Ram Pai <linuxram@xxxxxxxxxx> writes: > On Thu, Mar 19, 2020 at 09:25:48AM +0530, Aneesh Kumar K.V wrote: >> Fetch pkey from vma instead of linux page table. Also document the fact that in >> some cases the pkey returned in siginfo won't be the same as the one we took >> keyfault on. Even with linux page table walk, we can end up in a similar scenario. > > There is no way to correctly ensure that the key returned through > siginfo is actually the key that took the fault. Either get it > from page table or get it from the corresponding vma. That is correct. > > So we had to choose the lesser evil. Getting it from the page table was > faster, and did not involve taking any locks. That is because you are locks which need to be held on page table walk. >Getting it from the vma > was slower, since it needed locks. Also I faintly recall, there > is a scenario where the address that gets a key fault, has no > corresponding VMA associated with it yet. I would be interested in this. For now IIUC even x86 fetch the key from VMA. > > Hence the logic used was -- > if it is key-fault, than procure the key quickly > from the page table. In the unlikely event that the fault is > something else, but still has a non-permissive key associated > with it, get the key from the vma. I am fixing that logic further in the next patch. I do have a test case attached for that. We always check for the key in the vma and if it allows access, then we retry. > > A well written application should avoid changing the key of an address > space without synchronizing the corresponding threads that operate in > that address range. However, if the application ignores to do so, than > it is vulnerable to a undefined behavior. There is no way to prove that > the reported key is correct or incorrect, since there is no provable > order between the two events; the key-fault event and the key-change > event. > > Hence I think the change proposed in this patch may not be necessary. > RP The change is needed so that we can make the page table walk safer. -aneesh
