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Subject[PATCH 13/15] mm: get_user_pages check count
Most calls to get_user_pages soon release the pages and then return to
user space, but some are long term - notably Infiniband's ib_umem_get.
That's blessed with good locked_vm checking, but if the system were
misconfigured, then it might be possible to build up a huge page_count.

Guard against overflow by page_count_too_high checks in get_user_pages:
refuse with -ENOMEM once page count reaches its final PID_MAX_LIMIT
(which is why we allowed for 2*PID_MAX_LIMIT in the 23 bits of count).

Sorry, can't touch get_user_pages without giving it more cleanup.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---
include/linux/mm.h | 16 ++++++++++++++++
mm/memory.c | 40 +++++++++++++++++++++++-----------------
2 files changed, 39 insertions(+), 17 deletions(-)
--- mm12/include/linux/mm.h 2005-11-09 14:40:00.000000000 +0000
+++ mm13/include/linux/mm.h 2005-11-09 14:40:17.000000000 +0000
@@ -312,6 +312,7 @@ struct page {
*/
#define PCOUNT_SHIFT 23
#define PCOUNT_MASK ((1UL << PCOUNT_SHIFT) - 1)
+#define PCOUNT_HIGH (PCOUNT_MASK - PID_MAX_LIMIT)

/*
* Drop a ref, return true if the logical refcount fell to zero
@@ -377,8 +378,17 @@ static inline int page_mapped(struct pag
return (unsigned long)atomic64_read(&page->_pcount) > PCOUNT_MASK;
}

+static inline int page_count_too_high(struct page *page)
+{
+ /* get_user_pages check when nearing overflow */
+ return ((unsigned long)atomic64_read(&page->_pcount) & PCOUNT_MASK)
+ >= PCOUNT_HIGH;
+}
+
#else /* !ATOMIC64_INIT */

+#define PCOUNT_HIGH (INT_MAX - PID_MAX_LIMIT)
+
/*
* Drop a ref, return true if the logical refcount fell to zero
* (the page has no users)
@@ -437,6 +447,12 @@ static inline int page_mapped(struct pag
return atomic_read(&page->_mapcount) >= 0;
}

+static inline int page_count_too_high(struct page *page)
+{
+ /* get_user_pages check when nearing overflow */
+ return atomic_read(&page->_count) >= PCOUNT_HIGH;
+}
+
#endif /* !ATOMIC64_INIT */

void FASTCALL(__page_cache_release(struct page *));
--- mm12/mm/memory.c 2005-11-09 14:40:00.000000000 +0000
+++ mm13/mm/memory.c 2005-11-09 14:40:17.000000000 +0000
@@ -928,39 +928,43 @@ int get_user_pages(struct task_struct *t
do {
struct vm_area_struct *vma;
unsigned int foll_flags;
+ struct page *page;

vma = find_extend_vma(mm, start);
if (!vma && in_gate_area(tsk, start)) {
- unsigned long pg = start & PAGE_MASK;
- struct vm_area_struct *gate_vma = get_gate_vma(tsk);
pgd_t *pgd;
pud_t *pud;
pmd_t *pmd;
pte_t *pte;
+ pte_t ptent;
+
if (write) /* user gate pages are read-only */
return i ? : -EFAULT;
- if (pg > TASK_SIZE)
- pgd = pgd_offset_k(pg);
+ start &= PAGE_MASK; /* what needs that? */
+ if (start >= TASK_SIZE)
+ pgd = pgd_offset_k(start);
else
- pgd = pgd_offset_gate(mm, pg);
+ pgd = pgd_offset_gate(mm, start);
BUG_ON(pgd_none(*pgd));
- pud = pud_offset(pgd, pg);
+ pud = pud_offset(pgd, start);
BUG_ON(pud_none(*pud));
- pmd = pmd_offset(pud, pg);
+ pmd = pmd_offset(pud, start);
if (pmd_none(*pmd))
return i ? : -EFAULT;
- pte = pte_offset_map(pmd, pg);
- if (pte_none(*pte)) {
- pte_unmap(pte);
+ pte = pte_offset_map(pmd, start);
+ ptent = *pte;
+ pte_unmap(pte);
+ if (pte_none(ptent))
return i ? : -EFAULT;
- }
if (pages) {
- pages[i] = pte_page(*pte);
- get_page(pages[i]);
+ page = pte_page(ptent);
+ if (page_count_too_high(page))
+ return i ? : -ENOMEM;
+ get_page(page);
+ pages[i] = page;
}
- pte_unmap(pte);
if (vmas)
- vmas[i] = gate_vma;
+ vmas[i] = get_gate_vma(tsk);
i++;
start += PAGE_SIZE;
len--;
@@ -985,8 +989,6 @@ int get_user_pages(struct task_struct *t
foll_flags |= FOLL_ANON;

do {
- struct page *page;
-
if (write)
foll_flags |= FOLL_WRITE;

@@ -1020,6 +1022,10 @@ int get_user_pages(struct task_struct *t
}
}
if (pages) {
+ if (page_count_too_high(page)) {
+ put_page(page);
+ return i ? : -ENOMEM;
+ }
pages[i] = page;
flush_dcache_page(page);
}
-
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