Messages in this thread | | | Subject | Re: [PATCH v2 1/4] powerpc/64s: implement probe_kernel_read/write without touching AMR | From | Daniel Borkmann <> | Date | Tue, 7 Apr 2020 11:09:50 +0200 |
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Hey Nicholas,
On 4/7/20 6:01 AM, Nicholas Piggin wrote: > Nicholas Piggin's on April 3, 2020 9:05 pm: >> Christophe Leroy's on April 3, 2020 8:31 pm: >>> Le 03/04/2020 à 11:35, Nicholas Piggin a écrit : >>>> There is no need to allow user accesses when probing kernel addresses. >>> >>> I just discovered the following commit >>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=75a1a607bb7e6d918be3aca11ec2214a275392f4 >>> >>> This commit adds probe_kernel_read_strict() and probe_kernel_write_strict(). >>> >>> When reading the commit log, I understand that probe_kernel_read() may >>> be used to access some user memory. Which will not work anymore with >>> your patch. >> >> Hmm, I looked at _strict but obviously not hard enough. Good catch. >> >> I don't think probe_kernel_read() should ever access user memory, >> the comment certainly says it doesn't, but that patch sort of implies >> that they do. >> >> I think it's wrong. The non-_strict maybe could return userspace data to >> you if you did pass a user address? I don't see why that shouldn't just >> be disallowed always though. >> >> And if the _strict version is required to be safe, then it seems like a >> bug or security issue to just allow everyone that doesn't explicitly >> override it to use the default implementation. >> >> Also, the way the weak linkage is done in that patch, means parisc and >> um archs that were previously overriding probe_kernel_read() now get >> the default probe_kernel_read_strict(), which would be wrong for them. > > The changelog in commit 75a1a607bb7 makes it a bit clearer. If the > non-_strict variant is used on non-kernel addresses, then it might not > return -EFAULT or it might cause a kernel warning. The _strict variant > is supposed to be usable with any address and it will return -EFAULT if > it was not a valid and mapped kernel address. > > The non-_strict variant can not portably access user memory because it > uses KERNEL_DS, and its documentation says its only for kernel pointers. > So powerpc should be fine to run that under KUAP AFAIKS. > > I don't know why the _strict behaviour is not just made default, but > the implementation of it does seem to be broken on the archs that > override the non-_strict variant.
Yeah, we should make it default and only add a "opt out" for the old legacy cases; there was also same discussion started over here just recently [0].
Thanks, Daniel
[0] https://lore.kernel.org/lkml/20200403133533.GA3424@infradead.org/T/
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