[PATCH 5.15.y 3/3] splice: avoid stale f_pos writeback on error

[Sasha Levin <sashal@kernel.org>]

From: Daehyeon Ko <4ncienth@gmail.com>

[ Upstream commit 79b496ebe382866e3a05090b7dc9ae95e85e65b2 ]

do_splice() snapshots an implicit file position before calling the splice
helper. Since splice does not take f_pos_lock, another operation on the
same open file description can update f_pos while the helper is blocked.
If the helper then fails, the unconditional writeback restores the stale
snapshot.

For a directory without ->splice_read, a file-to-pipe splice fails with
-EINVAL. A concurrent directory mutation and getdents() call can advance
f_pos and refresh the filesystem's revalidation cookie before that failure
restores only the old position. The next ext2 readdir can then trust an
offset inside a directory entry and copy data from the adjacent page to
userspace.

Only update an implicit file position when splice returns a non-negative
result, matching read and write. Keep explicit offset updates unchanged.

Fixes: 7995bd287134 ("splice: don't pass the address of ->f_pos to methods")
Cc: stable@vger.kernel.org
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Link: https://patch.msgid.link/20261018051708.2422394-1-4ncienth@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/splice.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/splice.c b/fs/splice.c
index 70008e08998f5..3630fd18a12e8 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1107,10 +1107,10 @@ long do_splice(struct file *in, loff_t *off_in, struct file *out,
 		ret = do_splice_from(ipipe, out, &offset, len, flags);
 		file_end_write(out);
 
-		if (!off_out)
-			out->f_pos = offset;
-		else
+		if (off_out)
 			*off_out = offset;
+		else if (ret >= 0)
+			out->f_pos = offset;
 	} else if (opipe) {
 		if (off_out)
 			return -ESPIPE;
@@ -1127,10 +1127,10 @@ long do_splice(struct file *in, loff_t *off_in, struct file *out,
 
 		ret = splice_file_to_pipe(in, opipe, &offset, len, flags);
 
-		if (!off_in)
-			in->f_pos = offset;
-		else
+		if (off_in)
 			*off_in = offset;
+		else if (ret >= 0)
+			in->f_pos = offset;
 	} else {
 		ret = -EINVAL;
 	}
-- 
2.53.0