[PATCH] ARM: entry: omit FP emulation for UND exceptions taken in kernel mode
Ard Biesheuvel
ardb at kernel.org
Mon Oct 26 14:33:23 EDT 2020
On Mon, 26 Oct 2020 at 19:13, Nick Desaulniers <ndesaulniers at google.com> wrote:
>
> On Mon, Oct 26, 2020 at 7:37 AM Ard Biesheuvel <ardb at kernel.org> wrote:
> >
> > 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.
> >
> > Cc: Russell King - ARM Linux admin <linux at armlinux.org.uk>
> > Cc: Linus Walleij <linus.walleij at linaro.org>
> > Cc: Dmitry Osipenko <digetx at gmail.com>
> > Cc: Kees Cook <keescook at chromium.org>
> > Cc: Nick Desaulniers <ndesaulniers at google.com>
> > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> > ---
> > Related discussion here:
> > https://lore.kernel.org/linux-arm-kernel/20201021225737.739-1-digetx@gmail.com/
> >
> > Instead of installing the undef hook just to print the additional error
> > message, we might decide to simply drop that entirely, and rely on the
> > undefined exception splat to be sufficient to figure out what is going
> > on.
> >
> > arch/arm/kernel/entry-armv.S | 23 +---------
> > arch/arm/vfp/vfphw.S | 5 --
> > arch/arm/vfp/vfpmodule.c | 48 +++++++++++++++++++-
> > 3 files changed, 47 insertions(+), 29 deletions(-)
> >
> > diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> > index 55a47df04773..1bda8b57e0bb 100644
> > --- a/arch/arm/kernel/entry-armv.S
> > +++ b/arch/arm/kernel/entry-armv.S
> > @@ -252,35 +252,14 @@ __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]
> > + mov r1, #4 @ PC correction to apply
> > #else
> > mov r1, #2
>
> No comment on the issue the patch is addressing, just a minor
> stylistic drive by comment.
>
> Prefer:
> if x:
> foo()
> else:
> bar()
>
> to:
> if !x:
> bar()
> else:
> foo()
>
I disagree. Either could make sense, given the context. But more
importantly, this patch does not introduce the pattern, and therefore,
changing one into the other would make the patch, which is already
non-trivial, more complicated than necessary.
So you looked at the patch and had no comments, right? Can I take that
as an ack? Or were you only looking for minor stylistic drive-by
issues? :-)
> > - 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:
> > mov r0, sp @ struct pt_regs *regs
> > bl __und_fault
> >
> > -__und_svc_finish:
> > get_thread_info tsk
> > ldr r5, [sp, #S_PSR] @ Get SVC cpsr
> > svc_exit r5 @ return from exception
> > diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > index 4fcff9f59947..d5837bf05a9a 100644
> > --- a/arch/arm/vfp/vfphw.S
> > +++ b/arch/arm/vfp/vfphw.S
> > @@ -79,11 +79,6 @@ ENTRY(vfp_support_entry)
> > DBGSTR3 "instr %08x pc %08x state %p", r0, r2, r10
> >
> > .fpu vfpv2
> > - ldr r3, [sp, #S_PSR] @ Neither lazy restore nor FP exceptions
> > - and r3, r3, #MODE_MASK @ are supported in kernel mode
> > - teq r3, #USR_MODE
> > - bne vfp_kmode_exception @ Returns through lr
> > -
> > VFPFMRX r1, FPEXC @ Is the VFP enabled?
> > DBGSTR1 "fpexc %08x", r1
> > tst r1, #FPEXC_EN
> > diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
> > index 8c9e7f9f0277..30d1f089f890 100644
> > --- a/arch/arm/vfp/vfpmodule.c
> > +++ b/arch/arm/vfp/vfpmodule.c
> > @@ -23,6 +23,7 @@
> > #include <asm/cputype.h>
> > #include <asm/system_info.h>
> > #include <asm/thread_notify.h>
> > +#include <asm/traps.h>
> > #include <asm/vfp.h>
> >
> > #include "vfpinstr.h"
> > @@ -642,7 +643,9 @@ static int vfp_starting_cpu(unsigned int unused)
> > return 0;
> > }
> >
> > -void vfp_kmode_exception(void)
> > +#ifdef CONFIG_KERNEL_MODE_NEON
> > +
> > +static int vfp_kmode_exception(struct pt_regs *regs, unsigned int instr)
> > {
> > /*
> > * If we reach this point, a floating point exception has been raised
> > @@ -660,9 +663,50 @@ void vfp_kmode_exception(void)
> > pr_crit("BUG: unsupported FP instruction in kernel mode\n");
> > else
> > pr_crit("BUG: FP instruction issued in kernel mode with FP unit disabled\n");
> > + return 1;
> > }
> >
> > -#ifdef CONFIG_KERNEL_MODE_NEON
> > +static struct undef_hook vfp_kmode_exception_hook[] = {{
> > + .instr_mask = 0xfe000000,
> > + .instr_val = 0xf2000000,
> > + .cpsr_mask = MODE_MASK | PSR_T_BIT,
> > + .cpsr_val = SVC_MODE,
> > + .fn = vfp_kmode_exception,
> > +}, {
> > + .instr_mask = 0xff100000,
> > + .instr_val = 0xf4000000,
> > + .cpsr_mask = MODE_MASK | PSR_T_BIT,
> > + .cpsr_val = SVC_MODE,
> > + .fn = vfp_kmode_exception,
> > +}, {
> > + .instr_mask = 0xef000000,
> > + .instr_val = 0xef000000,
> > + .cpsr_mask = MODE_MASK | PSR_T_BIT,
> > + .cpsr_val = SVC_MODE | PSR_T_BIT,
> > + .fn = vfp_kmode_exception,
> > +}, {
> > + .instr_mask = 0xff100000,
> > + .instr_val = 0xf9000000,
> > + .cpsr_mask = MODE_MASK | PSR_T_BIT,
> > + .cpsr_val = SVC_MODE | PSR_T_BIT,
> > + .fn = vfp_kmode_exception,
> > +}, {
> > + .instr_mask = 0x0c000e00,
> > + .instr_val = 0x0c000a00,
> > + .cpsr_mask = MODE_MASK,
> > + .cpsr_val = SVC_MODE,
> > + .fn = vfp_kmode_exception,
> > +}};
> > +
> > +static int __init vfp_kmode_exception_hook_init(void)
> > +{
> > + int i;
> > +
> > + for (i = 0; i < ARRAY_SIZE(vfp_kmode_exception_hook); i++)
> > + register_undef_hook(&vfp_kmode_exception_hook[i]);
> > + return 0;
> > +}
> > +core_initcall(vfp_kmode_exception_hook_init);
> >
> > /*
> > * Kernel-side NEON support functions
> > --
> > 2.17.1
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers
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