On Mon, Jun 03, 2019 at 11:30:54AM -0700, Xing, Cedric wrote: > > From: Christopherson, Sean J > > Sent: Monday, June 03, 2019 10:16 AM > > > > On Sun, Jun 02, 2019 at 12:29:35AM -0700, Xing, Cedric wrote: > > > Hi Sean, > > > > > > Generally I agree with your direction but think ALLOW_* flags are > > > completely internal to LSM because they can be both produced and > > > consumed inside an LSM module. So spilling them into SGX driver and > > > also user mode code makes the solution ugly and in some cases > > > impractical because not every enclave host process has a priori > > > knowledge on whether or not an enclave page would be EMODPE'd at > > runtime. > > > > In this case, the host process should tag *all* pages it *might* convert > > to executable as ALLOW_EXEC. LSMs can (and should/will) be written in > > such a way that denying ALLOW_EXEC is fatal to the enclave if and only > > if the enclave actually attempts mprotect(PROT_EXEC). > > What if those pages contain self-modifying code but the host doesn't know > ahead of time? Would it require ALLOW_WRITE|ALLOW_EXEC at EADD? Then would it > prevent those pages to start with PROT_EXEC? Without ALLOW_WRITE+ALLOW_EXEC, the enclave would build and launch, but fail at mprotect(..., PROT_WRITE), e.g. when it attempted to gain write access to do self-modifying code. And it would would fail irrespective of LSM restrictions. > Anyway, my point is that it is unnecessary even if it works. Unnecessary in an ideal world, yes. Realistically, it's the least bad option. > > Take the SELinux path for example. The only scenario in which > > PROT_WRITE is cleared from @allowed_prot is if the page *starts* with > > PROT_EXEC. > > If PROT_EXEC is denied on a page that starts RW, e.g. an EAUG'd page, > > then PROT_EXEC will be cleared from @allowed_prot. > > > > As Stephen pointed out, auditing the denials on @allowed_prot means the > > log will contain false positives of a sort. But this is more of a noise > > issue than true false positives. E.g. there are three possible outcomes > > for the enclave. > > > > - The enclave does not do EMODPE[PROT_EXEC] in any scenario, ever. > > Requesting ALLOW_EXEC is either a straightforward a userspace bug or > > a poorly written generic enclave loader. > > > > - The enclave conditionally performs EMODPE[PROT_EXEC]. In this case > > the denial is a true false positive. > > > > - The enclave does EMODPE[PROT_EXEC] and its host userspace then fails > > on mprotect(PROT_EXEC), i.e. the LSM denial is working as intended. > > The audit log will be noisy, but viewed as a whole the denials > > aren't > > false positives. > > What I was talking about was EMODPE[PROT_WRITE] on an RX page. As above, mprotect(..., PROT_WRITE) would fail without ALLOW_WRITE. > > The potential for noisy audit logs and/or false positives is unfortunate, > > but it's (by far) the lesser of many evils. > > > > > Theoretically speaking, what you really need is a per page flag (let's > > > name it WRITTEN?) indicating whether a page has ever been written to > > > (or more precisely, granted PROT_WRITE), which will be used to decide > > > whether to grant PROT_EXEC when requested in future. Given the fact > > > that all mprotect() goes through LSM and mmap() is limited to > > > PROT_NONE, it's easy for LSM to capture that flag by itself instead of > > asking user mode code to provide it. > > > > > > That said, here is the summary of what I think is a better approach. > > > * In hook security_file_alloc(), if @file is an enclave, allocate some > > data > > > structure to store for every page, the WRITTEN flag as described > > above. > > > WRITTEN is cleared initially for all pages. > > > > This would effectively require *every* LSM to duplicate the SGX driver's > > functionality, e.g. track per-page metadata, implement locking to > > prevent races between multiple mm structs, etc... > > Architecturally we shouldn't dictate how LSM makes decisions. ALLOW_* are no > difference than PROCESS__* or FILE__* flags, which are just artifacts to > assist particular LSMs in decision making. They are never considered part of > the LSM interface, even if other LSMs than SELinux may adopt the same/similar > approach. No, the flags are tracked and managed by SGX. We are not dictating LSM behavior in any way, e.g. an LSM could completely ignore @allowed_prot and nothing would break. > If code duplication is what you are worrying about, you can put them in a > library, or implement/export them in some new file (maybe > security/enclave.c?) as utility functions. Code duplication is the least of my concerns. Tracking file pointers would require a global list/tree of some form, along with a locking and/or RCU scheme to protect accesses to that container. Another lock would be needed to prevent races between mprotect() calls from different processes. > But spilling them into user mode is what I think is unacceptable. Why is it unacceptable? There's effectively no cost to userspace for SGX1. The ALLOW_* flags only come into play in the event of a noexec or LSM restriction, i.e. worst case scenario an enclave that wants to do arbitrary self-modifying code can declare RWX on everything.
