[PATCH] riscv: VMAP_STACK overflow detection thread-safe

Deepak Gupta
2022-11-24T00:50:28Z

commit 31da94c25aea835ceac00575a9fd206c5a833fed 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.

Following are the changes in this patch

- Defines an asm macro to obtain per-cpu symbols in destination
register.
- Computing per-cpu symbol requires a temporary register. When stack is
out of question, a place is needed to spill a register. `thread_info`
is good location to have spill register.
- In entry.S when overflow is detected x31 is spilled into thread_info.
x31 is used as temp reg for asm macro to locate per-cpu overflow stack

Other relevant disccussion on this
https://lore.kernel.org/linux-riscv/Y347B0x4VUNOd6V7@xhacker/T/#t

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

[ 286.223273] Insufficient stack space to handle exception!/debug/provoke-crash/DIRECT
[ 286.223878] Task stack: [0xff20000010a98000..0xff20000010a9c000]
[ 286.224411] Overflow stack: [0xff600001f7d98370..0xff600001f7d99370]
[ 286.226057] CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34
[ 286.227139] Hardware name: riscv-virtio,qemu (DT)
[ 286.228000] epc : __memset+0x60/0xfc
[ 286.229299] ra : recursive_loop+0x48/0xc6 [lkdtm]
[ 286.231457] epc : ffffffff808de0e4 ra : ffffffff0163a752 sp : ff20000010a97e80
[ 286.232207] gp : ffffffff815c0330 tp : ff600000820ea280 t0 : ff20000010a97e88
[ 286.233584] t1 : 000000000000002e t2 : 3233206874706564 s0 : ff20000010a982b0
[ 286.234293] s1 : 0000000000000012 a0 : ff20000010a97e88 a1 : 0000000000000000
[ 286.234998] a2 : 0000000000000400 a3 : ff20000010a98288 a4 : 0000000000000000
[ 286.235697] a5 : 0000000000000000 a6 : fffffffffffe43f0 a7 : 00007fffffffffff
[ 286.236384] s2 : ff20000010a97e88 s3 : ffffffff01644680 s4 : ff20000010a9be90
[ 286.237743] s5 : ff600000842ba6c0 s6 : 00aaaaaac29e42b0 s7 : 00fffffff0aa3684
[ 286.238691] s8 : 00aaaaaac2978040 s9 : 0000000000000065 s10: 00ffffff8a7cad10
[ 286.239591] s11: 00ffffff8a76a4e0 t3 : ffffffff815dbaf4 t4 : ffffffff815dbaf4
[ 286.240537] t5 : ffffffff815dbab8 t6 : ff20000010a9bb48
[ 286.241540] status: 0000000200000120 badaddr: ff20000010a97e88 cause: 000000000000000f
[ 286.242979] Kernel panic - not syncing: Kernel stack overflow
[ 286.244106] CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34
[ 286.245276] Hardware name: riscv-virtio,qemu (DT)
[ 286.245929] Call Trace:
[ 286.246954] [<ffffffff80006754>] dump_backtrace+0x30/0x38
[ 286.247813] [<ffffffff808de798>] show_stack+0x40/0x4c
[ 286.248429] [<ffffffff808ea2a8>] dump_stack_lvl+0x44/0x5c
[ 286.249439] [<ffffffff808ea2d8>] dump_stack+0x18/0x20
[ 286.250056] [<ffffffff808dec06>] panic+0x126/0x2fe
[ 286.250642] [<ffffffff800065ea>] walk_stackframe+0x0/0xf0
[ 286.251357] [<ffffffff0163a752>] recursive_loop+0x48/0xc6 [lkdtm]
[ 286.253321] SMP: stopping secondary CPUs
[ 286.256724] ---[ end Kernel panic - not syncing: Kernel stack overflow ]---

Fixes: 31da94c25aea835ceac00575a9fd206c5a833fed
Cc: Guo Ren <guoren@kernel.org>
Cc: Jisheng Zhang <jszhang@kernel.org>

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
arch/riscv/include/asm/asm.h | 11 ++++++
arch/riscv/include/asm/thread_info.h | 3 ++
arch/riscv/kernel/asm-offsets.c | 4 +++
arch/riscv/kernel/entry.S | 54 ++++------------------------
arch/riscv/kernel/traps.c | 12 +------
5 files changed, 26 insertions(+), 58 deletions(-)

diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
index 1b471ff73178..373eba843331 100644
--- a/arch/riscv/include/asm/asm.h
+++ b/arch/riscv/include/asm/asm.h
@@ -69,6 +69,7 @@

#ifdef __ASSEMBLY__

+#include <asm/asm-offsets.h>
/* Common assembly source macros */

/*
@@ -80,6 +81,16 @@
.endr
.endm

+.macro asm_per_cpu dst sym tmp
+ REG_L \tmp, TASK_TI_CPU_NUM(tp)
+ slli \tmp, \tmp, 0x3
+ la \dst, __per_cpu_offset
+ add \dst, \dst, \tmp
+ REG_L \tmp, 0(\dst)
+ la \dst, \sym
+ add \dst, \dst, \tmp
+.endm
+
#endif /* __ASSEMBLY__ */

#endif /* _ASM_RISCV_ASM_H */
diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index 67322f878e0d..7e17dc07cf11 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -65,6 +65,9 @@ struct thread_info {
*/
long kernel_sp; /* Kernel stack pointer */
long user_sp; /* User stack pointer */
+#ifdef CONFIG_VMAP_STACK
+ long spill_reg; /* per cpu scratch space to spill a single register */
+#endif
int cpu;
};

diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index df9444397908..bed3c83bfb8f 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -38,6 +38,10 @@ 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);
+#ifdef CONFIG_VMAP_STACK
+ OFFSET(TASK_TI_SPILL_REG, task_struct, thread_info.spill_reg);
+#endif
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 b9eda3fcbd6d..12f285cec136 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>

#if !IS_ENABLED(CONFIG_PREEMPTION)
.set resume_kernel, restore_all
@@ -404,54 +406,12 @@ handle_syscall_trace_exit:

#ifdef CONFIG_VMAP_STACK
handle_kernel_stack_overflow:
- la sp, shadow_stack
- addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE
+ REG_S x31, TASK_TI_SPILL_REG(tp)
+ asm_per_cpu sp, overflow_stack, x31
+ li x31, OVERFLOW_STACK_SIZE
+ add sp, sp, x31
+ REG_L x31, TASK_TI_SPILL_REG(tp)

- //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)
-
- //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 f3e96d60a2ff..eef3a87514c7 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -208,18 +208,8 @@ int is_valid_bugaddr(unsigned long pc)
#endif /* CONFIG_GENERIC_BUG */

#ifdef CONFIG_VMAP_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);
-/*
- * shadow stack, handled_ kernel_ stack_ overflow(in kernel/entry.S) is used
- * to get per-cpu overflow stack(get_overflow_stack).
- */
-long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE/sizeof(long)];
-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)
{
--
2.25.1