Messages in this thread | | | Subject | Re: [PATCH V2 16/32] x86/sgx: Support restricting of enclave page permissions | Date | Thu, 17 Mar 2022 09:28:45 -0500 | From | "Haitao Huang" <> |
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Hi
On Thu, 17 Mar 2022 02:11:28 -0500, Jarkko Sakkinen <jarkko@kernel.org> wrote:
> On Thu, Mar 17, 2022 at 09:01:07AM +0200, Jarkko Sakkinen wrote: >> On Mon, Mar 14, 2022 at 10:39:36AM -0500, Haitao Huang wrote: >> > Hi Jarkko >> > >> > On Sun, 13 Mar 2022 21:58:51 -0500, Jarkko Sakkinen >> <jarkko@kernel.org> >> > wrote: >> > >> > > On Mon, Mar 14, 2022 at 04:50:56AM +0200, Jarkko Sakkinen wrote: >> > > > On Mon, Mar 14, 2022 at 04:49:37AM +0200, Jarkko Sakkinen wrote: >> > > > > On Fri, Mar 11, 2022 at 09:53:29AM -0800, Reinette Chatre wrote: >> > > > > >> > > > > > I saw Haitao's note that EMODPE requires "Read access >> permitted >> > > > by enclave". >> > > > > > This motivates that EMODPR->PROT_NONE should not be allowed >> > > > since it would >> > > > > > not be possible to relax permissions (run EMODPE) after that. >> > > > Even so, I >> > > > > > also found in the SDM that EACCEPT has the note "Read access >> > > > permitted >> > > > > > by enclave". That seems to indicate that EMODPR->PROT_NONE is >> > > > not practical >> > > > > > from that perspective either since the enclave will not be >> able to >> > > > > > EACCEPT the change. Does that match your understanding? >> > > > > >> > > > > Yes, PROT_NONE should not be allowed. >> > > > > >> > > > > This is however the real problem. >> > > > > >> > > > > The current kernel patch set has inconsistent API and EMODPR >> ioctl is >> > > > > simply unacceptable. It also requires more concurrency >> management >> > > > from >> > > > > user space run-time, which would be heck a lot easier to do in >> the >> > > > kernel. >> > > > > >> > > > > If you really want EMODPR as ioctl, then for consistencys sake, >> > > > then EAUG >> > > > > should be too. Like this when things go opposite directions, >> this >> > > > patch set >> > > > > plain and simply will not work out. >> > > > > >> > > > > I would pick EAUG's strategy from these two as it requires half >> > > > the back >> > > > > calls to host from an enclave. I.e. please combine mprotect() >> and >> > > > EMODPR, >> > > > > either in the #PF handler or as part of mprotect(), which ever >> > > > suits you >> > > > > best. >> > > > > >> > > > > I'll try demonstrate this with two examples. >> > > > > >> > > > > mmap() could go something like this() (simplified): >> > > > > 1. Execution #UD's to SYSCALL. >> > > > > 2. Host calls enclave's mmap() handler with mmap() parameters. >> > > > > 3. Enclave up-calls host's mmap(). >> > > > > 4. Loops the range with EACCEPTCOPY. >> > > > > >> > > > > mprotect() has to be done like this: >> > > > > 1. Execution #UD's to SYSCALL. >> > > > > 2. Host calls enclave's mprotect() handler. >> > > > > 3. Enclave up-calls host's mprotect(). >> > > > > 4. Enclave up-calls host's ioctl() to >> SGX_IOC_ENCLAVE_PERMISSIONS. >> > >> > I assume up-calls here are ocalls as we call them in our >> implementation, >> > which are the calls enclave make to untrusted side via EEXIT. >> >ar >> > If so, can your implementation combine this two up-calls into one, >> then host >> > side just do ioctl() and mprotect to kernel? If so, would that >> address your >> > concern about extra up-calls? >> > >> > >> > > > > 3. Loops the range with EACCEPT. >> > > > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ >> > > > 5. Loops the range with EACCEPT + EMODPE. >> > > > >> > > > > This is just terrible IMHO. I hope these examples bring some >> insight. >> > > >> > > E.g. in Enarx we have to add a special up-call (so called enarxcall >> in >> > > intermediate that we call sallyport, which provides shared buffer to >> > > communicate with the enclave) just for reseting the range with >> PROT_READ. >> > > Feel very redundant, adds ugly cruft and is completely opposite >> strategy >> > > to >> > > what you've chosen to do with EAUG, which is I think correct choice >> as >> > > far >> > > as API is concerned. >> > >> > The problem with EMODPR on #PF is that kernel needs to know what >> permissions >> > requested from enclave at the time of #PF. So enclave has to make at >> least >> > one call to kernel (again via ocall in our case, I assume up-call in >> your >> > case) to make the change. >> >> The #PF handler should do unconditionally EMODPR with PROT_READ. > > Or mprotect(), as long as secinfo contains PROT_READ. I don't care about > this detail hugely anymore because it does not affect uapi. > > Using EMODPR as a permission control mechanism is a ridiculous idea, and > I cannot commit to maintain a broken uapi. >
Jarkko, how would automatically forcing PROT_READ on #PF work for this sequence?
1) EAUG a page (has to be RW) 2) EACCEPT(RW) 3) enclave copies some data to page 4) enclave wants to change permission to R
If you are proposing mprotect, then as I indicated earlier, please address concerns raised by Reinette: https://lore.kernel.org/linux-sgx/e1c04077-0165-c5ec-53be-7fd732965e80@intel.com/
Thanks Haitao
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