Re: [PATCH STABLE 4.9 1/1] mm, gup: add missing refcount overflow checks on x86 and s390

On Fri, 2019-11-29 at 10:03 +0100, Vlastimil Babka wrote:

The mainline commit 8fde12ca79af ("mm: prevent get_user_pages() from overflowing page refcount") was backported to 4.9.y stable as commit 2ed768cfd895. The backport however missed that in 4.9, there are several arch-specific gup.c versions with fast gup implementations, so these do not prevent refcount overflow.

This is partially fixed for x86 in stable-only commit d73af79742e7 ("x86, mm, gup: prevent get_page() race with munmap in paravirt guest"). This stable-only commit adds missing parts to x86 version, as well as s390 version, both taken from the SUSE SLES/openSUSE 4.12-based kernels.

The remaining architectures with own gup.c are sparc, mips, sh. It's unlikely the known overflow scenario based on FUSE, which needs 140GB of RAM, is a problem for those architectures, and I don't feel confident enough to patch them.

Signed-off-by: Vlastimil Babka vbabka@suse.cz

arch/s390/mm/gup.c | 9 ++++++--- arch/x86/mm/gup.c | 10 ++++++++-- 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/s390/mm/gup.c b/arch/s390/mm/gup.c index 97fc449a7470..33a940389a6d 100644 --- a/arch/s390/mm/gup.c +++ b/arch/s390/mm/gup.c @@ -38,7 +38,8 @@ static inline int gup_pte_range(pmd_t *pmdp, pmd_t pmd, unsigned long addr, VM_BUG_ON(!pfn_valid(pte_pfn(pte))); page = pte_page(pte); head = compound_head(page);

• if (!page_cache_get_speculative(head))
  

• if (unlikely(WARN_ON_ONCE(page_ref_count(head) < 0)
  

No need for unlikely(); WARN_ON() includes that.

•     || !page_cache_get_speculative(head)))
  return 0;
  

  if (unlikely(pte_val(pte) != pte_val(*ptep))) { put_page(head);

[...]

--- a/arch/x86/mm/gup.c +++ b/arch/x86/mm/gup.c @@ -202,10 +202,12 @@ static int __gup_device_huge_pmd(pmd_t pmd, unsigned long addr, undo_dev_pagemap(nr, nr_start, pages); return 0; }

• if (unlikely(!try_get_page(page))) {
  

• 	put_dev_pagemap(pgmap);
  

• 	return 0;
  

• }
  

  SetPageReferenced(page); pages[*nr] = page;

• get_page(page);
  

• put_dev_pagemap(pgmap);
  

This leaks a pgmap reference on success!

(*nr)++;
pfn++;

} while (addr += PAGE_SIZE, addr != end); @@ -230,6 +232,8 @@ static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr, refs = 0; head = pmd_page(pmd);

• if (WARN_ON_ONCE(page_ref_count(head) <= 0))

Why <= 0, given we use < 0 elsewhere?

• return 0;
  

  page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT); do { VM_BUG_ON_PAGE(compound_head(page) != head, page);

@@ -289,6 +293,8 @@ static noinline int gup_huge_pud(pud_t pud, unsigned long addr, refs = 0; head = pud_page(pud);

• if (WARN_ON_ONCE(page_ref_count(head) <= 0))

Same question here.

Ben.

• return 0;
  

  page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT); do { VM_BUG_ON_PAGE(compound_head(page) != head, page);