On Thu, Jan 13, 2022 at 10:47 AM Daniel Thompson <daniel.thompson@xxxxxxxxxx> wrote: > On Wed, Jan 12, 2022 at 06:08:17PM +0000, Russell King (Oracle) wrote: > > > The kernel attempted to access an address that is in the userspace > > domain (NULL pointer) and took an exception. > > > > I suppose we should handle a domain fault more gracefully - what are > > the required semantics if the kernel attempts a userspace access > > using one of the _nofault() accessors? > > I think the best answer might well be that, if the arch provides > implementations of hooks such as copy_from_kernel_nofault_allowed() > then the kernel should never attempt a userspace access using the > _nofault() accessors. That means they can do whatever they like! > > In other words something like the patch below looks like a promising > approach. Right, it seems this is the same as on x86. > From f66a63b504ff582f261a506c54ceab8c0e77a98c Mon Sep 17 00:00:00 2001 > From: Daniel Thompson <daniel.thompson@xxxxxxxxxx> > Date: Thu, 13 Jan 2022 09:34:45 +0000 > Subject: [PATCH] arm: mm: Implement copy_from_kernel_nofault_allowed() > > Currently copy_from_kernel_nofault() can actually fault (due to software > PAN) if we attempt userspace access. In any case, the documented > behaviour for this function is to return -ERANGE if we attempt an access > outside of kernel space. > > Implementing copy_from_kernel_nofault_allowed() solves both these > problems. > > Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx> Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>
