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Message-ID: <7556bdd0-1015-4175-a810-d2d53662ba64@ghiti.fr>
Date: Fri, 21 Mar 2025 10:09:32 +0100
From: Alexandre Ghiti <alex@...ti.fr>
To: Cyril Bur <cyrilbur@...storrent.com>, palmer@...belt.com,
aou@...s.berkeley.edu, paul.walmsley@...ive.com, charlie@...osinc.com,
jrtc27@...c27.com, ben.dooks@...ethink.co.uk
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
jszhang@...nel.org, syzbot+e74b94fe601ab9552d69@...kaller.appspotmail.com
Subject: Re: [PATCH v5 1/5] riscv: save the SR_SUM status over switches
Hi Cyril/Ben,
On 20/03/2025 23:44, Cyril Bur wrote:
> 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 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 separate 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.
The whole changelog is outdated, it needs to be reworded:
"To prevent the evaluation of preemptible functions in
unsafe_get/put_XXX() which could clear SUM bit set by
get_user_access()... etc etc"
>
> 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..0de05d652e0f 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 flags;
I would prefer the use of status since it stores the sstatus csr.
> 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 e89455a6a0e5..556ebcbb7e22 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_FLAGS, task_struct, thread.flags);
>
> OFFSET(TASK_TI_CPU, task_struct, thread_info.cpu);
> OFFSET(TASK_TI_FLAGS, task_struct, thread_info.flags);
> @@ -347,6 +348,10 @@ 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 33a5a9f2a0d4..c278b3ac37b9 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 (and disable the user space access flag) */
> + li s0, SR_SUM
> + csrrc s1, CSR_STATUS, s0
Here (again), I don't think we need to clear sstatus.
> + REG_S s1, TASK_THREAD_FLAGS_RA(a3)
> +
> /* Save the kernel shadow call stack pointer */
> scs_save_current
> /* 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)
Thanks,
Alex
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