[PATCH] arm64: extable: fix null deref in load_unaligned_zeropad.

[Mark Rutland]

On Tue, Jan 25, 2022 at 03:23:20PM +0000, Mark Rutland wrote:
> On Fri, Jan 21, 2022 at 06:34:47PM -0800, Evgenii Stepanov wrote:
> > ex_handler_load_unaligned_zeropad extracts the source and data register
> > numbers from the wrong field of the exception table.
> 
> Ouch. Did you find this by inspection, or did this show up in testing?
> 
> Sorry about this.
> 
> I think we should be a little more explicit as to exactly what goes wrong. How
> about:
> 
> | In ex_handler_load_unaligned_zeropad() we erroneously extract the data and
> | addr register indices from ex->type rather than ex->data. As ex->type will
> | contain EX_TYPE_LOAD_UNALIGNED_ZEROPAD (i.e. 4):
> | 
> | * We'll always treat X0 as the address register, since EX_DATA_REG_ADDR is
> |   extracted from bits [9:5]. Thus, we may attempt to dereference an arbitrary
> |   address as X0 may hold an arbitary value.
> | 
> | * We'll always treat X4 as the data register, since EX_DATA_REG_DATA is
> |   extracted from bits [4:0]. Thus we will corrupt X4 and cause arbitrary
> |   behaviour within load_unaligned_zeropad() and its caller.
> | 
> | Fix this by extracting both values from ex->data as originally intended.
> 
> > Fixes: 753b3236
> 
> That should be expanded, e.g.
> 
>   Fixes: 753b32368705c396 ("arm64: extable: add load_unaligned_zeropad() handler")
> 
> With those changes:
> 
> Reviewed-by: Mark Rutland <mark.rutland at arm.com>

Looking again, sicne this isn't jsut a null-deref, can we also rework the
title, something like:

| arm64: extable: fix load_unaligned_zeropad() reg indices

Thanks,
Mark.