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

From: Suzuki K Poulose
Date: Tue Apr 07 2020 - 06:42:29 EST


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