fscrypt: report correct st_size for encrypted symlinks

This series makes the stat() family of syscalls start reporting the correct size for encrypted symlinks.

See patch 1 for a detailed explanation of the problem and solution.

Patch 1 adds a helper function that computes the correct size for an encrypted symlink. Patches 2-4 make the filesystems with fscrypt support use it, and patch 5 updates the documentation.

This series applies to mainline commit 3dbdb38e2869.

Eric Biggers (5):
fscrypt: add fscrypt_symlink_getattr() for computing st_size
ext4: report correct st_size for encrypted symlinks
f2fs: report correct st_size for encrypted symlinks
ubifs: report correct st_size for encrypted symlinks
fscrypt: remove mention of symlink st_size quirk from documentation

 Documentation/filesystems/fscrypt.rst |  5 ---
 fs/crypto/hooks.c                     | 44 +++++++++++++++++++++++++++
 fs/ext4/symlink.c                     | 12 +++++++-
 fs/f2fs/namei.c                       | 12 +++++++-
 fs/ubifs/file.c                       | 13 +++++++-
 include/linux/fscrypt.h               |  7 +++++
 6 files changed, 85 insertions(+), 8 deletions(-)

base-commit: 3dbdb38e286903ec220aaf1fb29a8d94297da246

2.32.0

From: Eric Biggers <[email protected]>

Add a helper function fscrypt_symlink_getattr() which will be called from the various filesystems' ->getattr() methods to read and decrypt the target of encrypted symlinks in order to report the correct st_size.

Detailed explanation:

As required by POSIX and as documented in various man pages, st_size for a symlink is supposed to be the length of the symlink target. Unfortunately, st_size has always been wrong for encrypted symlinks because st_size is populated from i_size from disk, which intentionally contains the length of the encrypted symlink target. That's slightly greater than the length of the decrypted symlink target (which is the symlink target that userspace usually sees), and usually won't match the length of the no-key encoded symlink target either.

This hadn't been fixed yet because reporting the correct st_size would require reading the symlink target from disk and decrypting or encoding it, which historically has been considered too heavyweight to do in ->getattr(). Also historically, the wrong st_size had only broken a test (LTP lstat03) and there were no known complaints from real users. (This is probably because the st_size of symlinks isn't used too often, and when it is, typically it's for a hint for what buffer size to pass to readlink() -- which a slightly-too-large size still works for.)

However, a couple things have changed now. First, there have recently been complaints about the current behavior from real users:

- Breakage in rpmbuild:
https://github.com/rpm-software-management/rpm/issues/1682
https://github.com/google/fscrypt/issues/305

- Breakage in toybox cpio:
https://www.mail-archive.com/[email protected]/msg07193.html

- Breakage in libgit2: https://issuetracker.google.com/issues/189629152
(on Android public issue tracker, requires login)

Second, we now cache decrypted symlink targets in ->i_link. Therefore, taking the performance hit of reading and decrypting the symlink target in ->getattr() wouldn't be as big a deal as it used to be, since usually it will just save having to do the same thing later.

Also note that eCryptfs ended up having to read and decrypt symlink targets in ->getattr() as well, to fix this same issue; see commit 3a60a1686f0d ("eCryptfs: Decrypt symlink target for stat size").

So, let's just bite the bullet, and read and decrypt the symlink target in ->getattr() in order to report the correct st_size. Add a function fscrypt_symlink_getattr() which the filesystems will call to do this.

(Alternatively, we could store the decrypted size of symlinks on-disk. But there isn't a great place to do so, and encryption is meant to hide the original size to some extent; that property would be lost.)

