6.1-stable review patch. If anyone has any objections, please let me know.
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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 Signed-off-by: Sasha Levin sashal@kernel.org --- 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;