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>
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@ -2273,6 +2273,9 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
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struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
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int ret;
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if (!(filp->f_mode & FMODE_WRITE))
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return -EBADF;
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if (!inode_owner_or_capable(inode))
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return -EACCES;
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@ -2343,6 +2346,9 @@ static int f2fs_ioc_commit_atomic_write(struct file *filp)
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struct inode *inode = file_inode(filp);
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int ret;
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if (!(filp->f_mode & FMODE_WRITE))
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return -EBADF;
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if (!inode_owner_or_capable(inode))
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return -EACCES;
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@ -2385,6 +2391,9 @@ static int f2fs_ioc_start_volatile_write(struct file *filp)
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struct inode *inode = file_inode(filp);
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int ret;
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if (!(filp->f_mode & FMODE_WRITE))
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return -EBADF;
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if (!inode_owner_or_capable(inode))
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return -EACCES;
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@ -2420,6 +2429,9 @@ static int f2fs_ioc_release_volatile_write(struct file *filp)
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struct inode *inode = file_inode(filp);
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int ret;
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if (!(filp->f_mode & FMODE_WRITE))
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return -EBADF;
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if (!inode_owner_or_capable(inode))
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return -EACCES;
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@ -2449,6 +2461,9 @@ static int f2fs_ioc_abort_volatile_write(struct file *filp)
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struct inode *inode = file_inode(filp);
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int ret;
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if (!(filp->f_mode & FMODE_WRITE))
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return -EBADF;
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if (!inode_owner_or_capable(inode))
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return -EACCES;
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