---
 fs/crypto/hooks.c       | 44 +++++++++++++++++++++++++++++++++++++++++
 include/linux/fscrypt.h |  7 +++++++
 2 files changed, 51 insertions(+)

index a73b0376e6f3..af74599ae1cf 100644
@@ -384,3 +384,47 @@ const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
 	return ERR_PTR(err);
 }
 EXPORT_SYMBOL_GPL(fscrypt_get_symlink);
+
+/**
+ * fscrypt_symlink_getattr() - set the correct st_size for encrypted symlinks
+ * @path: the path for the encrypted symlink being queried
+ * @stat: the struct being filled with the symlink's attributes
+ *
+ * Override st_size of encrypted symlinks to be the length of the decrypted
+ * symlink target (or the no-key encoded symlink target, if the key is
+ * unavailable) rather than the length of the encrypted symlink target.  This is
+ * necessary for st_size to match the symlink target that userspace actually
+ * sees.  POSIX requires this, and some userspace programs depend on it.
+ *
+ * This requires reading the symlink target from disk if needed, setting up the
+ * inode's encryption key if possible, and then decrypting or encoding the
+ * symlink target.  This makes lstat() more heavyweight than is normally the
+ * case.  However, decrypted symlink targets will be cached in ->i_link, so
+ * usually the symlink won't have to be read and decrypted again later if/when
+ * it is actually followed, readlink() is called, or lstat() is called again.
+ *
+ * Return: 0 on success, -errno on failure
+ */
+int fscrypt_symlink_getattr(const struct path *path, struct kstat *stat)
+{
+	struct dentry *dentry = path->dentry;
+	struct inode *inode = d_inode(dentry);
+	const char *link;
+	DEFINE_DELAYED_CALL(done);
+
+	/*
+	 * To get the symlink target that userspace will see (whether it's the
+	 * decrypted target or the no-key encoded target), we can just get it in
+	 * the same way the VFS does during path resolution and readlink().
+	 */
+	link = READ_ONCE(inode->i_link);
+	if (!link) {
+		link = inode->i_op->get_link(dentry, inode, &done);
+		if (IS_ERR(link))
+			return PTR_ERR(link);
+	}
+	stat->size = strlen(link);
+	do_delayed_call(&done);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(fscrypt_symlink_getattr);

index 2ea1387bb497..b7bfd0cd4f3e 100644
@@ -253,6 +253,7 @@ int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
 const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
 				unsigned int max_size,
 				struct delayed_call *done);
+int fscrypt_symlink_getattr(const struct path *path, struct kstat *stat);
 static inline void fscrypt_set_ops(struct super_block *sb,
 				   const struct fscrypt_operations *s_cop)
 {

@@ -583,6 +584,12 @@ static inline const char *fscrypt_get_symlink(struct inode *inode,
 	return ERR_PTR(-EOPNOTSUPP);
 }
 
+static inline int fscrypt_symlink_getattr(const struct path *path,
+					  struct kstat *stat)
+{
+	return -EOPNOTSUPP;
+}
+
 static inline void fscrypt_set_ops(struct super_block *sb,
 				   const struct fscrypt_operations *s_cop)
 {

2.32.0

From: Eric Biggers <[email protected]>

The stat() family of syscalls report the wrong size for encrypted symlinks, which has caused breakage in several userspace programs.

Fix this by calling fscrypt_symlink_getattr() after ext4_getattr() for encrypted symlinks. This function computes the correct size by reading and decrypting the symlink target (if it's not already cached).

For more details, see the commit which added fscrypt_symlink_getattr().

---
 fs/ext4/symlink.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

index dd05af983092..69109746e6e2 100644
@@ -52,10 +52,20 @@ static const char *ext4_encrypted_get_link(struct dentry *dentry,
 	return paddr;
 }
 
+static int ext4_encrypted_symlink_getattr(struct user_namespace *mnt_userns,
+					  const struct path *path,
+					  struct kstat *stat, u32 request_mask,
+					  unsigned int query_flags)
+{
+	ext4_getattr(mnt_userns, path, stat, request_mask, query_flags);
+
+	return fscrypt_symlink_getattr(path, stat);
+}
+
 const struct inode_operations ext4_encrypted_symlink_inode_operations = {
 	.get_link	= ext4_encrypted_get_link,
 	.setattr	= ext4_setattr,
-	.getattr	= ext4_getattr,
+	.getattr	= ext4_encrypted_symlink_getattr,
 	.listxattr	= ext4_listxattr,
 };
 

2.32.0

From: Eric Biggers <[email protected]>

The stat() family of syscalls report the wrong size for encrypted symlinks, which has caused breakage in several userspace programs.

