From: Thomas Hellstrom thellstrom@vmware.com Subject: mm/memory.c: fix a huge pud insertion race during faulting

A huge pud page can theoretically be faulted in racing with pmd_alloc() in __handle_mm_fault(). That will lead to pmd_alloc() returning an invalid pmd pointer. Fix this by adding a pud_trans_unstable() function similar to pmd_trans_unstable() and check whether the pud is really stable before using the pmd pointer.

Race: Thread 1: Thread 2: Comment create_huge_pud() Fallback - not taken. create_huge_pud() Taken. pmd_alloc() Returns an invalid pointer.

This will result in user-visible huge page data corruption.

Note that this was caught during a code audit rather than a real experienced problem. It looks to me like the only implementation that currently creates huge pud pagetable entries is dev_dax_huge_fault() which doesn't appear to care much about private (COW) mappings or write-tracking which is, I believe, a prerequisite for create_huge_pud() falling back on thread 1, but not in thread 2.

Link: http://lkml.kernel.org/r/20191115115808.21181-2-thomas_os@shipmail.org Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages") Signed-off-by: Thomas Hellstrom thellstrom@vmware.com Acked-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Arnd Bergmann arnd@arndb.de Cc: Matthew Wilcox willy@infradead.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/asm-generic/pgtable.h | 25 +++++++++++++++++++++++++ mm/memory.c | 6 ++++++ 2 files changed, 31 insertions(+)

--- a/include/asm-generic/pgtable.h~mm-fix-a-huge-pud-insertion-race-during-faulting +++ a/include/asm-generic/pgtable.h @@ -938,6 +938,31 @@ static inline int pud_trans_huge(pud_t p } #endif

+/* See pmd_none_or_trans_huge_or_clear_bad for discussion. */ +static inline int pud_none_or_trans_huge_or_dev_or_clear_bad(pud_t *pud) +{ + pud_t pudval = READ_ONCE(*pud); + + if (pud_none(pudval) || pud_trans_huge(pudval) || pud_devmap(pudval)) + return 1; + if (unlikely(pud_bad(pudval))) { + pud_clear_bad(pud); + return 1; + } + return 0; +} + +/* See pmd_trans_unstable for discussion. */ +static inline int pud_trans_unstable(pud_t *pud) +{ +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && \ + defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD) + return pud_none_or_trans_huge_or_dev_or_clear_bad(pud); +#else + return 0; +#endif +} + #ifndef pmd_read_atomic static inline pmd_t pmd_read_atomic(pmd_t *pmdp) { --- a/mm/memory.c~mm-fix-a-huge-pud-insertion-race-during-faulting +++ a/mm/memory.c @@ -4010,6 +4010,7 @@ static vm_fault_t __handle_mm_fault(stru vmf.pud = pud_alloc(mm, p4d, address); if (!vmf.pud) return VM_FAULT_OOM; +retry_pud: if (pud_none(*vmf.pud) && __transparent_hugepage_enabled(vma)) { ret = create_huge_pud(&vmf); if (!(ret & VM_FAULT_FALLBACK)) @@ -4036,6 +4037,11 @@ static vm_fault_t __handle_mm_fault(stru vmf.pmd = pmd_alloc(mm, vmf.pud, address); if (!vmf.pmd) return VM_FAULT_OOM; + + /* Huge pud page fault raced with pmd_alloc? */ + if (pud_trans_unstable(vmf.pud)) + goto retry_pud; + if (pmd_none(*vmf.pmd) && __transparent_hugepage_enabled(vma)) { ret = create_huge_pmd(&vmf); if (!(ret & VM_FAULT_FALLBACK)) _