1 files changed, 9 insertions, 4 deletions

diff --git a/ipc/shm.c b/ipc/shm.c
index d7805acb44fd..06ea9ef7f54a 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1091,8 +1091,8 @@ out_unlock1:
  * "raddr" thing points to kernel space, and there has to be a wrapper around
  * this.
  */
-long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
-              unsigned long shmlba)
+long do_shmat(int shmid, char __user *shmaddr, int shmflg,
+              ulong *raddr, unsigned long shmlba)
 {
         struct shmid_kernel *shp;
         unsigned long addr;
@@ -1113,8 +1113,13 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
                 goto out;
         else if ((addr = (ulong)shmaddr)) {
                 if (addr & (shmlba - 1)) {
-                        if (shmflg & SHM_RND)
-                                addr &= ~(shmlba - 1);     /* round down */
+                        /*
+                         * Round down to the nearest multiple of shmlba.
+                         * For sane do_mmap_pgoff() parameters, avoid
+                         * round downs that trigger nil-page and MAP_FIXED.
+                         */
+                        if ((shmflg & SHM_RND) && addr >= shmlba)
+                                addr &= ~(shmlba - 1);
                         else
 #ifndef __ARCH_FORCE_SHMLBA
                                 if (addr & ~PAGE_MASK)