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Message-Id: <20250410070526.3160847-2-cyrilbur@tenstorrent.com>
Date: Thu, 10 Apr 2025 07:05:22 +0000
From: Cyril Bur <cyrilbur@...storrent.com>
To: palmer@...belt.com,
aou@...s.berkeley.edu,
paul.walmsley@...ive.com,
charlie@...osinc.com,
jrtc27@...c27.com,
ben.dooks@...ethink.co.uk,
alex@...ti.fr
Cc: linux-riscv@...ts.infradead.org,
linux-kernel@...r.kernel.org,
jszhang@...nel.org,
syzbot+e74b94fe601ab9552d69@...kaller.appspotmail.com
Subject: [PATCH v6 1/5] riscv: save the SR_SUM status over switches
From: Ben Dooks <ben.dooks@...ethink.co.uk>
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 285a76bb2cf5 ("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: syzbot+e74b94fe601ab9552d69@...kaller.appspotmail.com
Signed-off-by: Ben Dooks <ben.dooks@...ethink.co.uk>
Signed-off-by: Cyril Bur <cyrilbur@...storrent.com>
---
arch/riscv/include/asm/processor.h | 1 +
arch/riscv/kernel/asm-offsets.c | 5 +++++
arch/riscv/kernel/entry.S | 8 ++++++++
3 files changed, 14 insertions(+)
diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 5f56eb9d114a..58fd11c89fe9 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/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;
diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index 16490755304e..969c65b1fe41 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/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])
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 33a5a9f2a0d4..00bd0de9faa2 100644
--- a/arch/riscv/kernel/entry.S
+++ b/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)
--
2.34.1
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