uprobes: ensure that uprobe->offset and ->ref_ctr_offset are properly aligned

uprobe_write_opcode() must not cross page boundary; prepare_uprobe()
relies on arch_uprobe_analyze_insn() which should validate "vaddr" but
some architectures (csky, s390, and sparc) don't do this.

We can remove the BUG_ON() check in prepare_uprobe() and validate the
offset early in __uprobe_register(). The new IS_ALIGNED() check matches
the alignment check in arch_prepare_kprobe() on supported architectures,
so I think that all insns must be aligned to UPROBE_SWBP_INSN_SIZE.

Another problem is __update_ref_ctr() which was wrong from the very
beginning, it can read/write outside of kmap'ed page unless "vaddr" is
aligned to sizeof(short), __uprobe_register() should check this too.

Reported-by: Linus Torvalds <[e-mail]>
Suggested-by: Linus Torvalds <[e-mail]>
Signed-off-by: Oleg Nesterov <[e-mail]>
Reviewed-by: Srikar Dronamraju <[e-mail]>
Acked-by: Christian Borntraeger <[e-mail]>
Tested-by: Sven Schnelle <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: [e-mail]
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/events/uprobes.c

···
 	if (ret)
 		goto out;
 
-	/* uprobe_write_opcode() assumes we don't cross page boundary */
-	BUG_ON((uprobe->offset & ~PAGE_MASK) +
-			UPROBE_SWBP_INSN_SIZE > PAGE_SIZE);
-
 	smp_wmb(); /* pairs with the smp_rmb() in handle_swbp() */
 	set_bit(UPROBE_COPY_INSN, &uprobe->flags);
 
···
 		return -EIO;
 	/* Racy, just to catch the obvious mistakes */
 	if (offset > i_size_read(inode))
+		return -EINVAL;
+
+	/*
+	 * This ensures that copy_from_page(), copy_to_page() and
+	 * __update_ref_ctr() can't cross page boundary.
+	 */
+	if (!IS_ALIGNED(offset, UPROBE_SWBP_INSN_SIZE))
+		return -EINVAL;
+	if (!IS_ALIGNED(ref_ctr_offset, sizeof(short)))
 		return -EINVAL;
 
  retry:
···
 	struct page *page;
 	uprobe_opcode_t opcode;
 	int result;
+
+	if (WARN_ON_ONCE(!IS_ALIGNED(vaddr, UPROBE_SWBP_INSN_SIZE)))
+		return -EINVAL;
 
 	pagefault_disable();
 	result = __get_user(opcode, (uprobe_opcode_t __user *)vaddr);