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Subject[patch 03/50] read_zero_pagealigned() locking fix
-stable review patch.  If anyone has any objections, please let us know.
------------------

From: Hugh Dickins <hugh@veritas.com>

Ramiro Voicu hits the BUG_ON(!pte_none(*pte)) in zeromap_pte_range: kernel
bugzilla 7645. Right: read_zero_pagealigned uses down_read of mmap_sem,
but another thread's racing read of /dev/zero, or a normal fault, can
easily set that pte again, in between zap_page_range and zeromap_page_range
getting there. It's been wrong ever since 2.4.3.

The simple fix is to use down_write instead, but that would serialize reads
of /dev/zero more than at present: perhaps some app would be badly
affected. So instead let zeromap_page_range return the error instead of
BUG_ON, and read_zero_pagealigned break to the slower clear_user loop in
that case - there's no need to optimize for it.

Use -EEXIST for when a pte is found: BUG_ON in mmap_zero (the other user of
zeromap_page_range), though it really isn't interesting there. And since
mmap_zero wants -EAGAIN for out-of-memory, the zeromaps better return that
than -ENOMEM.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Ramiro Voicu: <Ramiro.Voicu@cern.ch>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
---

drivers/char/mem.c | 12 ++++++++----
mm/memory.c | 32 +++++++++++++++++++++-----------
2 files changed, 29 insertions(+), 15 deletions(-)

--- linux-2.6.19.1.orig/drivers/char/mem.c
+++ linux-2.6.19.1/drivers/char/mem.c
@@ -646,7 +646,8 @@ static inline size_t read_zero_pagealign
count = size;

zap_page_range(vma, addr, count, NULL);
- zeromap_page_range(vma, addr, count, PAGE_COPY);
+ if (zeromap_page_range(vma, addr, count, PAGE_COPY))
+ break;

size -= count;
buf += count;
@@ -713,11 +714,14 @@ out:

static int mmap_zero(struct file * file, struct vm_area_struct * vma)
{
+ int err;
+
if (vma->vm_flags & VM_SHARED)
return shmem_zero_setup(vma);
- if (zeromap_page_range(vma, vma->vm_start, vma->vm_end - vma->vm_start, vma->vm_page_prot))
- return -EAGAIN;
- return 0;
+ err = zeromap_page_range(vma, vma->vm_start,
+ vma->vm_end - vma->vm_start, vma->vm_page_prot);
+ BUG_ON(err == -EEXIST);
+ return err;
}
#else /* CONFIG_MMU */
static ssize_t read_zero(struct file * file, char * buf,
--- linux-2.6.19.1.orig/mm/memory.c
+++ linux-2.6.19.1/mm/memory.c
@@ -1110,23 +1110,29 @@ static int zeromap_pte_range(struct mm_s
{
pte_t *pte;
spinlock_t *ptl;
+ int err = 0;

pte = pte_alloc_map_lock(mm, pmd, addr, &ptl);
if (!pte)
- return -ENOMEM;
+ return -EAGAIN;
arch_enter_lazy_mmu_mode();
do {
struct page *page = ZERO_PAGE(addr);
pte_t zero_pte = pte_wrprotect(mk_pte(page, prot));
+
+ if (unlikely(!pte_none(*pte))) {
+ err = -EEXIST;
+ pte++;
+ break;
+ }
page_cache_get(page);
page_add_file_rmap(page);
inc_mm_counter(mm, file_rss);
- BUG_ON(!pte_none(*pte));
set_pte_at(mm, addr, pte, zero_pte);
} while (pte++, addr += PAGE_SIZE, addr != end);
arch_leave_lazy_mmu_mode();
pte_unmap_unlock(pte - 1, ptl);
- return 0;
+ return err;
}

static inline int zeromap_pmd_range(struct mm_struct *mm, pud_t *pud,
@@ -1134,16 +1140,18 @@ static inline int zeromap_pmd_range(stru
{
pmd_t *pmd;
unsigned long next;
+ int err;

pmd = pmd_alloc(mm, pud, addr);
if (!pmd)
- return -ENOMEM;
+ return -EAGAIN;
do {
next = pmd_addr_end(addr, end);
- if (zeromap_pte_range(mm, pmd, addr, next, prot))
- return -ENOMEM;
+ err = zeromap_pte_range(mm, pmd, addr, next, prot);
+ if (err)
+ break;
} while (pmd++, addr = next, addr != end);
- return 0;
+ return err;
}

static inline int zeromap_pud_range(struct mm_struct *mm, pgd_t *pgd,
@@ -1151,16 +1159,18 @@ static inline int zeromap_pud_range(stru
{
pud_t *pud;
unsigned long next;
+ int err;

pud = pud_alloc(mm, pgd, addr);
if (!pud)
- return -ENOMEM;
+ return -EAGAIN;
do {
next = pud_addr_end(addr, end);
- if (zeromap_pmd_range(mm, pud, addr, next, prot))
- return -ENOMEM;
+ err = zeromap_pmd_range(mm, pud, addr, next, prot);
+ if (err)
+ break;
} while (pud++, addr = next, addr != end);
- return 0;
+ return err;
}

int zeromap_page_range(struct vm_area_struct *vma,
--
-
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