On Sat, Feb 23, 2019 at 12:03 AM Dave Hansen [off-list ref] wrote:

On 2/22/19 4:53 AM, Andrey Konovalov wrote:

quoted

--- a/fs/namespace.c
+++ b/fs/namespace.c

@@ -2730,7 +2730,7 @@ void *copy_mount_options(const void __user * data)
       * the remainder of the page.
       */
      /* copy_from_user cannot cross TASK_SIZE ! */
-     size = TASK_SIZE - (unsigned long)data;
+     size = TASK_SIZE - (unsigned long)untagged_addr(data);
      if (size > PAGE_SIZE)
              size = PAGE_SIZE;

I would have thought that copy_from_user() *is* entirely capable of
detecting and returning an error in the case that its arguments cross
TASK_SIZE.  It will fail and return an error, but that's what it's
supposed to do.

I'd question why this code needs to be doing its own checking in the
first place.  Is there something subtle going on?

The comment above exact_copy_from_user() states:

Some copy_from_user() implementations do not return the exact number of
bytes remaining to copy on a fault.  But copy_mount_options() requires that.
Note that this function differs from copy_from_user() in that it will oops
on bad values of `to', rather than returning a short copy.

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