Re: [RFC PATCH v2 1/5] fs: add O_ENCODED open flag

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-10-30, Omar Sandoval <osandov@osandov.com> wrote:
> On Sat, Oct 19, 2019 at 03:50:57PM +1100, Aleksa Sarai wrote:
> > On 2019-10-15, Omar Sandoval <osandov@osandov.com> wrote:
> > > From: Omar Sandoval <osandov@fb.com>
> > > 
> > > The upcoming RWF_ENCODED operation introduces some security concerns:
> > > 
> > > 1. Compressed writes will pass arbitrary data to decompression
> > >    algorithms in the kernel.
> > > 2. Compressed reads can leak truncated/hole punched data.
> > > 
> > > Therefore, we need to require privilege for RWF_ENCODED. It's not
> > > possible to do the permissions checks at the time of the read or write
> > > because, e.g., io_uring submits IO from a worker thread. So, add an open
> > > flag which requires CAP_SYS_ADMIN. It can also be set and cleared with
> > > fcntl(). The flag is not cleared in any way on fork or exec; it should
> > > probably be used with O_CLOEXEC in most cases.
> > > 
> > > Note that the usual issue that unknown open flags are ignored doesn't
> > > really matter for O_ENCODED; if the kernel doesn't support O_ENCODED,
> > > then it doesn't support RWF_ENCODED, either.
> > > 
> > > Signed-off-by: Omar Sandoval <osandov@fb.com>
> > > ---
> > >  fs/fcntl.c                       | 10 ++++++++--
> > >  fs/namei.c                       |  4 ++++
> > >  include/linux/fcntl.h            |  2 +-
> > >  include/uapi/asm-generic/fcntl.h |  4 ++++
> > >  4 files changed, 17 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/fs/fcntl.c b/fs/fcntl.c
> > > index 3d40771e8e7c..45ebc6df078e 100644
> > > --- a/fs/fcntl.c
> > > +++ b/fs/fcntl.c
> > > @@ -30,7 +30,8 @@
> > >  #include <asm/siginfo.h>
> > >  #include <linux/uaccess.h>
> > >  
> > > -#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT | O_NOATIME)
> > > +#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT | O_NOATIME | \
> > > +		    O_ENCODED)
> > >  
> > >  static int setfl(int fd, struct file * filp, unsigned long arg)
> > >  {
> > > @@ -49,6 +50,11 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
> > >  		if (!inode_owner_or_capable(inode))
> > >  			return -EPERM;
> > >  
> > > +	/* O_ENCODED can only be set by superuser */
> > > +	if ((arg & O_ENCODED) && !(filp->f_flags & O_ENCODED) &&
> > > +	    !capable(CAP_SYS_ADMIN))
> > > +		return -EPERM;
> > 
> > I have a feeling the error should probably be an EACCES and not EPERM.
> 
> Shrug, I wanted to make this consistent with O_NOATIME, which uses
> EPERM. EACCES seems more appropriate for lacking permissions for a
> particular path rather than for an operation, but the lines are blurry.

Fair enough, though I would also argue that O_NOATIME should've also
been EACCES (and there are plenty of examples throughout the kernel
where EPERM was used where EACCES makes more sense). ;)

> > > +
> > >  	/* required for strict SunOS emulation */
> > >  	if (O_NONBLOCK != O_NDELAY)
> > >  	       if (arg & O_NDELAY)
> > > @@ -1031,7 +1037,7 @@ static int __init fcntl_init(void)
> > >  	 * Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY
> > >  	 * is defined as O_NONBLOCK on some platforms and not on others.
> > >  	 */
> > > -	BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ !=
> > > +	BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ !=
> > >  		HWEIGHT32(
> > >  			(VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) |
> > >  			__FMODE_EXEC | __FMODE_NONOTIFY));
> > > diff --git a/fs/namei.c b/fs/namei.c
> > > index 671c3c1a3425..ae86b125888a 100644
> > > --- a/fs/namei.c
> > > +++ b/fs/namei.c
> > > @@ -2978,6 +2978,10 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> > >  	if (flag & O_NOATIME && !inode_owner_or_capable(inode))
> > >  		return -EPERM;
> > >  
> > > +	/* O_ENCODED can only be set by superuser */
> > > +	if ((flag & O_ENCODED) && !capable(CAP_SYS_ADMIN))
> > > +		return -EPERM;
> > 
> > I would suggest that this check be put into build_open_flags() rather
> > than putting it this late in open(). Also, same nit about the error
> > return as above.
> 
> This is where we check permissions for O_NOATIME, shouldn't we keep all
> of those permission checks in the same place? build_open_flags() only
> checks for flag validity.

Right, but O_NOATIME can't be checked earlier -- you need to have
resolved the inode in order to do the permission check. O_ENCODED only
depends on the capability set, and IMHO checking it earlier seems
cleaner to me.

> > > +
> > >  	return 0;
> > >  }
> > >  
> > > diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
> > > index d019df946cb2..5fac02479639 100644
> > > --- a/include/linux/fcntl.h
> > > +++ b/include/linux/fcntl.h
> > > @@ -9,7 +9,7 @@
> > >  	(O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \
> > >  	 O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __O_SYNC | O_DSYNC | \
> > >  	 FASYNC	| O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \
> > > -	 O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE)
> > > +	 O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_ENCODED)
> > >  
> > >  #ifndef force_o_largefile
> > >  #define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T))
> > > diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
> > > index 9dc0bf0c5a6e..8c5cbd5942e3 100644
> > > --- a/include/uapi/asm-generic/fcntl.h
> > > +++ b/include/uapi/asm-generic/fcntl.h
> > > @@ -97,6 +97,10 @@
> > >  #define O_NDELAY	O_NONBLOCK
> > >  #endif
> > >  
> > > +#ifndef O_ENCODED
> > > +#define O_ENCODED	040000000
> > > +#endif
> > 
> > You should also define this for all of the architectures which don't use
> > the generic O_* flag values. On alpha, O_PATH is equal to the value you
> > picked (just be careful on sparc -- 0x4000000 is the next free bit, but
> > it's used by FMODE_NONOTIFY.)
> 
> Good catch, I'll fix that. Thanks!

Oh, and please add a one-line comment in the sparc header to ensure
nobody accidentally breaks open() on sparc in the future.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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