On Fri, Aug 16, 2024 at 02:52:28PM +0000, Edgecombe, Rick P wrote:
On Fri, 2024-08-16 at 09:44 +0100, Catalin Marinas wrote:
After a token is consumed normally, it doesn't set it to zero. Instead it sets it to a "previous-ssp token". I don't think we actually want to do that here though because it involves the old SSP, which doesn't really apply in this case. I don't see any problem with zero, but was there any special thinking behind it?
BTW, since it's the parent setting up the shadow stack in its own address space before forking, I think at least the read can avoid access_remote_vm() and we could do it earlier, even before the new process is created.
Hmm. Makes sense. It's a bit racy since the parent could consume that token from another thread, but it would be a race in any case.
More on the race below. If we handle it properly, we don't need the separate checks.
+ if (access_remote_vm(mm, addr, &val, sizeof(val), + FOLL_FORCE | FOLL_WRITE) != sizeof(val)) + goto out;
The GUPs still seem a bit unfortunate for a couple reasons: - We could do a CMPXCHG version and are just not (I see ARM has identical code in gcs_consume_token()). It's not the only race like this though FWIW. - I *think* this is the only unprivileged FOLL_FORCE that can write to the current process in the kernel. As is, it could be used on normal RO mappings, at least in a limited way. Maybe another point for the VMA check. We'd want to check that it is normal shadow stack? - Lingering doubts about the wisdom of doing GUPs during task creation.
I don't like the access_remote_vm() either. In the common (practically only) case with CLONE_VM, the mm is actually current->mm, so no need for a GUP.
On the x86 side, we don't have a shadow stack access CMPXCHG. We will have to GUP and do a normal CMPXCHG off of the direct map to handle it fully properly in any case (CLONE_VM or not).
I guess we could do the same here and for the arm64 gcs_consume_token(). Basically get_user_page_vma_remote() gives us the page together with the vma that you mentioned needs checking. We can then do a cmpxchg directly on the page_address(). It's probably faster anyway than doing GUP twice.