[PATCH] read_zero_pagealigned() locking fix

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: Linus Torvalds <torvalds@osdl.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 089020e0ee5a..4f1813e04754 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -646,7 +646,8 @@ static inline size_t read_zero_pagealigned(char __user * buf, size_t size)
                         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,