Re: [RFC PATCH v3 1/1] x86/sgx: Explicitly give up the CPU in EDMM's ioctl() to avoid softlockup

[Jarkko Sakkinen]

On Thu May 16, 2024 at 12:55 AM EEST, Haitao Huang wrote:
> On Wed, 15 May 2024 01:55:21 -0500, Bojun Zhu <zhubojun.zbj@xxxxxxxxxxxx>  
> wrote:
>
> > EDMM's ioctl()s support batch operations, which may be
> > time-consuming. Try to explicitly give up the CPU as the prefix
> > operation at the every begin of "for loop" in
> > sgx_enclave_{ modify_types | restrict_permissions | remove_pages}
> > to give other tasks a chance to run, and avoid softlockup warning.
> >
> > Additionally perform pending signals check as the prefix operation,
> > and introduce sgx_check_signal_and_resched(),
> > which wraps all the checks.
> >
> > The following has been observed on Linux v6.9-rc5 with kernel
> > preemptions disabled(by configuring "PREEMPT_NONE=y"), when kernel
> > is requested to restrict page permissions of a large number of EPC pages.
> >
> >     ------------[ cut here ]------------
> >     watchdog: BUG: soft lockup - CPU#45 stuck for 22s!
> >     ...
> >     RIP: 0010:sgx_enclave_restrict_permissions+0xba/0x1f0
> >     ...
> >     Call Trace:
> >      sgx_ioctl
> >      __x64_sys_ioctl
> >      x64_sys_call
> >      do_syscall_64
> >      entry_SYSCALL_64_after_hwframe
> >     ------------[ end trace ]------------
> >
> > Signed-off-by: Bojun Zhu <zhubojun.zbj@xxxxxxxxxxxx>
> > ---
> >  arch/x86/kernel/cpu/sgx/ioctl.c | 40 +++++++++++++++++++++++----------
> >  1 file changed, 28 insertions(+), 12 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c  
> > b/arch/x86/kernel/cpu/sgx/ioctl.c
> > index b65ab214bdf5..6199f483143e 100644
> > --- a/arch/x86/kernel/cpu/sgx/ioctl.c
> > +++ b/arch/x86/kernel/cpu/sgx/ioctl.c
> > @@ -365,6 +365,20 @@ static int sgx_validate_offset_length(struct  
> > sgx_encl *encl,
> >  	return 0;
> >  }
> > +/*
> > + * Check signals and invoke scheduler. Return true for a pending signal.
> > + */
> > +static bool sgx_check_signal_and_resched(void)
> > +{
> > +	if (signal_pending(current))
> > +		return true;
> > +
> > +	if (need_resched())
> > +		cond_resched();
> > +
> > +	return false;
> > +}
> > +
> >  /**
> >   * sgx_ioc_enclave_add_pages() - The handler for  
> > %SGX_IOC_ENCLAVE_ADD_PAGES
> >   * @encl:       an enclave pointer
> > @@ -409,7 +423,7 @@ static long sgx_ioc_enclave_add_pages(struct  
> > sgx_encl *encl, void __user *arg)
> >  	struct sgx_enclave_add_pages add_arg;
> >  	struct sgx_secinfo secinfo;
> >  	unsigned long c;
> > -	int ret;
> > +	int ret = -ERESTARTSYS;
> > 	if (!test_bit(SGX_ENCL_CREATED, &encl->flags) ||
> >  	    test_bit(SGX_ENCL_INITIALIZED, &encl->flags))
> > @@ -432,15 +446,8 @@ static long sgx_ioc_enclave_add_pages(struct  
> > sgx_encl *encl, void __user *arg)
> >  		return -EINVAL;
> > 	for (c = 0 ; c < add_arg.length; c += PAGE_SIZE) {
> > -		if (signal_pending(current)) {
> > -			if (!c)
> > -				ret = -ERESTARTSYS;
> > -
> > +		if (sgx_check_signal_and_resched())
> >  			break;
> > -		}
>
> ERESTARTSYS is only appropriate if we have not EADDed any pages yet.
> If we got interrupted in the middle, we should return 0. User space would  
> check the 'count' returned and decide to recall this ioctl() with  
> 'offset'  reset to the next page, and adjust length.

Good catch! Thanks Haitao for the remark.

BR, Jarkko