Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable

On Sat, Aug 28, 2021 at 10:04:41PM +0000, Al Viro wrote:

On Sat, Aug 28, 2021 at 11:47:03PM +0200, Thomas Gleixner wrote:

quoted

  /* Try to handle #PF, but anything else is fatal. */
  if (ret != -EFAULT)
     return -EINVAL;

quoted

which all end up in user_insn(). user_insn() returns 0 or the negated
trap number, which results in -EFAULT for #PF, but for #MC the negated
trap number is -18 i.e. != -EFAULT. IOW, there is no endless loop.

This used to be a problem before commit:

  aee8c67a4faa ("x86/fpu: Return proper error codes from user access functions")

and as the changelog says the initial reason for this was #GP going into
the fault path, but I'm pretty sure that I also discussed the #MC angle with
Borislav back then. Should have added some more comments there
obviously.

... or at least have that check spelled

	if (ret != -X86_TRAP_PF)
		return -EINVAL;

Unless I'm misreading your explanation, that is...

BTW, is #MC triggered on stored to a poisoned cacheline?  Existence of CLZERO
would seem to argue against that...