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Message-ID: <20170510065301.GC4115@infradead.org>
Date: Tue, 9 May 2017 23:53:01 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Al Viro <viro@...IV.linux.org.uk>
Cc: Christoph Hellwig <hch@...radead.org>,
Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>,
Greg KH <greg@...ah.com>, Thomas Garnier <thgarnie@...gle.com>,
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René Nyffenegger <mail@...enyffenegger.ch>,
Andrew Morton <akpm@...ux-foundation.org>,
"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
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Linus Torvalds <torvalds@...ux-foundation.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: Re: [PATCH v9 1/4] syscalls: Verify address
limit before returning to user-mode
On Wed, May 10, 2017 at 04:12:54AM +0100, Al Viro wrote:
> What's the point? What's wrong with having kernel_read()/kernel_readv()/etc.?
> You still have set_fs() in there; doing that one level up in call chain would
> be just fine... IDGI.
The problem is that they modify the address limit, which the whole
subthread here wants to get rid of.
> Broken commit: "net: don't play with address limits in kernel_recvmsg".
> It would be OK if it was only about data. Unfortunately, that's not
> true in one case: svc_udp_recvfrom() wants ->msg_control.
Dropped, but we'll need to fix that eventually.
> Another delicate place: you can't assume that write() always advances
> file position by its (positive) return value. btrfs stuff is sensitive
> to that.
If we don't want to assume that we need to pass pointer to pos to
kernel_read/write. Which might be a good idea in general.
> ashmem probably _is_ OK with demanding ->read_iter(), but I'm not sure
> about blind asma->file->f_pos += ret. That's begging for races. Actually,
> scratch that - it *is* racy.
I think the proper fix is to not even bother to maintain f_pos of the
backing file, as we don't ever use it - all reads from it pass in
an explicit position anyway.
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