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Date:   Fri, 6 Mar 2020 02:29:15 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Jann Horn <jannh@google.com>,
        Alexander Potapenko <glider@google.com>,
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Subject: Re: [PATCH v2 2/3] binder: do not initialize locals passed to
 copy_from_user()
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On Thu, Mar 05, 2020 at 10:03:25AM +0100, Rasmus Villemoes wrote:

> Does copy_from_user guarantee to zero-initialize the remaining buffer if
> copying fails partway through?

That's guaranteed, short of raw_copy_from_user() being completely broken.
What raw_copy_from_user() implementation must guarantee is that if
raw_copy_from_user(to, from, N) returns N - n, then
	* 0 <= n <= N
	* all attempted reads had been within the range [from .. from + N - 1]
	* all stores had been to the range [to .. to + n - 1] and every byte
within that range had been overwritten
	* for all k in [0 .. n - 1], the value stored at to[k] by the end of
the call is equal to the value that would've been possible to read from
from[k] at some point during the call.  In particular, for all bytes in
range [from .. from + n - 1] there had been a successful read of some
object containing that byte.
	* if everything in [from .. from + N - 1] is readable, the call
will copy the entire range into [to .. to + N - 1] and return 0.

Provided that, copy_from_user() will leave no uninitialized data in
destination object in any case, success or no success.