[PATCH] RFC: riscv: save the SR_SUM status over switches

[Ben Dooks]

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 is 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()

This means the task_pid_vnr() is being called with user-access enabled
which itself is not a good idea, but that is a seperate issue. Here we
have a function that /might/ sleep being called with the SR_SUM and if
it does, then it returns with the SR_SUM flag possibly cleared thus
causing the above abort.

To try and deal with this, and stop the SR_SUM leaking out into other
threads (this has also been tested and see under stress. It can rarely
happen but it /does/ under load) make sure the __switch_to() will save
and restore the SR_SUM flag, and clear it possibly for the next thread
if it does not need it.

Note, test code to be supplied once other checks have been finished.

There may be further issues with the mstatus flags with this, this
can be discussed further once some initial testing has been done.

Reported-by: syzbot+e74b94fe601ab9552d69 at syzkaller.appspotmail.com
Signed-off-by: Ben Dooks <ben.dooks at codethink.co.uk>
Cc: Dmitry Vyukov <dvyukov at google.com>
Cc: Javier Jardon <javier.jardon at codethink.co.uk>
---
 arch/riscv/include/asm/processor.h | 1 +
 arch/riscv/kernel/asm-offsets.c    | 5 +++++
 arch/riscv/kernel/entry.S          | 7 +++++++
 3 files changed, 13 insertions(+)

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 021ed64ee608..e2398c6e7af9 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -35,6 +35,7 @@ struct thread_struct {
 	unsigned long s[12];	/* s[0]: frame pointer */
 	struct __riscv_d_ext_state fstate;
 	unsigned long bad_cause;
+	unsigned long flags;
 };
 
 #define INIT_THREAD {					\
diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index 9ef33346853c..0036570752d9 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -29,6 +29,7 @@ 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_FLAGS, task_struct, thread.flags);
 	OFFSET(TASK_TI_FLAGS, task_struct, thread_info.flags);
 	OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
 	OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
@@ -172,6 +173,10 @@ void asm_offsets(void)
 		  offsetof(struct task_struct, thread.s[11])
 		- offsetof(struct task_struct, thread.ra)
 	);
+	DEFINE(TASK_THREAD_FLAGS_RA,
+		  offsetof(struct task_struct, thread.flags)
+		- offsetof(struct task_struct, thread.ra)
+	);
 
 	DEFINE(TASK_THREAD_F0_F0,
 		  offsetof(struct task_struct, thread.fstate.f[0])
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 744f3209c48d..6137f6c2a2ad 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -417,7 +417,14 @@ ENTRY(__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 (and disable the user space access flag) */
+	li    s0, SR_SUM
+	csrrc s1, CSR_STATUS, s0
+	REG_S s1,  TASK_THREAD_FLAGS_RA(a3)
+
 	/* Restore context from next->thread */
+	REG_L s0,  TASK_THREAD_FLAGS_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)
-- 
2.30.2