[PATCH v2 1/6] riscv: VMAP_STACK overflow detection thread-safe

From: Sami Tolvanen
Date: Tue Aug 15 2023 - 16:35:57 EST

From: Deepak Gupta <debug@xxxxxxxxxxxx>

commit 31da94c25aea ("riscv: add VMAP_STACK overflow detection") added
support for CONFIG_VMAP_STACK. If overflow is detected, CPU switches to
`shadow_stack` temporarily before switching finally to per-cpu
`overflow_stack`.

If two CPUs/harts are racing and end up in over flowing kernel stack, one
or both will end up corrupting each other state because `shadow_stack` is
not per-cpu. This patch optimizes per-cpu overflow stack switch by
directly picking per-cpu `overflow_stack` and gets rid of `shadow_stack`.

Following are the changes in this patch

- Defines an asm macro to obtain per-cpu symbols in destination
register.
- In entry.S, when overflow is detected, per-cpu overflow stack is
located using per-cpu asm macro. Computing per-cpu symbol requires
a temporary register. x31 is saved away into CSR_SCRATCH
(CSR_SCRATCH is anyways zero since we're in kernel).

Please see Links for additional relevant disccussion and alternative
solution.

Tested by `echo EXHAUST_STACK > /sys/kernel/debug/provoke-crash/DIRECT`
Kernel crash log below

Insufficient stack space to handle exception!/debug/provoke-crash/DIRECT
Task stack: [0xff20000010a98000..0xff20000010a9c000]
Overflow stack: [0xff600001f7d98370..0xff600001f7d99370]
CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34
Hardware name: riscv-virtio,qemu (DT)
epc : __memset+0x60/0xfc
ra : recursive_loop+0x48/0xc6 [lkdtm]
epc : ffffffff808de0e4 ra : ffffffff0163a752 sp : ff20000010a97e80
gp : ffffffff815c0330 tp : ff600000820ea280 t0 : ff20000010a97e88
t1 : 000000000000002e t2 : 3233206874706564 s0 : ff20000010a982b0
s1 : 0000000000000012 a0 : ff20000010a97e88 a1 : 0000000000000000
a2 : 0000000000000400 a3 : ff20000010a98288 a4 : 0000000000000000
a5 : 0000000000000000 a6 : fffffffffffe43f0 a7 : 00007fffffffffff
s2 : ff20000010a97e88 s3 : ffffffff01644680 s4 : ff20000010a9be90
s5 : ff600000842ba6c0 s6 : 00aaaaaac29e42b0 s7 : 00fffffff0aa3684
s8 : 00aaaaaac2978040 s9 : 0000000000000065 s10: 00ffffff8a7cad10
s11: 00ffffff8a76a4e0 t3 : ffffffff815dbaf4 t4 : ffffffff815dbaf4
t5 : ffffffff815dbab8 t6 : ff20000010a9bb48
status: 0000000200000120 badaddr: ff20000010a97e88 cause: 000000000000000f
Kernel panic - not syncing: Kernel stack overflow
CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34
Hardware name: riscv-virtio,qemu (DT)
Call Trace:
[<ffffffff80006754>] dump_backtrace+0x30/0x38
[<ffffffff808de798>] show_stack+0x40/0x4c
[<ffffffff808ea2a8>] dump_stack_lvl+0x44/0x5c
[<ffffffff808ea2d8>] dump_stack+0x18/0x20
[<ffffffff808dec06>] panic+0x126/0x2fe
[<ffffffff800065ea>] walk_stackframe+0x0/0xf0
[<ffffffff0163a752>] recursive_loop+0x48/0xc6 [lkdtm]
SMP: stopping secondary CPUs
---[ end Kernel panic - not syncing: Kernel stack overflow ]---

