ARM: 9030/1: entry: omit FP emulation for UND exceptions taken in kernel mode

There are a couple of problems with the exception entry code that deals
with FP exceptions (which are reported as UND exceptions) when building
the kernel in Thumb2 mode:
- the conditional branch to vfp_kmode_exception in vfp_support_entry()
  may be out of range for its target, depending on how the linker decides
  to arrange the sections;
- when the UND exception is taken in kernel mode, the emulation handling
  logic is entered via the 'call_fpe' label, which means we end up using
  the wrong value/mask pairs to match and detect the NEON opcodes.

Since UND exceptions in kernel mode are unlikely to occur on a hot path
(as opposed to the user mode version which is invoked for VFP support
code and lazy restore), we can use the existing undef hook machinery for
any kernel mode instruction emulation that is needed, including calling
the existing vfp_kmode_exception() routine for unexpected cases. So drop
the call to call_fpe, and instead, install an undef hook that will get
called for NEON and VFP instructions that trigger an UND exception in
kernel mode.

While at it, make sure that the PC correction is accurate for the
execution mode where the exception was taken, by checking the PSR
Thumb bit.

Cc: Dmitry Osipenko <digetx@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

1 files changed, 2 insertions, 23 deletions

diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index c4220f51fcf3..0ea8529a4872 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -252,31 +252,10 @@ __und_svc:
 #else
 	svc_entry
 #endif
-	@
-	@ call emulation code, which returns using r9 if it has emulated
-	@ the instruction, or the more conventional lr if we are to treat
-	@ this as a real undefined instruction
-	@
-	@  r0 - instruction
-	@
-#ifndef CONFIG_THUMB2_KERNEL
-	ldr	r0, [r4, #-4]
-#else
-	mov	r1, #2
-	ldrh	r0, [r4, #-2]			@ Thumb instruction at LR - 2
-	cmp	r0, #0xe800			@ 32-bit instruction if xx >= 0
-	blo	__und_svc_fault
-	ldrh	r9, [r4]			@ bottom 16 bits
-	add	r4, r4, #2
-	str	r4, [sp, #S_PC]
-	orr	r0, r9, r0, lsl #16
-#endif
-	badr	r9, __und_svc_finish
-	mov	r2, r4
-	bl	call_fpe
 
 	mov	r1, #4				@ PC correction to apply
-__und_svc_fault:
+ THUMB(	tst	r5, #PSR_T_BIT		)	@ exception taken in Thumb mode?
+ THUMB(	movne	r1, #2			)	@ if so, fix up PC correction
 	mov	r0, sp				@ struct pt_regs *regs
 	bl	__und_fault