[PATCH v2] ARM: entry: omit FP emulation for UND exceptions taken in kernel mode
Ard Biesheuvel
ardb at kernel.org
Wed Nov 18 08:08:49 EST 2020
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 at gmail.com>
Cc: Kees Cook <keescook at chromium.org>
Cc: Nick Desaulniers <ndesaulniers at google.com>
Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
Reviewed-by: Linus Walleij <linus.walleij at linaro.org>
---
NOTE: this supersedes 9018/2 "vfp: force non-conditional encoding for external
Thumb2" which is currently queued in the patch system - the out-of-range
branch to vfp_kmode_exception() is dropped entirely in this patch
v2: - use the PSR T bit to select the right PC correction
- add Linus's ack
arch/arm/kernel/entry-armv.S | 25 +---------
arch/arm/vfp/vfphw.S | 5 --
arch/arm/vfp/vfpmodule.c | 49 +++++++++++++++++++-
3 files changed, 49 insertions(+), 30 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
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..c3b6451c18bd 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 +663,51 @@ 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");
+ pr_crit("FPEXC == 0x%08x\n", fmrx(FPEXC));
+ 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
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