Cc: Guo Ren <guoren@xxxxxxxxxx>
Cc: Jisheng Zhang <jszhang@xxxxxxxxxx>
Link: https://lore.kernel.org/linux-riscv/Y347B0x4VUNOd6V7@xhacker/T/#t
Link: https://lore.kernel.org/lkml/20221124094845.1907443-1-debug@xxxxxxxxxxxx/
Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
Co-developed-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Acked-by: Guo Ren <guoren@xxxxxxxxxx>
---
arch/riscv/include/asm/asm.h | 22 +++++++++
arch/riscv/include/asm/thread_info.h | 3 --
arch/riscv/kernel/asm-offsets.c | 1 +
arch/riscv/kernel/entry.S | 70 ++++------------------------
arch/riscv/kernel/traps.c | 36 +-------------
5 files changed, 34 insertions(+), 98 deletions(-)

diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
index 114bbadaef41..bfb4c26f113c 100644
--- a/arch/riscv/include/asm/asm.h
+++ b/arch/riscv/include/asm/asm.h
@@ -82,6 +82,28 @@
.endr
.endm

+#ifdef CONFIG_SMP
+#ifdef CONFIG_32BIT
+#define PER_CPU_OFFSET_SHIFT 2
+#else
+#define PER_CPU_OFFSET_SHIFT 3
+#endif
+
+.macro asm_per_cpu dst sym tmp
+ REG_L \tmp, TASK_TI_CPU_NUM(tp)
+ slli \tmp, \tmp, PER_CPU_OFFSET_SHIFT
+ la \dst, __per_cpu_offset
+ add \dst, \dst, \tmp
+ REG_L \tmp, 0(\dst)
+ la \dst, \sym
+ add \dst, \dst, \tmp
+.endm
+#else /* CONFIG_SMP */
+.macro asm_per_cpu dst sym tmp
+ la \dst, \sym
+.endm
+#endif /* CONFIG_SMP */
+
/* save all GPs except x1 ~ x5 */
.macro save_from_x6_to_x31
REG_S x6, PT_T1(sp)
diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index 1833beb00489..d18ce0113ca1 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -34,9 +34,6 @@

#ifndef __ASSEMBLY__

-extern long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE / sizeof(long)];
-extern unsigned long spin_shadow_stack;
-
#include <asm/processor.h>
#include <asm/csr.h>

diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index d6a75aac1d27..9f535d5de33f 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -39,6 +39,7 @@ void asm_offsets(void)
OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
OFFSET(TASK_TI_USER_SP, task_struct, thread_info.user_sp);

+ OFFSET(TASK_TI_CPU_NUM, task_struct, thread_info.cpu);
OFFSET(TASK_THREAD_F0, task_struct, thread.fstate.f[0]);
OFFSET(TASK_THREAD_F1, task_struct, thread.fstate.f[1]);
OFFSET(TASK_THREAD_F2, task_struct, thread.fstate.f[2]);
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 143a2bb3e697..3d11aa3af105 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -10,9 +10,11 @@
#include <asm/asm.h>
#include <asm/csr.h>
#include <asm/unistd.h>
+#include <asm/page.h>
#include <asm/thread_info.h>
#include <asm/asm-offsets.h>
#include <asm/errata_list.h>
+#include <linux/sizes.h>

SYM_CODE_START(handle_exception)
/*
@@ -170,67 +172,15 @@ SYM_CODE_END(ret_from_exception)

