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

[Eric Paris]

On Wed, 2014-07-23 at 13:46 -0700, Andy Lutomirski wrote:
> 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?

right when I sent it I knew I forgot to do the -v2 type stuff.

The new portions are fixes 3 and 4 below.  Which consists of masking
unknown caps from sys_setcap() and executing files with unknown
filecaps.

-Eric

> --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@xxxxxxxxxx>
> > Cc: Andrew Vagin <avagin@xxxxxxxxxx>
> > Cc: Andrew G. Morgan <morgan@xxxxxxxxxx>
> > Cc: Serge E. Hallyn <serge.hallyn@xxxxxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Steve Grubb <sgrubb@xxxxxxxxxx>
> > Cc: Dan Walsh <dwalsh@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> >  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;
> >  }
> >  
> > 
> 


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