linux-kernel - Re: [PATCH] make 'user_access_begin()' do 'access

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Message-ID: <CAHk-=wiHe4dNHKpE4oGhwwmy23jNTuuFQAgWTGCjjxyOVYjG_w@mail.gmail.com>
Date:   Sun, 6 Jan 2019 11:18:05 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Guenter Roeck <linux@...ck-us.net>,
        Matt Turner <mattst88@...il.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Cc:     Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] make 'user_access_begin()' do 'access_ok()'

[ Re-sending the message because my first reply bounced - Guenther had
mis-typed the lkml address ]

On Sun, Jan 6, 2019 at 10:09 AM Guenter Roeck <linux@...ck-us.net> wrote:
>
> All alpha and sh4 (big and little endian) images fail to boot in qemu
> with this patch applied. Reverting it fixes the problem.

Funky. 99% of that patch is a complete no-op on non-x86.

The one exception is the strncpy_from_user() and strnlen_user() cases,
which didn't use to do access_ok() at all, and now essentially do.

But I think I see what may be the problem. I think the alpha version
of "access_ok()" is buggy.

Lookie here:

  #define __access_ok(addr, size) \
        ((get_fs().seg & (addr | size | (addr+size))) == 0)

and what it basically tests is of any of the high bits get set (the
USER_DS value is 0xfffffc0000000000).

And that's completely wrong for the "addr+size" check. It's off-by-one
for the case where we check to the very end of the user address space,
which is exactly what the strn*_user() functions do.

Why? Because "addr+size" will be exactly the size of the address
space, so trying to access the last byte of the user address space
will *fail* the __access_ok() check, even though it shouldn't.

So it's not really that that commit is buggy in itself, but it
triggers that off-by-one error in access_ok().

Side note: that alpha macro is buggy for another reason too: it
re-uses the arguments twice.

And SH has almost the exact same bug:

  #define __addr_ok(addr) \
        ((unsigned long __force)(addr) < current_thread_info()->addr_limit.seg)

so far so good: yes, a user address must be below the limit. But then:

  #define __access_ok(addr, size)         \
        (__addr_ok((addr) + (size)))

is wrong with the exact same off-by-one case: the case when
"addr+size" is exactly _equal_ to the limit is actually perfectly
fine.

The SH version is actually seriously buggy in another way: it doesn't
actually check for overflow, even though it did copy the _comment_
that talks about overflow.

So it turns out that both SH and alpha actually have completely
buggered implementations of access_ok(), but they happened to work
(although the SH overflow one is a serious serious security bug, not
that anybody likely cares about SH security)

Ho humm.

Maybe something like the attached patch? Entirely untested, I don't
have a cross-build environment, much less a boot setup.

It isn't trying to be clever, the end address is based on this logic:

        unsigned long __ao_end = __ao_a + __ao_b - !!__ao_b;    \

which basically says "subtract one unless the length was zero".

For a lot of access_ok() users the length is a constant, so this isn't
actually as expensive as it initially looks.

Does that fix things for you?

                   Linus

Download attachment "patch.diff" of type "application/x-patch" (1669 bytes)