Re: [BUG] arm64: an infinite loop in generic_perform_write()

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Jun 23, 2021 at 04:27:37AM +0000, Al Viro wrote:
> On Wed, Jun 23, 2021 at 11:24:54AM +0800, Xiaoming Ni wrote:
> > On 2021/6/23 10:50, Al Viro wrote:
> > > On Wed, Jun 23, 2021 at 10:39:31AM +0800, Chen Huang wrote:
> > > 
> > > > Then when kernel handles the alignment_fault, it will not panic. As the
> > > > arm64 memory model spec said, when the address is not a multiple of the
> > > > element size, the access is unaligned. Unaligned accesses are allowed to
> > > > addresses marked as Normal, but not to Device regions. An unaligned access
> > > > to a Device region will trigger an exception (alignment fault).
> > > > 	
> > > > do_alignment_fault
> > > >      do_bad_area
> > > > 	__do_kernel_fault
> > > >             fixup_exception
> > > > 
> > > > But that fixup cann't handle the unaligned copy, so the
> > > > copy_page_from_iter_atomic returns 0 and traps in loop.
> > > 
> > > Looks like you need to fix your raw_copy_from_user(), then...
> > 
> > Exit loop when iov_iter_copy_from_user_atomic() returns 0.
> > This should solve the problem, too, and it's easier.
> 
> It might be easier, but it's not going to work correctly.
> If the page gets evicted by memory pressure, you are going
> to get spurious short write.
> 
> Besides, it's simply wrong - write(2) does *NOT* require an
> aligned source.  It (and raw_copy_from_user()) should act the
> same way memcpy(3) does.

On arm64, neither memcpy() nor raw_copy_from_user() are expected to work
on Device mappings, we have memcpy_fromio() for this but only for
ioremap(). There's no (easy) way to distinguish in the write() syscall
how the source buffer is mapped. generic_perform_write() does an
iov_iter_fault_in_readable() check but that's not sufficient and it also
breaks the cases where you can get intra-page faults (arm64 MTE or SPARC
ADI). I think in the general case it's racy anyway (another thread doing
an mprotect(PROT_NONE) after the readable check passed).

So I think generic_perform_write() returning -EFAULT if copied == 0
would make sense (well, unless it breaks other cases I'm not aware of).

-- 
Catalin