linux-kernel - Re: [PATCH v6 4/4] mm: vmalloc: convert vread() to vread

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <3e2d1f69-5cd8-8824-0a2e-a1c2c9029f66@redhat.com>
Date:   Wed, 22 Mar 2023 18:02:34 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Baoquan He <bhe@...hat.com>, Uladzislau Rezki <urezki@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Liu Shixin <liushixin2@...wei.com>,
        Jiri Olsa <jolsa@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v6 4/4] mm: vmalloc: convert vread() to vread_iter()

On 22.03.23 15:55, Lorenzo Stoakes wrote:
> Having previously laid the foundation for converting vread() to an iterator
> function, pull the trigger and do so.
> 
> This patch attempts to provide minimal refactoring and to reflect the
> existing logic as best we can, for example we continue to zero portions of
> memory not read, as before.
> 
> Overall, there should be no functional difference other than a performance
> improvement in /proc/kcore access to vmalloc regions.
> 
> Now we have eliminated the need for a bounce buffer in read_kcore_iter(),
> we dispense with it, and try to write to user memory optimistically but
> with faults disabled via copy_page_to_iter_nofault(). We already have
> preemption disabled by holding a spin lock.
> 
> If this fails, we fault in and retry a single time. This is a conservative
> approach intended to avoid spinning on vread_iter() if we repeatedly
> encouter issues reading from it.

I have to ask again: Can you comment why that is ok? You might end up 
signaling -EFAULT to user space simply because swapping/page 
migration/whatever triggered at the wrong time.

That could break existing user space or which important part am I missing?

-- 
Thanks,

David / dhildenb