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Date: Thu, 22 Oct 2020 20:27:22 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move
 rw_copy_check_uvector() into lib/iov_iter.c"
Message-ID: <20201022192722.GW3576660@ZenIV.linux.org.uk>
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Sender: Al Viro <viro@ftp.linux.org.uk>
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To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>, David Laight <David.Laight@aculab.com>, Christoph Hellwig <hch@lst.de>, David Hildenbrand <david@redhat.com>, Greg KH <gregkh@linuxfoundation.org>, "kernel-team@android.com" <kernel-team@android.com>, Andrew Morton <akpm@linux-foundation.org>, Jens Axboe <axboe@kernel.dk>, David Howells <dhowells@redhat.com>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>, "linux-parisc@vger.kernel.org" <linux-parisc@vger.kernel.org>, "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>, "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>, "sparclinux@vger.kernel.org" <sparclinux@vger.kernel.org>, "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>, "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>, "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>, "linux-aio@kvack.org" <linux-aio@kvack.org>, "io-uring@vger.kernel.org" <io-uring@vger.kernel.org>, "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "netdev@vger.kernel.org" <netdev@vger.kernel.org>, "keyrings@vger.kernel.org" <keyrings@vger.kernel.org>, "linux-security-module@vger.kernel.org" <linux-security-module@vger.kernel.org>

On Thu, Oct 22, 2020 at 08:24:58PM +0100, Al Viro wrote:
> On Thu, Oct 22, 2020 at 12:04:52PM -0700, Nick Desaulniers wrote:
> 
> > Passing an `unsigned long` as an `unsigned int` does no such
> > narrowing: https://godbolt.org/z/TvfMxe (same vice-versa, just tail
> > calls, no masking instructions).
> > So if rw_copy_check_uvector() is inlined into import_iovec() (looking
> > at the mainline@1028ae406999), then children calls of
> > `rw_copy_check_uvector()` will be interpreting the `nr_segs` register
> > unmodified, ie. garbage in the upper 32b.
> 
> FWIW,
> 
> void f(unsinged long v)

s/unsinged/unsigned/, obviously...