Re: [PATCH] arm64: armv8_deprecated: Fix undef_hook mask for thumb setend

[Suzuki K Poulose]

On 04/07/2020 10:27 AM, Catalin Marinas wrote:
> On Mon, Apr 06, 2020 at 04:16:05PM +0200, Fredrik Strupe wrote:
> > Use a full 32-bit mask to prevent accidental matchings of thumb32
> > instructions where the second half-word is equal to the thumb16 setend
> > encoding.
> >
> > This fixes the same problem as the following patch:
> >
> >      https://lkml.org/lkml/2020/3/16/341
>
> This link is not guaranteed to be stable and the commit should have the
> full description rather than referring to another email.
>
> > but for setend emulation instead.
> >
> > Signed-off-by: Fredrik Strupe <fredrik@xxxxxxxxxx>
>
> It also needs Fixes: and Cc: stable tags.
>
> > ---
> >   arch/arm64/kernel/armv8_deprecated.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
> > index 9d3442d62..8c06dfee0 100644
> > --- a/arch/arm64/kernel/armv8_deprecated.c
> > +++ b/arch/arm64/kernel/armv8_deprecated.c
> > @@ -609,7 +609,7 @@ static struct undef_hook setend_hooks[] = {
> >   	},
> >   	{
> >   		/* Thumb mode */
> > -		.instr_mask	= 0x0000fff7,
> > +		.instr_mask	= 0xfffffff7,
> >   		.instr_val	= 0x0000b650,
>
> I can see how this could happen but it would be useful to provide a
> concrete example in the commit log.
>
> The instruction opcode built by call_undef_hook() first reads a u16 as a
> T16 instruction and the above should be fine. However, if this looks
> like a T32 opcode, it reads a subsequent u16 which becomes the lowest
> half-word and the above mask/val may inadvertently match it.

We also do a check on the pstate_val, along with the instr_val to
confirm the mode. So this should be fine as it is ?

Suzuki