iov_iter: Fix iov_iter_get_pages*() for folio_queue

p9_get_mapped_pages() uses iov_iter_get_pages_alloc2() to extract pages
from an iterator when performing a zero-copy request and under some
circumstances, this crashes with odd page errors[1], for example, I see:

page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0xbcf0
flags: 0x2000000000000000(zone=1)
...
page dumped because: VM_BUG_ON_FOLIO(((unsigned int) folio_ref_count(folio) + 127u <= 127u))
------------[ cut here ]------------
kernel BUG at include/linux/mm.h:1444!

This is because, unlike in iov_iter_extract_folioq_pages(), the
iter_folioq_get_pages() helper function doesn't skip the current folio
when iov_offset points to the end of it, but rather extracts the next
page beyond the end of the folio and adds it to the list. Reading will
then clobber the contents of this page, leading to system corruption,
and if the page is not in use, put_page() may try to clean up the unused
page.

This can be worked around by copying the iterator before each
extraction[2] and using iov_iter_advance() on the original as the
advance function steps over the page we're at the end of.

Fix this by skipping the page extraction if we're at the end of the
folio.

This was reproduced in the ktest environment[3] by forcing 9p to use the
fscache caching mode and then reading a file through 9p.

Fixes: db0aa2e9566f ("mm: Define struct folio_queue and ITER_FOLIOQ to handle a sequence of folios")
Reported-by: Antony Antony <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]/
Signed-off-by: David Howells <[e-mail]>
cc: Eric Van Hensbergen <[e-mail]>
cc: Latchesar Ionkov <[e-mail]>
cc: Dominique Martinet <[e-mail]>
cc: Christian Schoenebeck <[e-mail]>
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]/ [1]
Link: https://lore.kernel.org/r/[e-mail]/ [2]
Link: https://github.com/koverstreet/ktest.git [3]
Tested-by: Antony Antony <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

lib/iov_iter.c

···
 		size_t offset = iov_offset, fsize = folioq_folio_size(folioq, slot);
 		size_t part = PAGE_SIZE - offset % PAGE_SIZE;
 
-		part = umin(part, umin(maxsize - extracted, fsize - offset));
-		count -= part;
-		iov_offset += part;
-		extracted += part;
+		if (offset < fsize) {
+			part = umin(part, umin(maxsize - extracted, fsize - offset));
+			count -= part;
+			iov_offset += part;
+			extracted += part;
 
-		*pages = folio_page(folio, offset / PAGE_SIZE);
-		get_page(*pages);
-		pages++;
-		maxpages--;
+			*pages = folio_page(folio, offset / PAGE_SIZE);
+			get_page(*pages);
+			pages++;
+			maxpages--;
+		}
+
 		if (maxpages == 0 || extracted >= maxsize)
 			break;