Re: [PATCH] CAPABILITIES: remove undefined caps from all processes

[Andy Lutomirski <luto@amacapital.net>]

On 07/23/2014 12:36 PM, Eric Paris wrote:
> This is effectively a revert of 7b9a7ec565505699f503b4fcf61500dceb36e744
> plus fixing it a different way...

You sent something like this a couple days ago.  What changed?

--Andy

> 
> We found, when trying to run an application from an application which
> had dropped privs that the kernel does security checks on undefined
> capability bits.  This was ESPECIALLY difficult to debug as those
> undefined bits are hidden from /proc/$PID/status.
> 
> Consider a root application which drops all capabilities from ALL 4
> capability sets.  We assume, since the application is going to set
> eff/perm/inh from an array that it will clear not only the defined caps
> less than CAP_LAST_CAP, but also the higher 28ish bits which are
> undefined future capabilities.
> 
> The BSET gets cleared differently.  Instead it is cleared one bit at a
> time.  The problem here is that in security/commoncap.c::cap_task_prctl()
> we actually check the validity of a capability being read.  So any task
> which attempts to 'read all things set in bset' followed by 'unset all
> things set in bset' will not even attempt to unset the undefined bits
> higher than CAP_LAST_CAP.
> 
> So the 'parent' will look something like:
> CapInh:	0000000000000000
> CapPrm:	0000000000000000
> CapEff:	0000000000000000
> CapBnd:	ffffffc000000000
> 
> All of this 'should' be fine.  Given that these are undefined bits that
> aren't supposed to have anything to do with permissions.  But they do...
> 
> So lets now consider a task which cleared the eff/perm/inh completely
> and cleared all of the valid caps in the bset (but not the invalid caps
> it couldn't read out of the kernel).  We know that this is exactly what
> the libcap-ng library does and what the go capabilities library does.
> They both leave you in that above situation if you try to clear all of
> you capapabilities from all 4 sets.  If that root task calls execve()
> the child task will pick up all caps not blocked by the bset.  The bset
> however does not block bits higher than CAP_LAST_CAP.  So now the child
> task has bits in eff which are not in the parent.  These are
> 'meaningless' undefined bits, but still bits which the parent doesn't
> have.
> 
> The problem is now in cred_cap_issubset() (or any operation which does a
> subset test) as the child, while a subset for valid cap bits, is not a
> subset for invalid cap bits!  So now we set durring commit creds that
> the child is not dumpable.  Given it is 'more priv' than its parent.  It
> also means the parent cannot ptrace the child and other stupidity.
> 
> The solution here:
> 1) stop hiding capability bits in status
> 	This makes debugging easier!
> 
> 2) stop giving any task undefined capability bits.  it's simple, it you
> don't put those invalid bits in CAP_FULL_SET you won't get them in init
> and you won't get them in any other task either.
> 	This fixes the cap_issubset() tests and resulting fallout (which
> 	made the init task in a docker container untraceable among other
> 	things)
> 
> 3) mask out undefined bits when sys_capset() is called as it might use
> ~0, ~0 to denote 'all capabilities' for backward/forward compatibility.
> 	This lets 'capsh --caps="all=eip" -- -c /bin/bash' run.
> 
> 4) mask out undefined bit when we read a file capability off of disk as
> again likely all bits are set in the xattr for forward/backward
> compatibility.
> 	This lets 'setcap all+pe /bin/bash; /bin/bash' run
> 
> Signed-off-by: Eric Paris <eparis@redhat.com>
> Cc: Andrew Vagin <avagin@openvz.org>
> Cc: Andrew G. Morgan <morgan@kernel.org>
