Re: [RFC] raw_copy_from_user() semantics

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Jul 23, 2020 at 08:37:27AM +0000, David Laight wrote:
> From: Catalin Marinas
> > Sent: 22 July 2020 17:54
> > On Wed, Jul 22, 2020 at 01:14:21PM +0000, David Laight wrote:
> > > From: Catalin Marinas
> > > > Sent: 22 July 2020 12:37
> > > > 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.
> > >
> > > What about changing the mount code to loop calling get_user()
> > > to read aligned words until failure?
> > > Mount is fairly uncommon and the extra cost is probably small compared
> > > to the rest of doing a mount.
> > 
> > Before commit 12efec560274 ("saner copy_mount_options()"), it was using
> > single-byte get_user(). That could have been optimised for aligned words
> > reading but I don't really think it's worth the hassle. Since the source
> > and destination don't have the same alignment and some architecture
> > don't support unaligned accesses (for storing to the kernel buffer), it
> > would just make this function unnecessarily complicated.
> 
> It could do aligned words if the user buffer is aligned (it will be
> most of the time) and bytes otherwise.
> 
> Or just fallback to a byte loop if the full 4k read fails.

That's what I'm proposing here (needed for arm64 MTE):

https://lore.kernel.org/linux-fsdevel/20200715170844.30064-25-catalin.marinas@arm.com/

-- 
Catalin