[PATCH 3.2 017/153] hugetlbfs: fix offset overflow in hugetlbfs mmap

[Ben Hutchings]

3.2.102-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

commit 045c7a3f53d9403b62d396b6d051c4be5044cdb4 upstream.

If mmap() maps a file, it can be passed an offset into the file at which
the mapping is to start.  Offset could be a negative value when
represented as a loff_t.  The offset plus length will be used to update
the file size (i_size) which is also a loff_t.

Validate the value of offset and offset + length to make sure they do
not overflow and appear as negative.

Found by syzcaller with commit ff8c0c53c475 ("mm/hugetlb.c: don't call
region_abort if region_chg fails") applied.  Prior to this commit, the
overflow would still occur but we would luckily return ENOMEM.

To reproduce:

   mmap(0, 0x2000, 0, 0x40021, 0xffffffffffffffffULL, 0x8000000000000000ULL);

Resulted in,

  kernel BUG at mm/hugetlb.c:742!
  Call Trace:
   hugetlbfs_evict_inode+0x80/0xa0
   evict+0x24a/0x620
   iput+0x48f/0x8c0
   dentry_unlink_inode+0x31f/0x4d0
   __dentry_kill+0x292/0x5e0
   dput+0x730/0x830
   __fput+0x438/0x720
   ____fput+0x1a/0x20
   task_work_run+0xfe/0x180
   exit_to_usermode_loop+0x133/0x150
   syscall_return_slowpath+0x184/0x1c0
   entry_SYSCALL_64_fastpath+0xab/0xad

Fixes: ff8c0c53c475 ("mm/hugetlb.c: don't call region_abort if region_chg fails")
Link: http://lkml.kernel.org/r/1491951118-30678-1-git-send-email-mike.kravetz@xxxxxxxxxx
Reported-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/hugetlbfs/inode.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -94,17 +94,26 @@ static int hugetlbfs_file_mmap(struct fi
 	vma->vm_flags |= VM_HUGETLB | VM_RESERVED;
 	vma->vm_ops = &hugetlb_vm_ops;
 
+	/*
+	 * Offset passed to mmap (before page shift) could have been
+	 * negative when represented as a (l)off_t.
+	 */
+	if (((loff_t)vma->vm_pgoff << PAGE_SHIFT) < 0)
+		return -EINVAL;
+
 	if (vma->vm_pgoff & (~huge_page_mask(h) >> PAGE_SHIFT))
 		return -EINVAL;
 
 	vma_len = (loff_t)(vma->vm_end - vma->vm_start);
+	len = vma_len + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
+	/* check for overflow */
+	if (len < vma_len)
+		return -EINVAL;
 
 	mutex_lock(&inode->i_mutex);
 	file_accessed(file);
 
 	ret = -ENOMEM;
-	len = vma_len + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
-
 	if (hugetlb_reserve_pages(inode,
 				vma->vm_pgoff >> huge_page_order(h),
 				len >> huge_page_shift(h), vma,
@@ -114,7 +123,7 @@ static int hugetlbfs_file_mmap(struct fi
 	ret = 0;
 	hugetlb_prefault_arch_hook(vma->vm_mm);
 	if (vma->vm_flags & VM_WRITE && inode->i_size < len)
-		inode->i_size = len;
+		i_size_write(inode, len);
 out:
 	mutex_unlock(&inode->i_mutex);