[PATCH v3 1/2] ARM: mm: fix use-after-free in __do_user_fault() under CONFIG_DEBUG_USER

[Qi Xi]

When CONFIG_DEBUG_USER is enabled with user_debug=31 on 32-bit ARM,
a user page fault triggers show_pte() via __do_user_fault() after
do_page_fault() has already released mmap_read_lock. If another
thread concurrently calls munmap(), the page table pages can be
freed while show_pte() is still reading them, causing a
use-after-free in show_pte().

The race can be reproduced on multi_v7_defconfig with:
    CONFIG_DEBUG_USER=y
    CONFIG_ARM_LPAE=y
    kernel command line: user_debug=31

A delay inserted in show_pte() for testing widens the race window and
makes the UAF reliably reproducible. On LPAE, the race works as
follows:

  CPU 0 (fault path)                       CPU 1 (munmap)
  munmap(page 0) -> clears PTE[0]
  PTE/PMD pages remain

  read page 0 -> page fault
    -> do_DataAbort()
      -> do_page_fault()
        -> lock_mm_and_find_vma() -> no VMA
           (mmap_read_lock released)
        -> __do_user_fault()
          -> show_pte(tsk->mm, addr)
            -> *pgd (valid)
            -> p4d/pud checks pass

            -> [delay]                     munmap(page 1)
                                             -> clears PTE[1]
                                             -> PTE/PMD pages freed
                                             -> PGD cleared

            -> pmd_offset(pud, addr)
              -> *pud=0 -> __va(0)
              -> dereference
              -> secondary data abort (kernel)

Fix by taking mmap_read_lock() around show_pte() in __do_user_fault().
__do_user_fault() is called from process context with interrupts
enabled, so the context can sleep and mmap_read_lock() is safe here.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Tested-by: Yuanbin Xie <xieyuanbin1@xxxxxxxxxx>
Suggested-by: Yuanbin Xie <xieyuanbin1@xxxxxxxxxx>
Signed-off-by: Qi Xi <xiqi2@xxxxxxxxxx>
---
 arch/arm/mm/fault.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index e62cc4be5a..1f2a85e1fa 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -181,7 +181,9 @@ __do_user_fault(unsigned long addr, unsigned int fsr, unsigned int sig,
 		pr_err("8<--- cut here ---\n");
 		pr_err("%s: unhandled page fault (%d) at 0x%08lx, code 0x%03x\n",
 		       tsk->comm, sig, addr, fsr);
+		mmap_read_lock(tsk->mm);
 		show_pte(KERN_ERR, tsk->mm, addr);
+		mmap_read_unlock(tsk->mm);
 		show_regs(regs);
 	}
 #endif
-- 
2.33.0