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f2fs: Require FMODE_WRITE for atomic write ioctls

PATCH New Eric Biggers via Linux-f2fs-devel 1 participant 1 message Oct 4, 2024 Stable branch

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PATCH 1/1 f2fs: Require FMODE_WRITE for atomic write ioctls

From: Jann Horn <[email protected]>

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 [email protected]
Signed-off-by Jann Horn <[email protected]>
Reviewed-by Chao Yu <[email protected]>
Reviewed-by Eric Biggers <[email protected]>
Signed-off-by Jaegeuk Kim <[email protected]>
Signed-off-by Eric Biggers <[email protected]>
---
 fs/f2fs/file.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
fs/f2fs/file.c +15 -0
index 2330600dbe02e..738d65abde510 100644
@@ -1855,10 +1855,13 @@ 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;

@@ -1921,10 +1924,13 @@ 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)

@@ -1963,10 +1969,13 @@ 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;

@@ -1998,10 +2007,13 @@ 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)

@@ -2027,10 +2039,13 @@ 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: 661f109c057497c8baf507a2562ceb9f9fb3cbc2

Signature · 1 line
2.47.0.rc0.187.ge670bccf7e-goog