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@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>
This commit is contained in:
Jann Horn 2024-10-04 19:36:43 +00:00 committed by Ksawlii
parent ddcb438ef7
commit 3877a33618

View file

@ -2273,6 +2273,9 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
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;
@ -2343,6 +2346,9 @@ 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;
@ -2385,6 +2391,9 @@ 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;
@ -2420,6 +2429,9 @@ 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;
@ -2449,6 +2461,9 @@ 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;