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

[Mark Rutland]

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@xxxxxxx>

Thanks,
Mark.

> Signed-off-by: Evgenii Stepanov <eugenis@xxxxxxxxxx>
> ---
>  arch/arm64/mm/extable.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
> index c0181e60cc98..489455309695 100644
> --- a/arch/arm64/mm/extable.c
> +++ b/arch/arm64/mm/extable.c
> @@ -40,8 +40,8 @@ static bool
>  ex_handler_load_unaligned_zeropad(const struct exception_table_entry *ex,
>  				  struct pt_regs *regs)
>  {
> -	int reg_data = FIELD_GET(EX_DATA_REG_DATA, ex->type);
> -	int reg_addr = FIELD_GET(EX_DATA_REG_ADDR, ex->type);
> +	int reg_data = FIELD_GET(EX_DATA_REG_DATA, ex->data);
> +	int reg_addr = FIELD_GET(EX_DATA_REG_ADDR, ex->data);
>  	unsigned long data, addr, offset;
>  
>  	addr = pt_regs_read_reg(regs, reg_addr);
> -- 
> 2.35.0.rc0.227.g00780c9af4-goog
>