CPUS results in a page->ptl lock. If the lock is not + * initialized by pgtable_page_ctor() then a coredump of the vector page will + * fail. + */ +static int __init vectors_user_mapping_init_page(void) +{ + struct page *page; + unsigned long addr = 0xffff0000; + pgd_t *pgd; + pud_t *pud; + pmd_t *pmd; + + pgd = pgd_offset_k(addr); + pud = pud_offset(pgd, addr); + pmd = pmd_offset(pud, addr); + page = pmd_page(*(pmd)); + + pgtable_page_ctor(page); + + return 0; +} +late_initcall(vectors_user_mapping_init_page); + #ifdef CONFIG_KUSER_HELPERS /* * The vectors page is always readable from user space for the