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Date:   Tue, 8 Oct 2019 21:16:16 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH] Convert filldir[64]() from __put_user() to
 unsafe_put_user()
Message-ID: <20191008201616.GW26530@ZenIV.linux.org.uk>
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On Tue, Oct 08, 2019 at 08:58:58PM +0100, Al Viro wrote:

> That's powerpc.  And while the constant-sized bits are probably pretty
> useless there as well, note the allow_read_from_user()/prevent_read_from_user()
> part.  Looks suspiciously similar to user_access_begin()/user_access_end()...
> 
> The difference is, they have separate "for read" and "for write" primitives
> and they want the range in their user_access_end() analogue.  Separating
> the read and write isn't a problem for callers (we want them close to
> the actual memory accesses).  Passing the range to user_access_end() just
> might be tolerable, unless it makes you throw up...

	BTW, another related cleanup is futex_atomic_op_inuser() and
arch_futex_atomic_op_inuser().  In the former we have
        if (!access_ok(uaddr, sizeof(u32)))
                return -EFAULT;

        ret = arch_futex_atomic_op_inuser(op, oparg, &oldval, uaddr);
        if (ret)
                return ret;
and in the latter we've got STAC/CLAC pairs stuck into inlined bits
on x86.  As well as allow_write_to_user(uaddr, sizeof(*uaddr)) on
ppc...

I don't see anything in x86 one objtool would've barfed if we pulled
STAC/CLAC out and turned access_ok() into user_access_begin(),
with matching user_access_end() right after the call of 
arch_futex_atomic_op_inuser().  Everything is inlined there and
no scary memory accesses would get into the scope (well, we do
have
        if (!ret)
                *oval = oldval;
in the very end of arch_futex_atomic_op_inuser() there, but oval
is the address of a local variable in the sole caller; if we run
with kernel stack on ring 3 page, we are deeply fucked *and*
wouldn't have survived that far into futex_atomic_op_inuser() anyway ;-)