Re: [PATCH] Abort file_remove_privs() for non-reg. files

Jan Kara
2018-12-17T08:28:06Z

On Fri 14-12-18 11:55:52, Alexander Lochmann wrote:
>
> file_remove_privs() might be called for non-regular files, e.g.
> blkdev inode. There is no reason to do its job on things
> like blkdev inodes, pipes, or cdevs. Hence, abort if
> file does not refer to a regular inode.
> The following stacktrace shows how to get there:
> 13: entry_SYSENTER_32:460
> 12: do_fast_syscall_32:410
> 11: _static_cpu_has:146
> 10: do_syscall_32_irqs_on:322
> 09: SyS_pwrite64:636
> 08: SYSC_pwrite64:650
> 07: fdput:38
> 06: vfs_write:560
> 05: __vfs_write:512
> 04: new_sync_write:500
> 03: blkdev_write_iter:1977
> 02: __generic_file_write_iter:2897
> 01: file_remove_privs:1818
> 00: inode_has_no_xattr:3163
>
> Found by LockDoc (Alexander Lochmann, Horst Schirmeier and Olaf
> Spinczyk)
>
> Signed-off-by: Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
> Signed-off-by: Horst Schirmeier <horst.schirmeier@tu-dortmund.de>

The patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> fs/inode.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/fs/inode.c b/fs/inode.c
> index 35d2108d567c..682088190413 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -1820,8 +1820,13 @@ int file_remove_privs(struct file *file)
> int kill;
> int error = 0;
>
> - /* Fast path for nothing security related */
> - if (IS_NOSEC(inode))
> + /*
> + * Fast path for nothing security related.
> + * As well for non-regular files, e.g. blkdev inodes.
> + * For example, blkdev_write_iter() might get here
> + * trying to remove privs which it is not allowed to.
> + */
> + if (IS_NOSEC(inode) || !S_ISREG(inode->i_mode))
> return 0;
>
> kill = dentry_needs_remove_privs(dentry);
> --
> 2.19.2
>

--
Jan Kara <jack@suse.com>
SUSE Labs, CR