Messages in this thread | | | Date | Sat, 16 Oct 2021 09:14:46 +0200 | Subject | Re: [PATCH v2 2/2] x86: sgx_vepc: implement SGX_IOC_VEPC_REMOVE ioctl | From | Paolo Bonzini <> |
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On 16/10/21 00:29, Sean Christopherson wrote: > On Tue, Oct 12, 2021, Paolo Bonzini wrote: >> diff --git a/arch/x86/kernel/cpu/sgx/virt.c b/arch/x86/kernel/cpu/sgx/virt.c >> index 59cdf3f742ac..81a0a0f22007 100644 >> --- a/arch/x86/kernel/cpu/sgx/virt.c >> +++ b/arch/x86/kernel/cpu/sgx/virt.c >> @@ -150,6 +150,46 @@ static int sgx_vepc_free_page(struct sgx_epc_page *epc_page) >> return 0; >> } >> >> +static long sgx_vepc_remove_all(struct sgx_vepc *vepc) >> +{ >> + struct sgx_epc_page *entry; >> + unsigned long index; >> + long failures = 0; >> + >> + xa_for_each(&vepc->page_array, index, entry) { > > Might be worth a comment that xa_for_each() is safe to use concurrently with > xa_load/xa_store, i.e. this doesn't need to take vepc->lock.
I considered that to be part of the xarray contract (xa_store uses rcu_assign_pointer so it has release semantics, and vepc->page_array is essentially "store once").
> It does raise the > question of whether or not the kernel is responsible for providing deterministic > results if userspace/guest is accessing previously-unallocated pages.
Garbage in, garbage out -- but you're right below that garbage in, WARN out is not acceptable. I'm sending a v3 with documentation changes too.
Paolo
>> + int ret = sgx_vepc_remove_page(entry); > > I don't see anything that prevents userspace from doing SGX_IOC_VEPC_REMOVE_ALL > on multiple threads with the same vEPC. That means userspace can induce a #GP > due to concurrent access. Taking vepc->lock would solve that particular problem, > but I think that's a moot point because the EREMOVE locking rules are relative to > the SECS, not the individual page (because of refcounting). SGX_IOC_VEPC_REMOVE_ALL > on any two arbitrary vEPCs could induce a fault if they have children belonging to > the same enclave, i.e. share an SECS. > > Sadly, I think this needs to be: > > if (ret == SGX_CHILD_PRESENT) > failures++; > else if (ret) > return -EBUSY; > >> + switch (ret) { >> + case 0: >> + break; >> + >> + case SGX_CHILD_PRESENT: >> + failures++; >> + break; >> + >> + case SGX_ENCLAVE_ACT: >> + /* >> + * Unlike in sgx_vepc_free_page, userspace could be calling >> + * the ioctl while logical processors are running in the >> + * enclave; do not warn. >> + */ >> + return -EBUSY; >> + >> + default: >> + WARN_ONCE(1, EREMOVE_ERROR_MESSAGE, ret, ret); >> + failures++; >> + break; >> + } >> + cond_resched(); >> + } >> + >> + /* >> + * Return the number of pages that failed to be removed, so >> + * userspace knows that there are still SECS pages lying >> + * around. >> + */ >> + return failures; >> +} >
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