[PATCH] RFC: riscv: save the SR_SUM status over switches
Ben Dooks
ben.dooks at codethink.co.uk
Thu Mar 18 17:42:04 GMT 2021
On 18/03/2021 16:57, Alex Ghiti wrote:
> Hi Ben,
>
> Le 3/18/21 à 11:10 AM, Ben Dooks a écrit :
>> 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.
>
> Very nice, good catch !
Yes, this has taken about 3 days of work to find (thank you to Codethink
for not making me do more boring work).
I'll try and make a blog post, a talk and some of the tools/patches used
available for people to look at.
So far this patch has been running syz-stress under qemu-riscv64 for
about 2 hours without an issue. My heating bill might be a bit reduced
for today.
>>
>> There may be further issues with the mstatus flags with this, this
>> can be discussed further once some initial testing has been done.
>
> Yes, that must be discussed, I don't have opinion right now but if this
> is the only bit to save/restore, we may consider other options: disable
> preemption, using __typeof(x) like suggested by Dmitry to avoid being
> scheduled and maybe other options.
I had a go at the typeof, it made a lot of messy kernel warnings and I
am not sure if it will actually fully fix things as gcc may well move
things around anyway.
There really does need to be a fix for the amount of code that might
be executed under put_user(), but I think the correct thing to do is
to ensure that the flags are saved over the switch_to().
I'm not sure if disabling pre-emption is great either as there are now
kernel sections where the access is disabled for using the "unsafe"
versions of the user space access.
>>
>> 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;
>
> I would not call it "flags", I'd rather use "status", named after
> CSR_STATUS.
Ok, good idea. Not sure if it should be status or csr_status.
>
>> };
>> #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
>
> I don't think that clearing SR_SUM is needed here since you restore its
> value later on.
The restore I think requires the flag to have been cleared.
>> + 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)
>>
>
> Again, very nice finding, well done :)
>
> Alex
>
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--
Ben Dooks http://www.codethink.co.uk/
Senior Engineer Codethink - Providing Genius
https://www.codethink.co.uk/privacy.html
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