Fix this by calling fscrypt_symlink_getattr() after f2fs_getattr() for encrypted symlinks. This function computes the correct size by reading and decrypting the symlink target (if it's not already cached).

For more details, see the commit which added fscrypt_symlink_getattr().

---
 fs/f2fs/namei.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

index a9cd9cf97229..e2d540ae2293 100644
@@ -1305,9 +1305,19 @@ static const char *f2fs_encrypted_get_link(struct dentry *dentry,
 	return target;
 }
 
+static int f2fs_encrypted_symlink_getattr(struct user_namespace *mnt_userns,
+					  const struct path *path,
+					  struct kstat *stat, u32 request_mask,
+					  unsigned int query_flags)
+{
+	f2fs_getattr(mnt_userns, path, stat, request_mask, query_flags);
+
+	return fscrypt_symlink_getattr(path, stat);
+}
+
 const struct inode_operations f2fs_encrypted_symlink_inode_operations = {
 	.get_link	= f2fs_encrypted_get_link,
-	.getattr	= f2fs_getattr,
+	.getattr	= f2fs_encrypted_symlink_getattr,
 	.setattr	= f2fs_setattr,
 	.listxattr	= f2fs_listxattr,
 };

2.32.0

From: Eric Biggers <[email protected]>

The stat() family of syscalls report the wrong size for encrypted symlinks, which has caused breakage in several userspace programs.

Fix this by calling fscrypt_symlink_getattr() after ubifs_getattr() for encrypted symlinks. This function computes the correct size by reading and decrypting the symlink target (if it's not already cached).

For more details, see the commit which added fscrypt_symlink_getattr().

---
 fs/ubifs/file.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

index 2e4e1d159969..5cfa28cd00cd 100644
@@ -1630,6 +1630,17 @@ static const char *ubifs_get_link(struct dentry *dentry,
 	return fscrypt_get_symlink(inode, ui->data, ui->data_len, done);
 }
 
+static int ubifs_symlink_getattr(struct user_namespace *mnt_userns,
+				 const struct path *path, struct kstat *stat,
+				 u32 request_mask, unsigned int query_flags)
+{
+	ubifs_getattr(mnt_userns, path, stat, request_mask, query_flags);
+
+	if (IS_ENCRYPTED(d_inode(path->dentry)))
+		return fscrypt_symlink_getattr(path, stat);
+	return 0;
+}
+
 const struct address_space_operations ubifs_file_address_operations = {
 	.readpage       = ubifs_readpage,
 	.writepage      = ubifs_writepage,

@@ -1655,7 +1666,7 @@ const struct inode_operations ubifs_file_inode_operations = {
 const struct inode_operations ubifs_symlink_inode_operations = {
 	.get_link    = ubifs_get_link,
 	.setattr     = ubifs_setattr,
-	.getattr     = ubifs_getattr,
+	.getattr     = ubifs_symlink_getattr,
 	.listxattr   = ubifs_listxattr,
 	.update_time = ubifs_update_time,
 };

2.32.0

From: Eric Biggers <[email protected]>

Now that the correct st_size is reported for encrypted symlinks on all filesystems, update the documentation accordingly.

---
 Documentation/filesystems/fscrypt.rst | 5 -----
 1 file changed, 5 deletions(-)

index 44b67ebd6e40..02ec57818920 100644
@@ -1063,11 +1063,6 @@ astute users may notice some differences in behavior:
 
 - DAX (Direct Access) is not supported on encrypted files.
 
-- The st_size of an encrypted symlink will not necessarily give the
-  length of the symlink target as required by POSIX.  It will actually
-  give the length of the ciphertext, which will be slightly longer
-  than the plaintext due to NUL-padding and an extra 2-byte overhead.
-
 - The maximum length of an encrypted symlink is 2 bytes shorter than
   the maximum length of an unencrypted symlink.  For example, on an
   EXT4 filesystem with a 4K block size, unencrypted symlinks can be up

2.32.0

All applied to fscrypt.git#master for 5.15.

- Eric