Re: [PATCH v6 02/11] fs: add O_ALLOW_ENCODED open flag

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Nov 18, 2020 at 9:18 PM 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 must be
> combined with O_CLOEXEC when opening to avoid accidental leaks (if
> needed, it may be set without O_CLOEXEC by using fnctl()).
>
> Note that the usual issue that unknown open flags are ignored doesn't
> really matter for O_ALLOW_ENCODED; if the kernel doesn't support
> O_ALLOW_ENCODED, then it doesn't support RWF_ENCODED, either.
>
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
>  arch/alpha/include/uapi/asm/fcntl.h  |  1 +
>  arch/parisc/include/uapi/asm/fcntl.h |  1 +
>  arch/sparc/include/uapi/asm/fcntl.h  |  1 +
>  fs/fcntl.c                           | 10 ++++++++--
>  fs/namei.c                           |  4 ++++
>  fs/open.c                            |  7 +++++++
>  include/linux/fcntl.h                |  2 +-
>  include/uapi/asm-generic/fcntl.h     |  4 ++++
>  8 files changed, 27 insertions(+), 3 deletions(-)
>
> diff --git a/arch/alpha/include/uapi/asm/fcntl.h b/arch/alpha/include/uapi/asm/fcntl.h
> index 50bdc8e8a271..391e0d112e41 100644
> --- a/arch/alpha/include/uapi/asm/fcntl.h
> +++ b/arch/alpha/include/uapi/asm/fcntl.h
> @@ -34,6 +34,7 @@
>
>  #define O_PATH         040000000
>  #define __O_TMPFILE    0100000000
> +#define O_ALLOW_ENCODED        0200000000
>
>  #define F_GETLK                7
>  #define F_SETLK                8
> diff --git a/arch/parisc/include/uapi/asm/fcntl.h b/arch/parisc/include/uapi/asm/fcntl.h
> index 03dee816cb13..72ea9bdf5f04 100644
> --- a/arch/parisc/include/uapi/asm/fcntl.h
> +++ b/arch/parisc/include/uapi/asm/fcntl.h
> @@ -19,6 +19,7 @@
>
>  #define O_PATH         020000000
>  #define __O_TMPFILE    040000000
> +#define O_ALLOW_ENCODED        100000000
>
>  #define F_GETLK64      8
>  #define F_SETLK64      9
> diff --git a/arch/sparc/include/uapi/asm/fcntl.h b/arch/sparc/include/uapi/asm/fcntl.h
> index 67dae75e5274..ac3e8c9cb32c 100644
> --- a/arch/sparc/include/uapi/asm/fcntl.h
> +++ b/arch/sparc/include/uapi/asm/fcntl.h
> @@ -37,6 +37,7 @@
>
>  #define O_PATH         0x1000000
>  #define __O_TMPFILE    0x2000000
> +#define O_ALLOW_ENCODED        0x8000000
>
>  #define F_GETOWN       5       /*  for sockets. */
>  #define F_SETOWN       6       /*  for sockets. */
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index 19ac5baad50f..9302f68fe698 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_ALLOW_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_ALLOW_ENCODED can only be set by superuser */
> +       if ((arg & O_ALLOW_ENCODED) && !(filp->f_flags & O_ALLOW_ENCODED) &&
> +           !capable(CAP_SYS_ADMIN))
> +               return -EPERM;
> +
>         /* required for strict SunOS emulation */
>         if (O_NONBLOCK != O_NDELAY)
>                if (arg & O_NDELAY)
> @@ -1033,7 +1039,7 @@ 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 d4a6dd772303..fbf64ce61088 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2890,6 +2890,10 @@ 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_ALLOW_ENCODED can only be set by superuser */
> +       if ((flag & O_ALLOW_ENCODED) && !capable(CAP_SYS_ADMIN))
> +               return -EPERM;
> +
>         return 0;
>  }
>
> diff --git a/fs/open.c b/fs/open.c
> index 9af548fb841b..f2863aaf78e7 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -1040,6 +1040,13 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
>                 acc_mode = 0;
>         }
>
> +       /*
> +        * O_ALLOW_ENCODED must be combined with O_CLOEXEC to avoid accidentally
> +        * leaking encoded I/O privileges.
> +        */
> +       if ((how->flags & (O_ALLOW_ENCODED | O_CLOEXEC)) == O_ALLOW_ENCODED)
> +               return -EINVAL;
> +


dup() can also result in accidental leak.
We could fail dup() of fd without O_CLOEXEC. Should we?

If we should than what error code should it be? We could return EPERM,
but since we do allow to clear O_CLOEXEC or set O_ALLOW_ENCODED
after open, EPERM seems a tad harsh.
EINVAL seems inappropriate because the error has nothing to do with
input args of dup() and EBADF would also be confusing.

Thanks,
Amir.