riscv: save the SR_SUM status over switches

When threads/tasks are switched we need to ensure the old execution's
SR_SUM state is saved and the new thread has the old SR_SUM state
restored.

The issue was seen under heavy load especially with the syz-stress tool
running, with crashes as follows in schedule_tail:

Unable to handle kernel access to user memory without uaccess routines
at virtual address 000000002749f0d0
Oops [#1]
Modules linked in:
CPU: 1 PID: 4875 Comm: syz-executor.0 Not tainted
5.12.0-rc2-syzkaller-00467-g0d7588ab9ef9 #0
Hardware name: riscv-virtio,qemu (DT)
epc : schedule_tail+0x72/0xb2 kernel/sched/core.c:4264
 ra : task_pid_vnr include/linux/sched.h:1421 [inline]
 ra : schedule_tail+0x70/0xb2 kernel/sched/core.c:4264
epc : ffffffe00008c8b0 ra : ffffffe00008c8ae sp : ffffffe025d17ec0
 gp : ffffffe005d25378 tp : ffffffe00f0d0000 t0 : 0000000000000000
 t1 : 0000000000000001 t2 : 00000000000f4240 s0 : ffffffe025d17ee0
 s1 : 000000002749f0d0 a0 : 000000000000002a a1 : 0000000000000003
 a2 : 1ffffffc0cfac500 a3 : ffffffe0000c80cc a4 : 5ae9db91c19bbe00
 a5 : 0000000000000000 a6 : 0000000000f00000 a7 : ffffffe000082eba
 s2 : 0000000000040000 s3 : ffffffe00eef96c0 s4 : ffffffe022c77fe0
 s5 : 0000000000004000 s6 : ffffffe067d74e00 s7 : ffffffe067d74850
 s8 : ffffffe067d73e18 s9 : ffffffe067d74e00 s10: ffffffe00eef96e8
 s11: 000000ae6cdf8368 t3 : 5ae9db91c19bbe00 t4 : ffffffc4043cafb2
 t5 : ffffffc4043cafba t6 : 0000000000040000
status: 0000000000000120 badaddr: 000000002749f0d0 cause:
000000000000000f
Call Trace:
[<ffffffe00008c8b0>] schedule_tail+0x72/0xb2 kernel/sched/core.c:4264
[<ffffffe000005570>] ret_from_exception+0x0/0x14
Dumping ftrace buffer:
   (ftrace buffer empty)
---[ end trace b5f8f9231dc87dda ]---

The issue comes from the put_user() in schedule_tail
(kernel/sched/core.c) doing the following:

asmlinkage __visible void schedule_tail(struct task_struct *prev)
{
...
        if (current->set_child_tid)
                put_user(task_pid_vnr(current), current->set_child_tid);
...
}

the put_user() macro causes the code sequence to come out as follows:

1:	__enable_user_access()
2:	reg = task_pid_vnr(current);
3:	*current->set_child_tid = reg;
4:	__disable_user_access()

The problem is that we may have a sleeping function as argument which
could clear SR_SUM causing the panic above. This was fixed by
evaluating the argument of the put_user() macro outside the user-enabled
section in commit 285a76b ("riscv: evaluate put_user() arg before
enabling user access")"

In order for riscv to take advantage of unsafe_get/put_XXX() macros and
to avoid the same issue we had with put_user() and sleeping functions we
must ensure code flow can go through switch_to() from within a region of
code with SR_SUM enabled and come back with SR_SUM still enabled. This
patch addresses the problem allowing future work to enable full use of
unsafe_get/put_XXX() macros without needing to take a CSR bit flip cost
on every access. Make switch_to() save and restore SR_SUM.

Reported-by: [email protected]
Signed-off-by: Ben Dooks <[email protected]>
Signed-off-by: Cyril Bur <[email protected]>
Reviewed-by: Alexandre Ghiti <[email protected]>
Reviewed-by: Deepak Gupta <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: bjdooks-ct

arch/riscv/include/asm/processor.h

@@ -103,6 +103,7 @@ struct thread_struct {
 	struct __riscv_d_ext_state fstate;
 	unsigned long bad_cause;
 	unsigned long envcfg;
+	unsigned long status;
 	u32 riscv_v_flags;
 	u32 vstate_ctrl;
 	struct __riscv_v_ext_state vstate;

arch/riscv/kernel/asm-offsets.c

@@ -34,6 +34,7 @@ void asm_offsets(void)
 	OFFSET(TASK_THREAD_S9, task_struct, thread.s[9]);
 	OFFSET(TASK_THREAD_S10, task_struct, thread.s[10]);
 	OFFSET(TASK_THREAD_S11, task_struct, thread.s[11]);
+	OFFSET(TASK_THREAD_STATUS, task_struct, thread.status);
 
 	OFFSET(TASK_TI_CPU, task_struct, thread_info.cpu);
 	OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
@@ -346,6 +347,10 @@ void asm_offsets(void)
 		  offsetof(struct task_struct, thread.s[11])
 		- offsetof(struct task_struct, thread.ra)
 	);
+	DEFINE(TASK_THREAD_STATUS_RA,
+		  offsetof(struct task_struct, thread.status)
+		- offsetof(struct task_struct, thread.ra)
+	);
 
 	DEFINE(TASK_THREAD_F0_F0,
 		  offsetof(struct task_struct, thread.fstate.f[0])

arch/riscv/kernel/entry.S

@@ -397,9 +397,17 @@ SYM_FUNC_START(__switch_to)
 	REG_S s9,  TASK_THREAD_S9_RA(a3)
 	REG_S s10, TASK_THREAD_S10_RA(a3)
 	REG_S s11, TASK_THREAD_S11_RA(a3)
+
+	/* save the user space access flag */
+	li    s0, SR_SUM
+	csrr  s1, CSR_STATUS
+	REG_S s1, TASK_THREAD_STATUS_RA(a3)
+
 	/* Save the kernel shadow call stack pointer */
 	scs_save_current
 	/* Restore context from next->thread */
+	REG_L s0,  TASK_THREAD_STATUS_RA(a4)
+	csrs  CSR_STATUS, s0
 	REG_L ra,  TASK_THREAD_RA_RA(a4)
 	REG_L sp,  TASK_THREAD_SP_RA(a4)
 	REG_L s0,  TASK_THREAD_S0_RA(a4)