From: Eric Biggers <ebiggers@kernel.org>
To: stable@vger.kernel.org
Cc: linux-f2fs-devel@lists.sourceforge.net,
	Jann Horn <jannh@google.com>,
	Chao Yu <chao@kernel.org>,
	Jaegeuk Kim <jaegeuk@kernel.org>
Subject: [PATCH 5.10] f2fs: Require FMODE_WRITE for atomic write ioctls
Date: Fri,  4 Oct 2024 19:36:43 +0000
Message-ID: <20241004193643.190072-1-ebiggers@kernel.org>
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit

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(+)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 50514962771a1..e25788e643bbd 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -2047,10 +2047,13 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
 	struct inode *inode = file_inode(filp);
 	struct f2fs_inode_info *fi = F2FS_I(inode);
 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
 	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;
@@ -2117,10 +2120,13 @@ 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)
@@ -2159,10 +2165,13 @@ 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;
@@ -2194,10 +2203,13 @@ 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)
@@ -2223,10 +2235,13 @@ 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)

base-commit: ceb091e2c4ccf93b1ee0e0e8a202476a433784ff
-- 
2.47.0.rc0.187.ge670bccf7e-goog