Re: [bug report] dead loop in generic_perform_write() //Re: [PATCH v7 07/12] iov_iter: Convert iterate*() to inline funcs

[Linus Torvalds]

On Wed, 28 Feb 2024 at 13:21, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hmm. If the copy doesn't succeed and make any progress at all, then
> the code in generic_perform_write() after the "goto again"
>
>                 //[4]
>                 if (unlikely(fault_in_iov_iter_readable(i, bytes) ==
>                               bytes)) {
>
> should break out of the loop.

Ahh. I see the problem. Or at least part of it.

The iter is an ITER_BVEC.

And fault_in_iov_iter_readable() "knows" that an ITER_BVEC cannot
fail. Because obviously it's a kernel address, so no user page fault.

But for the machine check case, ITER_BVEC very much can fail.

This should never have worked in the first place.

What a crock.

Do we need to make iterate_bvec() always succeed fully, and make
copy_mc_to_kernel() zero out the end?

                   Linus