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Date: Fri, 30 Aug 2019 12:47:48 -0700
From: Kees Cook <>
To: Dave Chinner <>
Cc: Jason Yan <>,,

On Fri, Aug 30, 2019 at 02:29:58PM +1000, Dave Chinner wrote:
> On Thu, Aug 29, 2019 at 09:15:36AM -0700, Kees Cook wrote:
> > On Thu, Aug 29, 2019 at 08:42:30PM +0800, Jason Yan wrote:
> > > We found an issue of kernel bug related to HARDENED_USERCOPY.
> > > When copying an IO buffer to userspace, HARDENED_USERCOPY thought it is
> > > illegal to copy this buffer. Actually this is because this IO buffer was
> > > merged from two bio vectors, and the two bio vectors buffer was allocated
> > > with kmalloc() in the filesystem layer.
> > 
> > Ew. I thought the FS layer was always using page_alloc?
> No, they don't. It's perfectly legal to use heap memory for bio
> buffers - we've been doing it since, at least, XFS got merged all
> those years ago.

Okay, so I have some observations/thoughts about this:

- This "cross allocation merging" is going to continue being a problem
  in the future when we have hardware-backed allocation tagging (like
  ARM's MTE). It'll be exactly the same kind of detection: a tagged
  pointer crossed into a separately allocated region and access through
  it will be rejected.

- I don't think using _copy_to_user() unconditionally is correct here
  unless we can be absolutely sure that the size calculation really
  was correct. (i.e. is the merge close enough to the copy that the
  non-merge paths don't lose the validation?)

- If this has gone until now to get noticed (hardened usercopy was
  introduced in v4.8), is this optimization (and, frankly, layering
  violation) actually useful?

- We could just turn off allocation merging in the face of having
  hardened usercopy or allocation tagging enabled...

Kees Cook

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