On Mon, 03 Nov 2014 16:06:33 -0800 Dave Hansen <dave@xxxxxxxx> wrote: > > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> > > 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 Could be preserved in tools/testing/selftests/ipc/ > 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. > > ... > > --- a/ipc/shm.c~mm-shmdt-fix-over-aggressive-unmap 2014-11-03 14:32:09.479595152 -0800 > +++ b/ipc/shm.c 2014-11-03 16:04:28.340225666 -0800 > @@ -1229,6 +1229,7 @@ SYSCALL_DEFINE1(shmdt, char __user *, sh > 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 *, sh > * 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 *, sh > 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; Maybe we should have used i_size_read() here. I don't think i_mutex is held? > do_munmap(mm, vma->vm_start, vma->vm_end - vma->vm_start); > /* > * We discovered the size of the shm segment, so -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>