fscrypt: don't print name of busy file when removing key

Commit Message

Eric Biggers Jan. 20, 2020, 6:07 a.m. UTC

From: Eric Biggers <[e-mail]>

When an encryption key can't be fully removed due to file(s) protected
by it still being in-use, we shouldn't really print the path to one of
these files to the kernel log, since parts of this path are likely to be
encrypted on-disk, and (depending on how the system is set up) the
confidentiality of this path might be lost by printing it to the log.

This is a trade-off: a single file path often doesn't matter at all,
especially if it's a directory; the kernel log might still be protected
in some way; and I had originally hoped that any "inode(s) still busy"
bugs (which are security weaknesses in their own right) would be quickly
fixed and that to do so it would be super helpful to always know the
file path and not have to run 'find dir -inum $inum' after the fact.

But in practice, these bugs can be hard to fix (e.g. due to asynchronous
process killing that is difficult to eliminate, for performance
reasons), and also not tied to specific files, so knowing a file path
doesn't necessarily help.

So to be safe, for now let's just show the inode number, not the path.
If someone really wants to know a path they can use 'find -inum'.

Fixes: b1c0ec3599f4 ("fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl")
Cc: <[e-mail]> # v5.4+
Signed-off-by: Eric Biggers <[e-mail]>
---
 fs/crypto/keyring.c | 15 ++-------------
 1 file changed, 2 insertions(+), 13 deletions(-)

Comments

Eric Biggers Jan. 22, 2020, 10:59 p.m. UTC | #1

On Sun, Jan 19, 2020 at 10:07:32PM -0800, Eric Biggers wrote:
> From: Eric Biggers <[e-mail]>
> 
> When an encryption key can't be fully removed due to file(s) protected
> by it still being in-use, we shouldn't really print the path to one of
> these files to the kernel log, since parts of this path are likely to be
> encrypted on-disk, and (depending on how the system is set up) the
> confidentiality of this path might be lost by printing it to the log.
> 
> This is a trade-off: a single file path often doesn't matter at all,
> especially if it's a directory; the kernel log might still be protected
> in some way; and I had originally hoped that any "inode(s) still busy"
> bugs (which are security weaknesses in their own right) would be quickly
> fixed and that to do so it would be super helpful to always know the
> file path and not have to run 'find dir -inum $inum' after the fact.
> 
> But in practice, these bugs can be hard to fix (e.g. due to asynchronous
> process killing that is difficult to eliminate, for performance
> reasons), and also not tied to specific files, so knowing a file path
> doesn't necessarily help.
> 
> So to be safe, for now let's just show the inode number, not the path.
> If someone really wants to know a path they can use 'find -inum'.
> 
> Fixes: b1c0ec3599f4 ("fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl")
> Cc: <[e-mail]> # v5.4+
> Signed-off-by: Eric Biggers <[e-mail]>

Applied to fscrypt.git#master for 5.6.

- Eric

Patch

diff --git a/fs/crypto/keyring.c b/fs/crypto/keyring.c
index 098ff2e0f0bb4..ab41b25d4fa1b 100644
--- a/fs/crypto/keyring.c
+++ b/fs/crypto/keyring.c
@@ -776,9 +776,6 @@  static int check_for_busy_inodes(struct super_block *sb,
 	struct list_head *pos;
 	size_t busy_count = 0;
 	unsigned long ino;
-	struct dentry *dentry;
-	char _path[256];
-	char *path = NULL;
 
 	spin_lock(&mk->mk_decrypted_inodes_lock);
 
@@ -797,22 +794,14 @@  static int check_for_busy_inodes(struct super_block *sb,
 					 struct fscrypt_info,
 					 ci_master_key_link)->ci_inode;
 		ino = inode->i_ino;
-		dentry = d_find_alias(inode);
 	}
 	spin_unlock(&mk->mk_decrypted_inodes_lock);
 
-	if (dentry) {
-		path = dentry_path(dentry, _path, sizeof(_path));
-		dput(dentry);
-	}
-	if (IS_ERR_OR_NULL(path))
-		path = "(unknown)";
-
 	fscrypt_warn(NULL,
-		     "%s: %zu inode(s) still busy after removing key with %s %*phN, including ino %lu (%s)",
+		     "%s: %zu inode(s) still busy after removing key with %s %*phN, including ino %lu",
 		     sb->s_id, busy_count, master_key_spec_type(&mk->mk_spec),
 		     master_key_spec_len(&mk->mk_spec), (u8 *)&mk->mk_spec.u,
-		     ino, path);
+		     ino);
 	return -EBUSY;
 }