[PATCH v3] vfs: fix copy_file_range regression in cross-fs copies

Luis Henriques
2021-02-17T17:26:40Z

A regression has been reported by Nicolas Boichat, found while using the
copy_file_range syscall to copy a tracefs file. Before commit
5dae222a5ff0 ("vfs: allow copy_file_range to copy across devices") the
kernel would return -EXDEV to userspace when trying to copy a file across
different filesystems. After this commit, the syscall doesn't fail anymore
and instead returns zero (zero bytes copied), as this file's content is
generated on-the-fly and thus reports a size of zero.

This patch restores some cross-filesystems copy restrictions that existed
prior to commit 5dae222a5ff0 ("vfs: allow copy_file_range to copy across
devices"). It also introduces a flag (COPY_FILE_SPLICE) that can be used
by filesystems calling directly into the vfs copy_file_range to override
these restrictions. Right now, only NFS needs to set this flag.

Fixes: 5dae222a5ff0 ("vfs: allow copy_file_range to copy across devices")
Link: https://lore.kernel.org/linux-fsdevel/20210212044405.4120619-1-drinkcat@chromium.org/
Link: https://lore.kernel.org/linux-fsdevel/CANMq1KDZuxir2LM5jOTm0xx+BnvW=ZmpsG47CyHFJwnw7zSX6Q@mail.gmail.com/
Link: https://lore.kernel.org/linux-fsdevel/20210126135012.1.If45b7cdc3ff707bc1efa17f5366057d60603c45f@changeid/
Reported-by: Nicolas Boichat <drinkcat@chromium.org>
Signed-off-by: Luis Henriques <lhenriques@suse.de>
---
Ok, I've tried to address all the issues and comments. Hopefully this v3
is a bit closer to the final fix.

Changes since v2
- do all the required checks earlier, in generic_copy_file_checks(),
adding new checks for ->remap_file_range
- new COPY_FILE_SPLICE flag
- don't remove filesystem's fallback to generic_copy_file_range()
- updated commit changelog (and subject)
Changes since v1 (after Amir review)
- restored do_copy_file_range() helper
- return -EOPNOTSUPP if fs doesn't implement CFR
- updated commit description

fs/nfsd/vfs.c | 3 ++-
fs/read_write.c | 44 +++++++++++++++++++++++++++++++++++++++++---
include/linux/fs.h | 7 +++++++
3 files changed, 50 insertions(+), 4 deletions(-)

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 04937e51de56..14e55822c223 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -578,7 +578,8 @@ ssize_t nfsd_copy_file_range(struct file *src, u64 src_pos, struct file *dst,
* limit like this and pipeline multiple COPY requests.
*/
count = min_t(u64, count, 1 << 22);
- return vfs_copy_file_range(src, src_pos, dst, dst_pos, count, 0);
+ return vfs_copy_file_range(src, src_pos, dst, dst_pos, count,
+ COPY_FILE_SPLICE);
}

__be32 nfsd4_vfs_fallocate(struct svc_rqst *rqstp, struct svc_fh *fhp,
diff --git a/fs/read_write.c b/fs/read_write.c
index 75f764b43418..40a16003fb05 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1410,6 +1410,33 @@ static ssize_t do_copy_file_range(struct file *file_in, loff_t pos_in,
flags);
}

+/*
+ * This helper function checks whether copy_file_range can actually be used,
+ * depending on the source and destination filesystems being the same.
+ *
+ * In-kernel callers may set COPY_FILE_SPLICE to override these checks.
+ */
+static int fops_copy_file_checks(struct file *file_in, struct file *file_out,
+ unsigned int flags)
+{
+ if (WARN_ON_ONCE(flags & ~COPY_FILE_SPLICE))
+ return -EINVAL;
+
+ if (flags & COPY_FILE_SPLICE)
+ return 0;
+ /*
+ * We got here from userspace, so forbid copies if copy_file_range isn't
+ * implemented or if we're doing a cross-fs copy.
+ */
+ if (!file_out->f_op->copy_file_range)
+ return -EOPNOTSUPP;
+ else if (file_out->f_op->copy_file_range !=
+ file_in->f_op->copy_file_range)
+ return -EXDEV;
+
+ return 0;
+}
+
/*
* Performs necessary checks before doing a file copy
*
@@ -1427,6 +1454,14 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
loff_t size_in;
int ret;

+ /* Only check f_ops if we're not trying to clone */
+ if (!file_in->f_op->remap_file_range ||
+ (file_inode(file_in)->i_sb == file_inode(file_out)->i_sb)) {
+ ret = fops_copy_file_checks(file_in, file_out, flags);
+ if (ret)
+ return ret;
+ }
+
ret = generic_file_rw_checks(file_in, file_out);
if (ret)
return ret;
@@ -1474,9 +1509,6 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
{
ssize_t ret;

- if (flags != 0)
- return -EINVAL;
-
ret = generic_copy_file_checks(file_in, pos_in, file_out, pos_out, &len,
flags);
if (unlikely(ret))
@@ -1511,6 +1543,9 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
ret = cloned;
goto done;
}
+ ret = fops_copy_file_checks(file_in, file_out, flags);
+ if (ret)
+ return ret;
}

ret = do_copy_file_range(file_in, pos_in, file_out, pos_out, len,
@@ -1543,6 +1578,9 @@ SYSCALL_DEFINE6(copy_file_range, int, fd_in, loff_t __user *, off_in,
struct fd f_out;
ssize_t ret = -EBADF;

+ if (flags != 0)
+ return -EINVAL;
+
f_in = fdget(fd_in);
if (!f_in.file)
goto out2;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index fd47deea7c17..6f604926d955 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1815,6 +1815,13 @@ struct dir_context {
*/
#define REMAP_FILE_ADVISORY (REMAP_FILE_CAN_SHORTEN)

+/*
+ * This flag control the behavior of copy_file_range from internal (kernel)
+ * users. It can be used to override the policy of forbidding copies when
+ * source and destination filesystems are different.
+ */
+#define COPY_FILE_SPLICE (1 << 0)
+
struct iov_iter;

struct file_operations {