fs: Fix rw_hint validation

Reject values that are valid rw_hints after truncation but not before
truncation by passing an untruncated value to rw_hint_valid().
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 5657cb07 ("fs/fcntl: use copy_to/from_user() for u64 types")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240202203926.2478590-2-bvanassche@acm.orgSigned-off-by: Christian Brauner <brauner@kernel.org>

fs/fcntl.c

@@ -268,7 +268,7 @@ static int f_getowner_uids(struct file *filp, unsigned long arg)
 }
 #endif
 
-static bool rw_hint_valid(enum rw_hint hint)
+static bool rw_hint_valid(u64 hint)
 {
 	switch (hint) {
 	case RWH_WRITE_LIFE_NOT_SET:
@@ -288,19 +288,17 @@ static long fcntl_rw_hint(struct file *file, unsigned int cmd,
 {
 	struct inode *inode = file_inode(file);
 	u64 __user *argp = (u64 __user *)arg;
-	enum rw_hint hint;
-	u64 h;
+	u64 hint;
 
 	switch (cmd) {
 	case F_GET_RW_HINT:
-		h = inode->i_write_hint;
-		if (copy_to_user(argp, &h, sizeof(*argp)))
+		hint = inode->i_write_hint;
+		if (copy_to_user(argp, &hint, sizeof(*argp)))
 			return -EFAULT;
 		return 0;
 	case F_SET_RW_HINT:
-		if (copy_from_user(&h, argp, sizeof(h)))
+		if (copy_from_user(&hint, argp, sizeof(hint)))
 			return -EFAULT;
-		hint = (enum rw_hint) h;
 		if (!rw_hint_valid(hint))
 			return -EINVAL;