[PATCH 3.2 018/153] hugetlbfs: check for pgoff value overflow

[Ben Hutchings]

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

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

From: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

commit 63489f8e821144000e0bdca7e65a8d1cc23a7ee7 upstream.

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.

[mike.kravetz@xxxxxxxxxx: v3]
  Link: http://lkml.kernel.org/r/20180309002726.7248-1-mike.kravetz@xxxxxxxxxx
[akpm@xxxxxxxxxxxxxxxxxxxx: include mmdebug.h]
[akpm@xxxxxxxxxxxxxxxxxxxx: fix -ve left shift count on sh]
Link: http://lkml.kernel.org/r/20180308210502.15952-1-mike.kravetz@xxxxxxxxxx
Fixes: 045c7a3f53d9 ("hugetlbfs: fix offset overflow in hugetlbfs mmap")
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Reported-by: Nic Losby <blurbdust@xxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Yisheng Xie <xieyisheng1@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2:
 - Use a conditional WARN() instead of VM_WARN()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/hugetlbfs/inode.c | 17 ++++++++++++++---
 mm/hugetlb.c         |  7 +++++++
 2 files changed, 21 insertions(+), 3 deletions(-)

--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -76,6 +76,16 @@ static void huge_pagevec_release(struct
 	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 \
+	(((1UL << (PAGE_SHIFT + 1)) - 1) <<  (BITS_PER_LONG - (PAGE_SHIFT + 1)))
+
 static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	struct inode *inode = file->f_path.dentry->d_inode;
@@ -95,12 +105,13 @@ static int hugetlbfs_file_mmap(struct fi
 	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;
 
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -18,6 +18,7 @@
 #include <linux/bootmem.h>
 #include <linux/sysfs.h>
 #include <linux/slab.h>
+#include <linux/mmdebug.h>
 #include <linux/rmap.h>
 #include <linux/swap.h>
 #include <linux/swapops.h>
@@ -3097,6 +3098,14 @@ int hugetlb_reserve_pages(struct inode *
 	struct hstate *h = hstate_inode(inode);
 	struct hugepage_subpool *spool = subpool_inode(inode);
 
+	/* This should never happen */
+	if (from > to) {
+#ifdef CONFIG_DEBUG_VM
+		WARN(1, "%s called with a negative range\n", __func__);
+#endif
+		return -EINVAL;
+	}
+
 	/*
 	 * Only apply hugepage reservation if asked. At fault time, an
 	 * attempt will be made for VM_NORESERVE to allocate a page