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: sta...@vger.kernel.org
Signed-off-by: Jann Horn <ja...@google.com>
---
fs/f2fs/file.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 168f08507004..a662392c5d8b 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -2117,12 +2117,15 @@ static int f2fs_ioc_start_atomic_write(struct file
*filp, bool truncate)
struct f2fs_inode_info *fi = F2FS_I(inode);
struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
struct inode *pinode;
loff_t isize;
int ret;
+ if (!(filp->f_mode & FMODE_WRITE))
+ return -EBADF;
+
if (!inode_owner_or_capable(idmap, inode))
return -EACCES;
if (!S_ISREG(inode->i_mode))
return -EINVAL;
@@ -2225,12 +2228,15 @@ static int f2fs_ioc_start_atomic_write(struct file
*filp, bool truncate)
static int f2fs_ioc_commit_atomic_write(struct file *filp)
{
struct inode *inode = file_inode(filp);
struct mnt_idmap *idmap = file_mnt_idmap(filp);
int ret;
+ if (!(filp->f_mode & FMODE_WRITE))
+ return -EBADF;
+
if (!inode_owner_or_capable(idmap, inode))
return -EACCES;
ret = mnt_want_write_file(filp);
if (ret)
return ret;
@@ -2257,12 +2263,15 @@ static int f2fs_ioc_commit_atomic_write(struct file
*filp)
static int f2fs_ioc_abort_atomic_write(struct file *filp)
{
struct inode *inode = file_inode(filp);
struct mnt_idmap *idmap = file_mnt_idmap(filp);
int ret;
+ if (!(filp->f_mode & FMODE_WRITE))
+ return -EBADF;
+
if (!inode_owner_or_capable(idmap, inode))
return -EACCES;
ret = mnt_want_write_file(filp);
if (ret)
return ret;
---
base-commit: b446a2dae984fa5bd56dd7c3a02a426f87e05813
change-id: 20240806-f2fs-atomic-write-e019a47823de
--
Jann Horn <ja...@google.com>
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