The NUMA balancing logic uses an arch-specific PROT_NONE page table flag defined by pte_protnone() or pmd_protnone() to mark PTEs or huge page PMDs respectively as requiring balancing upon a subsequent page fault. User-defined PROT_NONE memory regions which also have this flag set will not normally invoke the NUMA balancing code as do_page_fault() will send a segfault to the process before handle_mm_fault() is even called. However if access_remote_vm() is invoked to access a PROT_NONE region of memory, handle_mm_fault() is called via faultin_page() and __get_user_pages() without any access checks being performed, meaning the NUMA balancing logic is incorrectly invoked on a non-NUMA memory region. A simple means of triggering this problem is to access PROT_NONE mmap'd memory using /proc/self/mem which reliably results in the NUMA handling functions being invoked when CONFIG_NUMA_BALANCING is set. This issue was reported in bugzilla (issue 99101) which includes some simple repro code. There are BUG_ON() checks in do_numa_page() and do_huge_pmd_numa_page() added at commit c0e7cad to avoid accidentally provoking strange behaviour by attempting to apply NUMA balancing to pages that are in fact PROT_NONE. The BUG_ON()'s are consistently triggered by the repro. This patch moves the PROT_NONE check into mm/memory.c rather than invoking BUG_ON() as faulting in these pages via faultin_page() is a valid reason for reaching the NUMA check with the PROT_NONE page table flag set and is therefore not always a bug. Link: https://bugzilla.kernel.org/show_bug.cgi?id=99101 Reported-by: Trevor Saunders <tbsaunde@xxxxxxxxxxxx> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx> --- mm/huge_memory.c | 3 --- mm/memory.c | 12 +++++++----- 2 files changed, 7 insertions(+), 8 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index d76700d..954be55 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1198,9 +1198,6 @@ int do_huge_pmd_numa_page(struct fault_env *fe, pmd_t pmd) bool was_writable; int flags = 0; - /* A PROT_NONE fault should not end up here */ - BUG_ON(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))); - fe->ptl = pmd_lock(vma->vm_mm, fe->pmd); if (unlikely(!pmd_same(pmd, *fe->pmd))) goto out_unlock; diff --git a/mm/memory.c b/mm/memory.c index 020226b..aebc04f 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3351,9 +3351,6 @@ static int do_numa_page(struct fault_env *fe, pte_t pte) bool was_writable = pte_write(pte); int flags = 0; - /* A PROT_NONE fault should not end up here */ - BUG_ON(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))); - /* * The "pte" at this point cannot be used safely without * validation through pte_unmap_same(). It's of NUMA type but @@ -3458,6 +3455,11 @@ static int wp_huge_pmd(struct fault_env *fe, pmd_t orig_pmd) return VM_FAULT_FALLBACK; } +static inline bool vma_is_accessible(struct vm_area_struct *vma) +{ + return vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE); +} + /* * These routines also need to handle stuff like marking pages dirty * and/or accessed for architectures that don't do it in hardware (most @@ -3524,7 +3526,7 @@ static int handle_pte_fault(struct fault_env *fe) if (!pte_present(entry)) return do_swap_page(fe, entry); - if (pte_protnone(entry)) + if (pte_protnone(entry) && vma_is_accessible(fe->vma)) return do_numa_page(fe, entry); fe->ptl = pte_lockptr(fe->vma->vm_mm, fe->pmd); @@ -3590,7 +3592,7 @@ static int __handle_mm_fault(struct vm_area_struct *vma, unsigned long address, barrier(); if (pmd_trans_huge(orig_pmd) || pmd_devmap(orig_pmd)) { - if (pmd_protnone(orig_pmd)) + if (pmd_protnone(orig_pmd) && vma_is_accessible(vma)) return do_huge_pmd_numa_page(&fe, orig_pmd); if ((fe.flags & FAULT_FLAG_WRITE) && -- 2.9.3 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>