[PATCH v3] hugetlbfs: check for pgoff value overflow

[Mike Kravetz]

A vma with vm_pgoff large enough to overflow a loff_t type when
converted to a byte offset can be passed via the remap_file_pages
system call.  The hugetlbfs mmap routine uses the byte offset to
calculate reservations and file size.

A sequence such as:
  mmap(0x20a00000, 0x600000, 0, 0x66033, -1, 0);
  remap_file_pages(0x20a00000, 0x600000, 0, 0x20000000000000, 0);
will result in the following when task exits/file closed,
  kernel BUG at mm/hugetlb.c:749!
Call Trace:
  hugetlbfs_evict_inode+0x2f/0x40
  evict+0xcb/0x190
  __dentry_kill+0xcb/0x150
  __fput+0x164/0x1e0
  task_work_run+0x84/0xa0
  exit_to_usermode_loop+0x7d/0x80
  do_syscall_64+0x18b/0x190
  entry_SYSCALL_64_after_hwframe+0x3d/0xa2

The overflowed pgoff value causes hugetlbfs to try to set up a
mapping with a negative range (end < start) that leaves invalid
state which causes the BUG.

The previous overflow fix to this code was incomplete and did not
take the remap_file_pages system call into account.

Fixes: 045c7a3f53d9 ("hugetlbfs: fix offset overflow in hugetlbfs mmap")
Cc: <stable@xxxxxxxxxxxxxxx>
Reported-by: Nic Losby <blurbdust@xxxxxxxxx>
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
Changes in v3
  * Use a simpler mask computation as suggested by Andrew Morton
Changes in v2
  * Use bitmask for overflow check as suggested by Yisheng Xie
  * Add explicit (from > to) check when setting up reservations
  * Cc stable

 fs/hugetlbfs/inode.c | 16 +++++++++++++---
 mm/hugetlb.c         |  6 ++++++
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 8fe1b0aa2896..e46117dc006a 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -108,6 +108,15 @@ static void huge_pagevec_release(struct pagevec *pvec)
 	pagevec_reinit(pvec);
 }
 
+/*
+ * Mask used when checking the page offset value passed in via system
+ * calls.  This value will be converted to a loff_t which is signed.
+ * Therefore, we want to check the upper PAGE_SHIFT + 1 bits of the
+ * value.  The extra bit (- 1 in the shift value) is to take the sign
+ * bit into account.
+ */
+#define PGOFF_LOFFT_MAX (PAGE_MASK << (BITS_PER_LONG - (2 * PAGE_SHIFT) - 1))
+
 static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	struct inode *inode = file_inode(file);
@@ -127,12 +136,13 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
 	vma->vm_ops = &hugetlb_vm_ops;
 
 	/*
-	 * Offset passed to mmap (before page shift) could have been
-	 * negative when represented as a (l)off_t.
+	 * page based offset in vm_pgoff could be sufficiently large to
+	 * overflow a (l)off_t when converted to byte offset.
 	 */
-	if (((loff_t)vma->vm_pgoff << PAGE_SHIFT) < 0)
+	if (vma->vm_pgoff & PGOFF_LOFFT_MAX)
 		return -EINVAL;
 
+	/* must be huge page aligned */
 	if (vma->vm_pgoff & (~huge_page_mask(h) >> PAGE_SHIFT))
 		return -EINVAL;
 
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 7c204e3d132b..8eeade0a0b7a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4374,6 +4374,12 @@ int hugetlb_reserve_pages(struct inode *inode,
 	struct resv_map *resv_map;
 	long gbl_reserve;
 
+	/* This should never happen */
+	if (from > to) {
+		VM_WARN(1, "%s called with a negative range\n", __func__);
+		return -EINVAL;
+	}
+
 	/*
 	 * Only apply hugepage reservation if asked. At fault time, an
 	 * attempt will be made for VM_NORESERVE to allocate a page
-- 
2.13.6