Messages in this thread | | | Date | Tue, 2 Jun 2020 16:32:55 -0400 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok() |
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On Tue, Jun 02, 2020 at 10:18:09AM -0700, Linus Torvalds wrote: > On Tue, Jun 2, 2020 at 9:33 AM Al Viro <viro@zeniv.linux.org.uk> wrote: > > > > > > > > It's not clear whether we need a new API, I think __uaccess_being() has the > > > assumption that the address has been validated by access_ok(). > > > > __uaccess_begin() is a stopgap, not a public API. > > Correct. It's just an x86 implementation detail. > > > The problem is real, but "let's add a public API that would do user_access_begin() > > with access_ok() already done" is no-go. > > Yeah, it's completely pointless. > > The solution to this is easy: remove the incorrect and useless early > "access_ok()". Boom, done.
Hmm are you sure we can drop it? access_ok is done in the context of the process. Access itself in the context of a kernel thread that borrows the same mm. IIUC if the process can be 32 bit while the kernel is 64 bit, access_ok in the context of the kernel thread will not DTRT.
> Then use user_access_begin() and the appropriate unsage_get/put_user() > sequence, and user_access_end(). > > The range test that user-access-begin does is not just part of the > ABI, it's just required in general. We have almost thirty years of > history of trying to avoid it, AND IT WAS ALL BOGUS. > > The fact is, the range check is pretty damn cheap, and not doing the > range check has always been a complete and utter disaster. > > You have exactly two cases: > > (a) the access_ok() would be right above the code and can't be missed > > (b) not > > and in (a) the solution is: remove the access_ok() and let > user_access_begin() do the range check. > > In (b), the solution is literally "DON'T DO THAT!" > > Because EVERY SINGLE TIME people have eventually noticed (possibly > after code movement) that "oops, we never did the access_ok at all, > and now we can be fooled into kernel corruption and a security issue". > > And even if that didn't happen, the worry was there. > > End result: use user_access_begin() and stop trying to remove the two > cycles or whatever of the limit checking cost. The "upside" of > removing that limit check just isn't. It's a downside. > > Linus
That's true. Limit check cost is measureable but very small. It's the speculation barrier that's costly.
-- MST
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