[PATCH 0/4] backport fscrypt symlink fixes to 4.19

[PATCH 0/4] backport fscrypt symlink fixes to 4.19

[Eric Biggers]

This series backports some patches that failed to apply to 4.19-stable
due to the prototype of inode_operations::getattr having changed in
v5.12, as well as several other conflicts.  Please apply to 4.19-stable.

Eric Biggers (4):
  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

 fs/crypto/hooks.c               | 44 +++++++++++++++++++++++++++++++++
 fs/ext4/symlink.c               | 11 ++++++++-
 fs/f2fs/namei.c                 | 11 ++++++++-
 fs/ubifs/file.c                 | 12 ++++++++-
 include/linux/fscrypt_notsupp.h |  6 +++++
 include/linux/fscrypt_supp.h    |  1 +
 6 files changed, 82 insertions(+), 3 deletions(-)

-- 
2.33.0.153.gba50c8fa24-goog

[PATCH 1/4] fscrypt: add fscrypt_symlink_getattr() for computing st_size

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

commit d18760560593e5af921f51a8c9b64b6109d634c2 upstream.

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/toybox@lists.landley.net/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.)

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210702065350.209646-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/crypto/hooks.c               | 44 +++++++++++++++++++++++++++++++++
 include/linux/fscrypt_notsupp.h |  6 +++++
 include/linux/fscrypt_supp.h    |  1 +
 3 files changed, 51 insertions(+)

diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
index aa86cb2db823..ae7a413c0bdd 100644
--- a/fs/crypto/hooks.c
+++ b/fs/crypto/hooks.c
@@ -279,3 +279,47 @@ 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);
diff --git a/include/linux/fscrypt_notsupp.h b/include/linux/fscrypt_notsupp.h
index 93304cfeb601..6e95c5a9b7fa 100644
--- a/include/linux/fscrypt_notsupp.h
+++ b/include/linux/fscrypt_notsupp.h
@@ -234,4 +234,10 @@ 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;
+}
+
 #endif	/* _LINUX_FSCRYPT_NOTSUPP_H */
diff --git a/include/linux/fscrypt_supp.h b/include/linux/fscrypt_supp.h
index 0409c14ae1de..f4cb4871a987 100644
--- a/include/linux/fscrypt_supp.h
+++ b/include/linux/fscrypt_supp.h
@@ -231,5 +231,6 @@ extern int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
 extern 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);
 
 #endif	/* _LINUX_FSCRYPT_SUPP_H */
-- 
2.33.0.153.gba50c8fa24-goog

[PATCH 2/4] ext4: report correct st_size for encrypted symlinks

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

commit 8c4bca10ceafc43b1ca0a9fab5fa27e13cbce99e upstream.

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().

Fixes: f348c252320b ("ext4 crypto: add symlink encryption")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210702065350.209646-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/ext4/symlink.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/symlink.c b/fs/ext4/symlink.c
index dd05af983092..a9457fed351e 100644
--- a/fs/ext4/symlink.c
+++ b/fs/ext4/symlink.c
@@ -52,10 +52,19 @@ static const char *ext4_encrypted_get_link(struct dentry *dentry,
 	return paddr;
 }
 
+static int ext4_encrypted_symlink_getattr(const struct path *path,
+					  struct kstat *stat, u32 request_mask,
+					  unsigned int query_flags)
+{
+	ext4_getattr(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,
 };
 
-- 
2.33.0.153.gba50c8fa24-goog

[PATCH 3/4] f2fs: report correct st_size for encrypted symlinks

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

commit 461b43a8f92e68e96c4424b31e15f2b35f1bbfa9 upstream.

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().

Fixes: cbaf042a3cc6 ("f2fs crypto: add symlink encryption")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210702065350.209646-4-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/f2fs/namei.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/namei.c b/fs/f2fs/namei.c
index e20a0f9e6845..edc80855974a 100644
--- a/fs/f2fs/namei.c
+++ b/fs/f2fs/namei.c
@@ -1219,9 +1219,18 @@ static const char *f2fs_encrypted_get_link(struct dentry *dentry,
 	return target;
 }
 
+static int f2fs_encrypted_symlink_getattr(const struct path *path,
+					  struct kstat *stat, u32 request_mask,
+					  unsigned int query_flags)
+{
+	f2fs_getattr(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,
 #ifdef CONFIG_F2FS_FS_XATTR
 	.listxattr	= f2fs_listxattr,
-- 
2.33.0.153.gba50c8fa24-goog

[PATCH 4/4] ubifs: report correct st_size for encrypted symlinks

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

commit 064c734986011390b4d111f1a99372b7f26c3850 upstream.

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().

Fixes: ca7f85be8d6c ("ubifs: Add support for encrypted symlinks")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210702065350.209646-5-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/ubifs/file.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
index d7d2fdda4bbd..3dbb5ac630e4 100644
--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -1642,6 +1642,16 @@ 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(const struct path *path, struct kstat *stat,
+				 u32 request_mask, unsigned int query_flags)
+{
+	ubifs_getattr(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,
@@ -1669,7 +1679,7 @@ 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,
 #ifdef CONFIG_UBIFS_FS_XATTR
 	.listxattr   = ubifs_listxattr,
 #endif
-- 
2.33.0.153.gba50c8fa24-goog

Re: [PATCH 0/4] backport fscrypt symlink fixes to 4.19

[Greg KH]

On Wed, Sep 08, 2021 at 02:50:29PM -0700, Eric Biggers wrote:
> This series backports some patches that failed to apply to 4.19-stable
> due to the prototype of inode_operations::getattr having changed in
> v5.12, as well as several other conflicts.  Please apply to 4.19-stable.
> 
> Eric Biggers (4):
>   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
> 
>  fs/crypto/hooks.c               | 44 +++++++++++++++++++++++++++++++++
>  fs/ext4/symlink.c               | 11 ++++++++-
>  fs/f2fs/namei.c                 | 11 ++++++++-
>  fs/ubifs/file.c                 | 12 ++++++++-
>  include/linux/fscrypt_notsupp.h |  6 +++++
>  include/linux/fscrypt_supp.h    |  1 +
>  6 files changed, 82 insertions(+), 3 deletions(-)
> 
> -- 
> 2.33.0.153.gba50c8fa24-goog
> 

All now queued up, thanks!

greg k-h