> Cc: Serge E. Hallyn <serge.hallyn@canonical.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Steve Grubb <sgrubb@redhat.com>
> Cc: Dan Walsh <dwalsh@redhat.com>
> Cc: stable@vger.kernel.org
> ---
>  fs/proc/array.c            | 11 +----------
>  include/linux/capability.h |  5 ++++-
>  kernel/audit.c             |  2 +-
>  kernel/capability.c        |  4 ++++
>  security/commoncap.c       |  3 +++
>  5 files changed, 13 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index 64db2bc..3e1290b 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -297,15 +297,11 @@ static void render_cap_t(struct seq_file *m, const char *header,
>  	seq_puts(m, header);
>  	CAP_FOR_EACH_U32(__capi) {
>  		seq_printf(m, "%08x",
> -			   a->cap[(_KERNEL_CAPABILITY_U32S-1) - __capi]);
> +			   a->cap[CAP_LAST_U32 - __capi]);
>  	}
>  	seq_putc(m, '\n');
>  }
>  
> -/* Remove non-existent capabilities */
> -#define NORM_CAPS(v) (v.cap[CAP_TO_INDEX(CAP_LAST_CAP)] &= \
> -				CAP_TO_MASK(CAP_LAST_CAP + 1) - 1)
> -
>  static inline void task_cap(struct seq_file *m, struct task_struct *p)
>  {
>  	const struct cred *cred;
> @@ -319,11 +315,6 @@ static inline void task_cap(struct seq_file *m, struct task_struct *p)
>  	cap_bset	= cred->cap_bset;
>  	rcu_read_unlock();
>  
> -	NORM_CAPS(cap_inheritable);
> -	NORM_CAPS(cap_permitted);
> -	NORM_CAPS(cap_effective);
> -	NORM_CAPS(cap_bset);
> -
>  	render_cap_t(m, "CapInh:\t", &cap_inheritable);
>  	render_cap_t(m, "CapPrm:\t", &cap_permitted);
>  	render_cap_t(m, "CapEff:\t", &cap_effective);
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index 84b13ad..aa93e5e 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -78,8 +78,11 @@ extern const kernel_cap_t __cap_init_eff_set;
>  # error Fix up hand-coded capability macro initializers
>  #else /* HAND-CODED capability initializers */
>  
> +#define CAP_LAST_U32			((_KERNEL_CAPABILITY_U32S) - 1)
> +#define CAP_LAST_U32_VALID_MASK		(CAP_TO_MASK(CAP_LAST_CAP + 1) -1)
> +
>  # define CAP_EMPTY_SET    ((kernel_cap_t){{ 0, 0 }})
> -# define CAP_FULL_SET     ((kernel_cap_t){{ ~0, ~0 }})
> +# define CAP_FULL_SET     ((kernel_cap_t){{ ~0, CAP_LAST_U32_VALID_MASK }})
>  # define CAP_FS_SET       ((kernel_cap_t){{ CAP_FS_MASK_B0 \
>  				    | CAP_TO_MASK(CAP_LINUX_IMMUTABLE), \
>  				    CAP_FS_MASK_B1 } })
> diff --git a/kernel/audit.c b/kernel/audit.c
> index 3ef2e0e..ba2ff5a 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -1677,7 +1677,7 @@ void audit_log_cap(struct audit_buffer *ab, char *prefix, kernel_cap_t *cap)
>  	audit_log_format(ab, " %s=", prefix);
>  	CAP_FOR_EACH_U32(i) {
>  		audit_log_format(ab, "%08x",
> -				 cap->cap[(_KERNEL_CAPABILITY_U32S-1) - i]);
> +				 cap->cap[CAP_LAST_U32 - i]);
>  	}
>  }
>  
> diff --git a/kernel/capability.c b/kernel/capability.c
> index a5cf13c..989f5bf 100644
> --- a/kernel/capability.c
> +++ b/kernel/capability.c
> @@ -258,6 +258,10 @@ SYSCALL_DEFINE2(capset, cap_user_header_t, header, const cap_user_data_t, data)
>  		i++;
>  	}
>  
> +	effective.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK;
> +	permitted.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK;
> +	inheritable.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK;
> +
>  	new = prepare_creds();
>  	if (!new)
>  		return -ENOMEM;
> diff --git a/security/commoncap.c b/security/commoncap.c
> index b9d613e..963dc59 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -421,6 +421,9 @@ int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data
>  		cpu_caps->inheritable.cap[i] = le32_to_cpu(caps.data[i].inheritable);
>  	}
>  
> +	cpu_caps->permitted.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK;
> +	cpu_caps->inheritable.cap[CAP_LAST_U32] &= CAP_LAST_U32_VALID_MASK;
> +
>  	return 0;
>  }
>  
>