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

[Al Viro]

On Thu, Jun 24, 2021 at 05:38:35PM +0100, Robin Murphy wrote:
> On 2021-06-24 17:27, Al Viro wrote:
> > On Thu, Jun 24, 2021 at 02:22:27PM +0100, Robin Murphy wrote:
> > 
> > > FWIW I think the only way to make the kernel behaviour any more robust here
> > > would be to make the whole uaccess API more expressive, such that rather
> > > than simply saying "I only got this far" it could actually differentiate
> > > between stopping due to a fault which may be recoverable and worth retrying,
> > > and one which definitely isn't.
> > 
> > ... and propagate that "more expressive" information through what, 3 or 4
> > levels in the call chain?
> > 
> >  From include/linux/uaccess.h:
> > 
> >   * If raw_copy_{to,from}_user(to, from, size) returns N, size - N bytes starting
> >   * at to must become equal to the bytes fetched from the corresponding area
> >   * starting at from.  All data past to + size - N must be left unmodified.
> >   *
> >   * If copying succeeds, the return value must be 0.  If some data cannot be
> >   * fetched, it is permitted to copy less than had been fetched; the only
> >   * hard requirement is that not storing anything at all (i.e. returning size)
> >   * should happen only when nothing could be copied.  In other words, you don't
> >   * have to squeeze as much as possible - it is allowed, but not necessary.
> > 
> > arm64 instances violate the aforementioned hard requirement.  Please, fix
> > it there; it's not hard.  All you need is an exception handler in .Ltiny15
> > that would fall back to (short) byte-by-byte copy if the faulting address
> > happened to be unaligned.  Or just do one-byte copy, not that it had been
> > considerably cheaper than a loop.  Will be cheaper than propagating that extra
> > information up the call chain, let alone paying for extra ->write_begin()
> > and ->write_end() for single byte in generic_perform_write().
> 
> And what do we do if we then continue to fault with an external abort
> because whatever it is that warranted being mapped as Device-type memory in
> the first place doesn't support byte accesses?

If it does not support byte access, it would've failed on fault-in.