From: Catalin Marinas <catalin.marinas@arm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
linux-arch <linux-arch@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC] raw_copy_from_user() semantics
Date: Wed, 22 Jul 2020 12:37:07 +0100 [thread overview]
Message-ID: <20200722113707.GC27540@gaia> (raw)
In-Reply-To: <CAHk-=wirA7zJJB17KJPCE-V9pKwn8VKxXTeiaM+F+Sa1Xd2SWA@mail.gmail.com>
On Sun, Jul 19, 2020 at 12:34:11PM -0700, Linus Torvalds wrote:
> On Sun, Jul 19, 2020 at 12:28 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> > I think we should try to get rid of the exact semantics.
>
> Side note: I think one of the historical reasons for the exact
> semantics was that we used to do things like the mount option copying
> with a "copy_from_user()" iirc.
>
> And that could take a fault at the end of the stack etc, because
> "copy_mount_options()" is nasty and doesn't get a size, and just
> copies "up to 4kB" of data.
>
> It's a mistake in the interface, but it is what it is. But we've
> always handled the inexact count there anyway by originally doing byte
> accesses, and at some point you optimized it to just look at where
> page boundaries might be..
And we may have to change this again since, with arm64 MTE, the page
boundary check is insufficient:
https://lore.kernel.org/linux-fsdevel/20200715170844.30064-25-catalin.marinas@arm.com/
While currently the fault path is unlikely to trigger, with MTE in user
space it's a lot more likely since the buffer (e.g. a string) is
normally less than 4K and the adjacent addresses would have a different
colour.
I looked (though briefly) into passing the copy_from_user() problem to
filesystems that would presumably know better how much to copy. In most
cases the options are string, so something like strncpy_from_user()
would work. For mount options as binary blobs (IIUC btrfs) maybe the fs
has a better way to figure out how much to copy.
> I think that was the only truly _valid_ case of "we actually copy data
> from user space, and we might need to handle a partial case", and
> exactly because of that, it had already long avoided the whole "assume
> copy_from_user gives us byte-accurate data before the fault".
With MTE, we didn't find any other instance of copy_from_user() where
the byte accuracy matters. The close relative, strncpy_from_user(),
already handles exact copying via a fall back to byte at a time.
--
Catalin
next prev parent reply other threads:[~2020-07-22 11:37 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-19 3:17 [RFC] raw_copy_from_user() semantics Al Viro
2020-07-19 19:28 ` Linus Torvalds
2020-07-19 19:34 ` Linus Torvalds
2020-07-22 11:37 ` Catalin Marinas [this message]
2020-07-22 13:14 ` David Laight
2020-07-22 16:53 ` Catalin Marinas
2020-07-23 8:37 ` David Laight
2020-07-23 10:18 ` Catalin Marinas
2020-07-23 10:34 ` David Laight
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