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    SubjectRe: [PATCH RFC] uaccess: user_access_begin_after_access_ok()
    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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