shm: fix null pointer deref when userspace specifies invalid hugepage size

Dave reported an oops triggered by trinity:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
  IP: newseg+0x10d/0x390
  PGD cf8c1067 PUD cf8c2067 PMD 0
  Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
  CPU: 2 PID: 7636 Comm: trinity-child2 Not tainted 3.9.0+#67
  ...
  Call Trace:
    ipcget+0x182/0x380
    SyS_shmget+0x5a/0x60
    tracesys+0xdd/0xe2

This bug was introduced by commit af73e4d9506d ("hugetlbfs: fix mmap
failure in unaligned size request").

Reported-by: Dave Jones <davej@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Li Zefan <lizfan@huawei.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 1ae21d645c68..f681e1842fad 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1367,9 +1367,13 @@ SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
                         len = ALIGN(len, huge_page_size(hstate_file(file)));
         } else if (flags & MAP_HUGETLB) {
                 struct user_struct *user = NULL;
+                struct hstate *hs = hstate_sizelog((flags >> MAP_HUGE_SHIFT) &
+                                                   SHM_HUGE_MASK);
 
-                len = ALIGN(len, huge_page_size(hstate_sizelog(
-                        (flags >> MAP_HUGE_SHIFT) & MAP_HUGE_MASK)));
+                if (!hs)
+                        return -EINVAL;
+
+                len = ALIGN(len, huge_page_size(hs));
                 /*
                  * VM_NORESERVE is used because the reservations will be
                  * taken when vm_ops->mmap() is called