#ifdef CONFIG_VMAP_STACK
SYM_CODE_START_LOCAL(handle_kernel_stack_overflow)
- /*
- * Takes the psuedo-spinlock for the shadow stack, in case multiple
- * harts are concurrently overflowing their kernel stacks. We could
- * store any value here, but since we're overflowing the kernel stack
- * already we only have SP to use as a scratch register. So we just
- * swap in the address of the spinlock, as that's definately non-zero.
- *
- * Pairs with a store_release in handle_bad_stack().
- */
-1: la sp, spin_shadow_stack
- REG_AMOSWAP_AQ sp, sp, (sp)
- bnez sp, 1b
-
- la sp, shadow_stack
- addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE
-
- //save caller register to shadow stack
- addi sp, sp, -(PT_SIZE_ON_STACK)
- REG_S x1, PT_RA(sp)
- REG_S x5, PT_T0(sp)
- REG_S x6, PT_T1(sp)
- REG_S x7, PT_T2(sp)
- REG_S x10, PT_A0(sp)
- REG_S x11, PT_A1(sp)
- REG_S x12, PT_A2(sp)
- REG_S x13, PT_A3(sp)
- REG_S x14, PT_A4(sp)
- REG_S x15, PT_A5(sp)
- REG_S x16, PT_A6(sp)
- REG_S x17, PT_A7(sp)
- REG_S x28, PT_T3(sp)
- REG_S x29, PT_T4(sp)
- REG_S x30, PT_T5(sp)
- REG_S x31, PT_T6(sp)
-
- la ra, restore_caller_reg
- tail get_overflow_stack
-
-restore_caller_reg:
- //save per-cpu overflow stack
- REG_S a0, -8(sp)
- //restore caller register from shadow_stack
- REG_L x1, PT_RA(sp)
- REG_L x5, PT_T0(sp)
- REG_L x6, PT_T1(sp)
- REG_L x7, PT_T2(sp)
- REG_L x10, PT_A0(sp)
- REG_L x11, PT_A1(sp)
- REG_L x12, PT_A2(sp)
- REG_L x13, PT_A3(sp)
- REG_L x14, PT_A4(sp)
- REG_L x15, PT_A5(sp)
- REG_L x16, PT_A6(sp)
- REG_L x17, PT_A7(sp)
- REG_L x28, PT_T3(sp)
- REG_L x29, PT_T4(sp)
- REG_L x30, PT_T5(sp)
- REG_L x31, PT_T6(sp)
+ /* we reach here from kernel context, sscratch must be 0 */
+ csrrw x31, CSR_SCRATCH, x31
+ asm_per_cpu sp, overflow_stack, x31
+ li x31, OVERFLOW_STACK_SIZE
+ add sp, sp, x31
+ /* zero out x31 again and restore x31 */
+ xor x31, x31, x31
+ csrrw x31, CSR_SCRATCH, x31

- //load per-cpu overflow stack
- REG_L sp, -8(sp)
addi sp, sp, -(PT_SIZE_ON_STACK)

//save context to overflow stack
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d..deb2144d9143 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -397,48 +397,14 @@ int is_valid_bugaddr(unsigned long pc)
#endif /* CONFIG_GENERIC_BUG */

#ifdef CONFIG_VMAP_STACK
-/*
- * Extra stack space that allows us to provide panic messages when the kernel
- * has overflowed its stack.
- */
-static DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)],
+DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)],
overflow_stack)__aligned(16);
-/*
- * A temporary stack for use by handle_kernel_stack_overflow. This is used so
- * we can call into C code to get the per-hart overflow stack. Usage of this
- * stack must be protected by spin_shadow_stack.
- */
-long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE/sizeof(long)] __aligned(16);
-
-/*
- * A pseudo spinlock to protect the shadow stack from being used by multiple
- * harts concurrently. This isn't a real spinlock because the lock side must
- * be taken without a valid stack and only a single register, it's only taken
- * while in the process of panicing anyway so the performance and error
- * checking a proper spinlock gives us doesn't matter.
- */
-unsigned long spin_shadow_stack;
-
-asmlinkage unsigned long get_overflow_stack(void)
-{
- return (unsigned long)this_cpu_ptr(overflow_stack) +
- OVERFLOW_STACK_SIZE;
-}

asmlinkage void handle_bad_stack(struct pt_regs *regs)
{
unsigned long tsk_stk = (unsigned long)current->stack;
unsigned long ovf_stk = (unsigned long)this_cpu_ptr(overflow_stack);

- /*
- * We're done with the shadow stack by this point, as we're on the
- * overflow stack. Tell any other concurrent overflowing harts that
- * they can proceed with panicing by releasing the pseudo-spinlock.
- *
- * This pairs with an amoswap.aq in handle_kernel_stack_overflow.
- */
- smp_store_release(&spin_shadow_stack, 0);
-
console_verbose();

pr_emerg("Insufficient stack space to handle exception!\n");
--
2.41.0.694.ge786442a9b-goog