Re: [PATCH] fs/proc/kcore: reinstate bounce buffer for KCORE_TEXT regions

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Tue, Aug 01, 2023 at 06:34:26PM +0200, David Hildenbrand wrote:
> On 01.08.23 18:33, Lorenzo Stoakes wrote:
> > On Tue, Aug 01, 2023 at 11:05:40AM +0200, David Hildenbrand wrote:
> > > On 31.07.23 23:50, Lorenzo Stoakes wrote:
> > > > Some architectures do not populate the entire range categorised by
> > > > KCORE_TEXT, so we must ensure that the kernel address we read from is
> > > > valid.
> > > >
> > > > Unfortunately there is no solution currently available to do so with a
> > > > purely iterator solution so reinstate the bounce buffer in this instance so
> > > > we can use copy_from_kernel_nofault() in order to avoid page faults when
> > > > regions are unmapped.
> > > >
> > > > This change partly reverts commit 2e1c0170771e ("fs/proc/kcore: avoid
> > > > bounce buffer for ktext data"), reinstating the bounce buffer, but adapts
> > > > the code to continue to use an iterator.
> > > >
> > > > Fixes: 2e1c0170771e ("fs/proc/kcore: avoid bounce buffer for ktext data")
> > > > Reported-by: Jiri Olsa <olsajiri@gmail.com>
> > > > Closes: https://lore.kernel.org/all/ZHc2fm+9daF6cgCE@krava
> > > > Cc: stable@vger.kernel.org
> > > > Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> > > > ---
> > > >    fs/proc/kcore.c | 26 +++++++++++++++++++++++++-
> > > >    1 file changed, 25 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> > > > index 9cb32e1a78a0..3bc689038232 100644
> > > > --- a/fs/proc/kcore.c
> > > > +++ b/fs/proc/kcore.c
> > > > @@ -309,6 +309,8 @@ static void append_kcore_note(char *notes, size_t *i, const char *name,
> > > >    static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
> > > >    {
> > > > +	struct file *file = iocb->ki_filp;
> > > > +	char *buf = file->private_data;
> > > >    	loff_t *fpos = &iocb->ki_pos;
> > > >    	size_t phdrs_offset, notes_offset, data_offset;
> > > >    	size_t page_offline_frozen = 1;
> > > > @@ -554,11 +556,22 @@ static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
> > > >    			fallthrough;
> > > >    		case KCORE_VMEMMAP:
> > > >    		case KCORE_TEXT:
> > > > +			/*
> > > > +			 * Sadly we must use a bounce buffer here to be able to
> > > > +			 * make use of copy_from_kernel_nofault(), as these
> > > > +			 * memory regions might not always be mapped on all
> > > > +			 * architectures.
> > > > +			 */
> > > > +			if (copy_from_kernel_nofault(buf, (void *)start, tsz)) {
> > > > +				if (iov_iter_zero(tsz, iter) != tsz) {
> > > > +					ret = -EFAULT;
> > > > +					goto out;
> > > > +				}
> > > >    			/*
> > > >    			 * We use _copy_to_iter() to bypass usermode hardening
> > > >    			 * which would otherwise prevent this operation.
> > > >    			 */
> > >
> > > Having a comment at this indentation level looks for the else case looks
> > > kind of weird.
> >
> > Yeah, but having it indented again would be weird and seem like it doesn't
> > apply to the block below, there's really no good spot for it and
> > checkpatch.pl doesn't mind so I think this is ok :)
> >
> > >
> > > (does that comment still apply?)
> >
> > Hm good point, actually, now we're using the bounce buffer we don't need to
> > avoid usermode hardening any more.
> >
> > However since we've established a bounce buffer ourselves its still
> > appropriate to use _copy_to_iter() as we know the source region is good to
> > copy from.
> >
> > To make life easy I'll just respin with an updated comment :)
>
> I'm not too picky this time, no need to resend if everybody else is fine :P
>

Haha you know the classic Lorenzo respin spiral and want to avoid it I see ;)

The comment is actually inaccurate now, so to avoid noise + make life easy
(maybe) for Andrew here's a fix patch that just corrects the comment:-

----8<----

From d2b8fb271f21b79048e5630699133f77a93d0481 Mon Sep 17 00:00:00 2001
From: Lorenzo Stoakes <lstoakes@gmail.com>
Date: Tue, 1 Aug 2023 17:36:08 +0100
Subject: [PATCH] fs/proc/kcore: correct comment

Correct comment to be strictly correct about reasoning.

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
---
 fs/proc/kcore.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index 3bc689038232..23fc24d16b31 100644
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -568,8 +568,8 @@ static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
 					goto out;
 				}
 			/*
-			 * We use _copy_to_iter() to bypass usermode hardening
-			 * which would otherwise prevent this operation.
+			 * We know the bounce buffer is safe to copy from, so
+			 * use _copy_to_iter() directly.
 			 */
 			} else if (_copy_to_iter(buf, tsz, iter) != tsz) {
 				ret = -EFAULT;
--
2.41.0