[PATCH] fuse: fix copy_file_range reply handling race

[Chengfeng Ye <nicoyip.dev@gmail.com>]

__fuse_copy_file_range() selects its request opcode and output buffer
using fc->no_copy_file_range_64, then reads the flag again to handle the
reply. Copies to different destination inodes can run concurrently on
the same connection, so the flag may change while a request is pending.

For example, task A submits FUSE_COPY_FILE_RANGE_64 and waits for its
reply. Task B submits the same opcode, receives -ENOSYS and sets
no_copy_file_range_64. When A receives a successful reply, it reads the
uninitialized outarg.size instead of outarg_64.bytes_copied. This can
produce an incorrect return value, page cache invalidation range and
inode size update, and disclose stack contents if automatic stack
initialization is disabled.

If A also receives -ENOSYS, it instead mistakes that reply for rejection
of the legacy opcode and sets no_copy_file_range, skipping the legacy
fallback and unnecessarily disabling copy offload for the connection.

KCSAN reported the conflicting accesses:

  BUG: KCSAN: data-race in __fuse_copy_file_range / __fuse_copy_file_range

  read to 0xffff8881014a4d4d of 1 bytes by task 123 on cpu 4:
   __fuse_copy_file_range+0x1a7/0x6e0
   fuse_copy_file_range+0x54/0xd0
   vfs_copy_file_range+0x290/0x9d0
   __do_sys_copy_file_range+0x10f/0x2f0

  write to 0xffff8881014a4d4d of 8 bytes by task 119 on cpu 0:
   __fuse_copy_file_range+0x633/0x6e0
   fuse_copy_file_range+0x54/0xd0
   vfs_copy_file_range+0x290/0x9d0
   __do_sys_copy_file_range+0x10f/0x2f0

The bug can lead to leak kernel base address for KASLR bypass, the 
corresponding reproducer can be provided per request.

Use args.opcode for both -ENOSYS handling and output buffer selection.
It records the opcode actually sent and is updated on fallback, keeping
each reply tied to its request regardless of concurrent flag updates.

Fixes: 7a37f55af7af ("fuse: add COPY_FILE_RANGE_64 that allows large copies")
Cc: stable@vger.kernel.org
Assisted-by: GPT-6 Astra
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 fs/fuse/file.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 9a36d0329e221..939391b48cc9b 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -3019,7 +3019,7 @@ static ssize_t __fuse_copy_file_range(struct file *file_in, loff_t pos_in,
 	}
 	err = fuse_simple_request(fm, &args);
 	if (err == -ENOSYS) {
-		if (fc->no_copy_file_range_64) {
+		if (args.opcode == FUSE_COPY_FILE_RANGE) {
 			fc->no_copy_file_range = 1;
 			err = -EOPNOTSUPP;
 		} else {
@@ -3030,7 +3030,7 @@ static ssize_t __fuse_copy_file_range(struct file *file_in, loff_t pos_in,
 	if (err)
 		goto out;
 
-	bytes_copied = fc->no_copy_file_range_64 ?
+	bytes_copied = args.opcode == FUSE_COPY_FILE_RANGE ?
 		outarg.size : outarg_64.bytes_copied;
 
 	if (bytes_copied > len) {
-- 
2.43.0