diff mbox series

[f2fs-dev,4.19] f2fs: Require FMODE_WRITE for atomic write ioctls

Message ID 20241004193749.190266-1-ebiggers@kernel.org (mailing list archive)
State New
Headers show
Series [f2fs-dev,4.19] f2fs: Require FMODE_WRITE for atomic write ioctls | expand

Commit Message

Eric Biggers Oct. 4, 2024, 7:37 p.m. UTC 
From: Jann Horn <jannh@google.com>

commit 4f5a100f87f32cb65d4bb1ad282a08c92f6f591e upstream.

The F2FS ioctls for starting and committing atomic writes check for
inode_owner_or_capable(), but this does not give LSMs like SELinux or
Landlock an opportunity to deny the write access - if the caller's FSUID
matches the inode's UID, inode_owner_or_capable() immediately returns true.

There are scenarios where LSMs want to deny a process the ability to write
particular files, even files that the FSUID of the process owns; but this
can currently partially be bypassed using atomic write ioctls in two ways:

 - F2FS_IOC_START_ATOMIC_REPLACE + F2FS_IOC_COMMIT_ATOMIC_WRITE can
   truncate an inode to size 0
 - F2FS_IOC_START_ATOMIC_WRITE + F2FS_IOC_ABORT_ATOMIC_WRITE can revert
   changes another process concurrently made to a file

Fix it by requiring FMODE_WRITE for these operations, just like for
F2FS_IOC_MOVE_RANGE. Since any legitimate caller should only be using these
ioctls when intending to write into the file, that seems unlikely to break
anything.

Fixes: 88b88a667971 ("f2fs: support atomic writes")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/f2fs/file.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)


base-commit: de2cffe297563c815c840cfa14b77a0868b61e53
diff mbox series

Patch

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 043ce96ac1270..0cc2f41e81243 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -1709,10 +1709,13 @@  static int f2fs_ioc_getversion(struct file *filp, unsigned long arg)
 static int f2fs_ioc_start_atomic_write(struct file *filp)
 {
 	struct inode *inode = file_inode(filp);
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(inode))
 		return -EACCES;
 
 	if (!S_ISREG(inode->i_mode))
 		return -EINVAL;
@@ -1766,10 +1769,13 @@  static int f2fs_ioc_start_atomic_write(struct file *filp)
 static int f2fs_ioc_commit_atomic_write(struct file *filp)
 {
 	struct inode *inode = file_inode(filp);
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(inode))
 		return -EACCES;
 
 	ret = mnt_want_write_file(filp);
 	if (ret)
@@ -1811,10 +1817,13 @@  static int f2fs_ioc_commit_atomic_write(struct file *filp)
 static int f2fs_ioc_start_volatile_write(struct file *filp)
 {
 	struct inode *inode = file_inode(filp);
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(inode))
 		return -EACCES;
 
 	if (!S_ISREG(inode->i_mode))
 		return -EINVAL;
@@ -1846,10 +1855,13 @@  static int f2fs_ioc_start_volatile_write(struct file *filp)
 static int f2fs_ioc_release_volatile_write(struct file *filp)
 {
 	struct inode *inode = file_inode(filp);
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(inode))
 		return -EACCES;
 
 	ret = mnt_want_write_file(filp);
 	if (ret)
@@ -1875,10 +1887,13 @@  static int f2fs_ioc_release_volatile_write(struct file *filp)
 static int f2fs_ioc_abort_volatile_write(struct file *filp)
 {
 	struct inode *inode = file_inode(filp);
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(inode))
 		return -EACCES;
 
 	ret = mnt_want_write_file(filp);
 	if (ret)