Re: ÐÑÐÐÑ: VFS, NFS security bug? Should CAP_MKNOD and CAP_LINUX_IMMUTABLE be added toCAP_FS_MASK?

[Serge E. Hallyn]

Quoting J. Bruce Fields (bfields@xxxxxxxxxxxx):
> Seems this isn't entirely obvious in the general case.  In the specific

I think the MKNOD part is (based on historical behavior as you
suggested) obvious:  both masks should include CAP_MKNOD and
CAP_LINUX_IMMUTABLE.

(references: http://lxr.linux.no/linux-old+v2.0.21/fs/ext2/ioctl.c#L60
for immutable and http://lxr.linux.no/linux-old+v2.0.21/fs/namei.c#L503
for mknod)

-serge

> case of nfsd, however, this is pretty obvious.  So I'm inclined to
> submit the following now (and leave it to be reverted by a later patch
> if CAP_FS_MASK ends up including CAP_MKNOD, as seems likely).
> 
> --b.
> 
> commit 2ec8f8f0c0005ffe3cf93bbf3d9976de76cf4652
> Author: J. Bruce Fields <bfields@xxxxxxxxxxxxxx>
> Date:   Mon Mar 16 18:34:20 2009 -0400
> 
>     nfsd: nfsd should drop CAP_MKNOD for non-root
>     
>     Since creating a device node is normally an operation requiring special
>     privilege, Igor Zhbanov points out that it is surprising (to say the
>     least) that a client can, for example, create a device node on a
>     filesystem exported with root_squash.
>     
>     So, make sure CAP_MKNOD is among the capabilities dropped when an nfsd
>     thread handles a request from a non-root user.
>     
>     Reported-by: Igor Zhbanov <izh1979@xxxxxxxxx>
>     Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxxxxxx>
> 
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index 02bdb76..7824483 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -393,8 +393,10 @@ struct cpu_vfs_cap_data {
>  # define CAP_FULL_SET     ((kernel_cap_t){{ ~0, ~0 }})
>  # define CAP_INIT_EFF_SET ((kernel_cap_t){{ ~CAP_TO_MASK(CAP_SETPCAP), ~0 }})
>  # define CAP_FS_SET       ((kernel_cap_t){{ CAP_FS_MASK_B0, CAP_FS_MASK_B1 } })
> -# define CAP_NFSD_SET     ((kernel_cap_t){{ CAP_FS_MASK_B0|CAP_TO_MASK(CAP_SYS_RESOURCE), \
> -					CAP_FS_MASK_B1 } })
> +# define CAP_NFSD_SET     ((kernel_cap_t){{ CAP_FS_MASK_B0 \
> +					    | CAP_TO_MASK(CAP_SYS_RESOURCE) \
> +					    | CAP_TO_MASK(CAP_MKNOD), \
> +					    CAP_FS_MASK_B1 } })
> 
>  #endif /* _KERNEL_CAPABILITY_U32S != 2 */
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