[RT,14/23] Revert "ARM: Initialize split page table locks for vector page"
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Message ID b1c7e28f28755f6bc202daf3b276f2874ac4c776.1582814004.git.zanussi@kernel.org
State New
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  • Linux v4.14.170-rt75-rc2
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Commit Message

Tom Zanussi Feb. 27, 2020, 2:33 p.m. UTC
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

v4.14.170-rt75-rc2 stable review patch.
If anyone has any objections, please let me know.


[ Upstream commit 247074c44d8c3e619dfde6404a52295d8d671d38 ]

I'm dropping this patch, with its original description:

|ARM: Initialize split page table locks for vector page
|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.

The patch named mm-shrink-the-page-frame-to-rt-size.patch was dropped
from the RT queue once the SPLIT_PTLOCK_CPUS feature (in a slightly
different shape) went upstream (somewhere between v3.12 and v3.14).

I can see that the patch still allocates a lock which wasn't there
before. However I can't trigger a kernel oops like described in the
patch by triggering a coredump.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
 arch/arm/kernel/process.c | 24 ------------------------
 1 file changed, 24 deletions(-)

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diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index cf4e1452d4b4..d96714e1858c 100644
--- a/arch/arm/kernel/process.c
+++ b/arch/arm/kernel/process.c
@@ -325,30 +325,6 @@  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;
  * The vectors page is always readable from user space for the