6.1-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jann Horn <jannh@xxxxxxxxxx>
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>
---
fs/f2fs/file.c | 9 +++++++++
1 file changed, 9 insertions(+)
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;
--
2.43.0
