[PATCH v6 19/24] netfs: Fix potential UAF in netfs_unlock_abandoned_read_pages()

netfs_unlock_abandoned_read_pages(rreq) accesses the index of the folios it
is wanting to unlock and compares that to rreq->no_unlock_folio so that it
doesn't unlock a folio being read for netfs_perform_write() or
netfs_write_begin().

However, given that netfs_unlock_abandoned_read_pages() is called _after_
NETFS_RREQ_IN_PROGRESS is cleared, the one folio that it's not allowed to
dereference is the one specified by ->no_unlock_folio as ownership
immediately reverts to the caller.

Fix this by storing the folio pointer instead and using that rather than
the index.  Also fix netfs_unlock_read_folio() where the same applies.

Fixes: ee4cdf7ba857 ("netfs: Speed up buffered reading")
Closes: https://sashiko.dev/#/patchset/20260414082004.3756080-1-dhowells%40redhat.com
Signed-off-by: David Howells <[email protected]>
cc: Paulo Alcantara <[email protected]>
cc: Viacheslav Dubeyko <[email protected]>
cc: Matthew Wilcox <[email protected]>
cc: [email protected]
cc: [email protected]
---
 fs/netfs/buffered_read.c | 4 ++--
 fs/netfs/read_collect.c  | 2 +-
 fs/netfs/read_retry.c    | 2 +-
 include/linux/netfs.h    | 2 +-
 4 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/netfs/buffered_read.c b/fs/netfs/buffered_read.c
index 004d426c02b4..83d0b8153e96 100644
--- a/fs/netfs/buffered_read.c
+++ b/fs/netfs/buffered_read.c
@@ -670,7 +670,7 @@ int netfs_write_begin(struct netfs_inode *ctx,
 		ret = PTR_ERR(rreq);
 		goto error;
 	}
-	rreq->no_unlock_folio	= folio->index;
+	rreq->no_unlock_folio	= folio;
 	__set_bit(NETFS_RREQ_NO_UNLOCK_FOLIO, &rreq->flags);
 
 	ret = netfs_begin_cache_read(rreq, ctx);
@@ -736,7 +736,7 @@ int netfs_prefetch_for_write(struct file *file, struct folio *folio,
 		goto error;
 	}
 
-	rreq->no_unlock_folio = folio->index;
+	rreq->no_unlock_folio = folio;
 	__set_bit(NETFS_RREQ_NO_UNLOCK_FOLIO, &rreq->flags);
 	ret = netfs_begin_cache_read(rreq, ctx);
 	if (ret == -ENOMEM || ret == -EINTR || ret == -ERESTARTSYS)
diff --git a/fs/netfs/read_collect.c b/fs/netfs/read_collect.c
index 3c9b847885c2..23660a590124 100644
--- a/fs/netfs/read_collect.c
+++ b/fs/netfs/read_collect.c
@@ -83,7 +83,7 @@ static void netfs_unlock_read_folio(struct netfs_io_request *rreq,
 	}
 
 just_unlock:
-	if (folio->index == rreq->no_unlock_folio &&
+	if (folio == rreq->no_unlock_folio &&
 	    test_bit(NETFS_RREQ_NO_UNLOCK_FOLIO, &rreq->flags)) {
 		_debug("no unlock");
 	} else {
diff --git a/fs/netfs/read_retry.c b/fs/netfs/read_retry.c
index e10eb5a07332..f59a70f3a086 100644
--- a/fs/netfs/read_retry.c
+++ b/fs/netfs/read_retry.c
@@ -292,7 +292,7 @@ void netfs_unlock_abandoned_read_pages(struct netfs_io_request *rreq)
 			struct folio *folio = folioq_folio(p, slot);
 
 			if (folio && !folioq_is_marked2(p, slot)) {
-				if (folio->index == rreq->no_unlock_folio &&
+				if (folio == rreq->no_unlock_folio &&
 				    test_bit(NETFS_RREQ_NO_UNLOCK_FOLIO,
 					     &rreq->flags)) {
 					_debug("no unlock");
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 4fd1d796ad73..243c0f737938 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -252,7 +252,7 @@ struct netfs_io_request {
 	unsigned long long	collected_to;	/* Point we've collected to */
 	unsigned long long	cleaned_to;	/* Position we've cleaned folios to */
 	unsigned long long	abandon_to;	/* Position to abandon folios to */
-	pgoff_t			no_unlock_folio; /* Don't unlock this folio after read */
+	const struct folio	*no_unlock_folio; /* Don't unlock this folio after read */
 	unsigned int		direct_bv_count; /* Number of elements in direct_bv[] */
 	unsigned int		debug_id;
 	unsigned int		rsize;		/* Maximum read size (0 for none) */