Patch "f2fs: Require FMODE_WRITE for atomic write ioctls" has been added to the 6.1-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    f2fs: Require FMODE_WRITE for atomic write ioctls

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     f2fs-require-fmode_write-for-atomic-write-ioctls.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 9d34bec0f710f7f3510130d4b55cf012ea674bb4
Author: Jann Horn <jannh@xxxxxxxxxx>
Date:   Fri Oct 4 19:34:41 2024 +0000

    f2fs: Require FMODE_WRITE for atomic write ioctls
    
    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@xxxxxxxxxxxxxxx
    Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
    Reviewed-by: Chao Yu <chao@xxxxxxxxxx>
    Reviewed-by: Eric Biggers <ebiggers@xxxxxxxxxx>
    Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
    Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 1e1f640bf1c5c..6a2b5fcbe6799 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -2108,6 +2108,9 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
 	loff_t isize;
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(mnt_userns, inode))
 		return -EACCES;
 
@@ -2209,6 +2212,9 @@ static int f2fs_ioc_commit_atomic_write(struct file *filp)
 	struct user_namespace *mnt_userns = file_mnt_user_ns(filp);
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(mnt_userns, inode))
 		return -EACCES;
 
@@ -2241,6 +2247,9 @@ static int f2fs_ioc_abort_atomic_write(struct file *filp)
 	struct user_namespace *mnt_userns = file_mnt_user_ns(filp);
 	int ret;
 
+	if (!(filp->f_mode & FMODE_WRITE))
+		return -EBADF;
+
 	if (!inode_owner_or_capable(mnt_userns, inode))
 		return -EACCES;