On 11/05/2017 11:35 AM, Florian Weimer wrote: > I'm working on adding memory protection key support to glibc. > > I don't think pkey_free, as it is implemented today, is very safe due to > key reuse by a subsequent pkey_alloc. I see two problems: > > (A) pkey_free allows reuse for they key while there are still mappings > that use it. > > (B) If a key is reused, existing threads retain their access rights, > while there is an expectation that pkey_alloc denies access for the > threads except the current one. I have a somewhat related question to API/documentation of pkeys, that came up from a customer interested in using the feature. The man page of mprotect/pkey_mprotect doesn't say how to remove a pkey from a set of pages, i.e. reset it to the default 0 (or the exec-only pkey), so initially they thought there's no way to do that. Calling pkey_mprotect() with pkey==0 will fail with EINVAL, because 0 was not allocated by pkey_alloc(). That's fair I guess. What seems to work to reset the pkey is either calling plain mprotect(), or calling pkey_mprotect() with pkey == -1, as the former is just wired to the latter. So, is plain mprotect() the intended way to reset a pkey and should it be explicitly documented in the man page? And, was the pkey == -1 internal wiring supposed to be exposed to the pkey_mprotect() signal, or should there have been a pre-check returning EINVAL in SYSCALL_DEFINE4(pkey_mprotect), before calling do_mprotect_pkey())? I assume it's too late to change it now anyway (or not?), so should we also document it? Thanks, Vlastimil
