Re: [RFC] raw_copy_from_user() semantics

[Catalin Marinas <catalin.marinas@arm.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