Subject: Re: [PATCH 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults

From: Catalin Marinas <[email-masked]>
Date: Fri, 26 Nov 2021 22:57:44 +0000

On Fri, Nov 26, 2021 at 11:29:45PM +0100, Andreas Gruenbacher wrote:
> On Thu, Nov 25, 2021 at 11:42 PM Catalin Marinas <[email-masked]> wrote:
> > As per Linus' reply, we can work around this by doing
> > a sub-page fault_in_writable(point_of_failure, align) where 'align'
> > should cover the copy_to_user() impreciseness.
> >
> > (of course, fault_in_writable() takes the full size argument but behind
> > the scene it probes the 'align' prefix at sub-page fault granularity)
> 
> That doesn't make sense; we don't want fault_in_writable() to fail or
> succeed depending on the alignment of the address range passed to it.

If we know that the arch copy_to_user() has an error of say maximum 16
bytes (or 15 rather on arm64), we can instead get fault_in_writeable()
to probe first 16 bytes rather than 1.

> Have a look at the below code to see what I mean.  Function
> copy_to_user_nofault_unaligned() should be further optimized, maybe as
> mm/maccess.c:copy_from_kernel_nofault() and/or per architecture
> depending on the actual alignment rules; I'm not sure.
[...]
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -2051,13 +2051,30 @@ static noinline int key_in_sk(struct btrfs_key *key,
>  	return 1;
>  }
>  
> +size_t copy_to_user_nofault_unaligned(void __user *to, void *from, size_t size)
> +{
> +	size_t rest = copy_to_user_nofault(to, from, size);
> +
> +	if (rest) {
> +		size_t n;
> +
> +		for (n = size - rest; n < size; n++) {
> +			if (copy_to_user_nofault(to + n, from + n, 1))
> +				break;
> +		}
> +		rest = size - n;
> +	}
> +	return rest;

That's what I was trying to avoid. That's basically a fall-back to byte
at a time copy (we do this in copy_mount_options(); at some point we
even had a copy_from_user_exact() IIRC).

Linus' idea (if I got it correctly) was instead to slightly extend the
probing in fault_in_writeable() for the beginning of the buffer from 1
byte to some per-arch range.

I attempted the above here and works ok:

https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=devel/btrfs-live-lock-fix

but too late to post it this evening, I'll do it in the next day or so
as an alternative to this series.

-- 
Catalin

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