ipc/shm.c: fix overly aggressive shmdt() when calls span multiple segments

This is a highly-contrived scenario.  But, a single shmdt() call can be
induced in to unmapping memory from mulitple shm segments.  Example code
is here:

	http://www.sr71.net/~dave/intel/shmfun.c

The fix is pretty simple: Record the 'struct file' for the first VMA we
encounter and then stick to it.  Decline to unmap anything not from the
same file and thus the same segment.

I found this by inspection and the odds of anyone hitting this in practice
are pretty darn small.

Lightly tested, but it's a pretty small patch.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 5 deletions

diff --git a/ipc/shm.c b/ipc/shm.c
index 01454796ba3c..e5dc7d246f05 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1229,6 +1229,7 @@ SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
         int retval = -EINVAL;
 #ifdef CONFIG_MMU
         loff_t size = 0;
+        struct file *file;
         struct vm_area_struct *next;
 #endif
 
@@ -1245,7 +1246,8 @@ SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
          *   started at address shmaddr. It records it's size and then unmaps
          *   it.
          * - Then it unmaps all shm vmas that started at shmaddr and that
-         *   are within the initially determined size.
+         *   are within the initially determined size and that are from the
+         *   same shm segment from which we determined the size.
          * Errors from do_munmap are ignored: the function only fails if
          * it's called with invalid parameters or if it's called to unmap
          * a part of a vma. Both calls in this function are for full vmas,
@@ -1271,8 +1273,14 @@ SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
                 if ((vma->vm_ops == &shm_vm_ops) &&
                         (vma->vm_start - addr)/PAGE_SIZE == vma->vm_pgoff) {
 
-
-                        size = file_inode(vma->vm_file)->i_size;
+                        /*
+                         * Record the file of the shm segment being
+                         * unmapped.  With mremap(), someone could place
+                         * page from another segment but with equal offsets
+                         * in the range we are unmapping.
+                         */
+                        file = vma->vm_file;
+                        size = file_inode(file)->i_size;
                         do_munmap(mm, vma->vm_start, vma->vm_end - vma->vm_start);
                         /*
                          * We discovered the size of the shm segment, so
@@ -1298,8 +1306,8 @@ SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
 
                 /* finding a matching vma now does not alter retval */
                 if ((vma->vm_ops == &shm_vm_ops) &&
-                        (vma->vm_start - addr)/PAGE_SIZE == vma->vm_pgoff)
-
+                    ((vma->vm_start - addr)/PAGE_SIZE == vma->vm_pgoff) &&
+                    (vma->vm_file == file))
                         do_munmap(mm, vma->vm_start, vma->vm_end - vma->vm_start);
                 vma = next;
         }