mm: avoid wrapping vm_pgoff in mremap()

The normal mmap paths all avoid creating a mapping where the pgoff
inside the mapping could wrap around due to overflow.  However, an
expanding mremap() can take such a non-wrapping mapping and make it
bigger and cause a wrapping condition.

Noticed by Robert Swiecki when running a system call fuzzer, where it
caused a BUG_ON() due to terminally confusing the vma_prio_tree code.  A
vma dumping patch by Hugh then pinpointed the crazy wrapped case.

Reported-and-tested-by: Robert Swiecki <robert@swiecki.net>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 2 deletions

diff --git a/mm/mremap.c b/mm/mremap.c
index 1de98d492ddc..a7c1f9f9b941 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -277,9 +277,16 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
         if (old_len > vma->vm_end - addr)
                 goto Efault;
 
-        if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP)) {
-                if (new_len > old_len)
+        /* Need to be careful about a growing mapping */
+        if (new_len > old_len) {
+                unsigned long pgoff;
+
+                if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
                         goto Efault;
+                pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
+                pgoff += vma->vm_pgoff;
+                if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
+                        goto Einval;
         }
 
         if (vma->vm_flags & VM_LOCKED) {