Re: [PATCH v2] define convenient securebits masks for prctl users

[Michael Kerrisk]

Hi Serge,

On Wed, Oct 14, 2009 at 11:15 PM, Serge E. Hallyn <serue@xxxxxxxxxx> wrote:
> The securebits are used by passing them to prctl with the
> PR_{S,G}ET_SECUREBITS commands.  But the defines must be
> shifted to be used in prctl, which begs to be confused and
> misused by userspace.  So define some more convenient
> values for userspace to specify.  This way userspace does
>
>        prctl(PR_SET_SECUREBITS, SECBIT_NOROOT);
>
> instead of
>
>        prctl(PR_SET_SECUREBITS, 1 << SECURE_NOROOT);
>
> Thanks to Michael for the idea.
>
> This patch also adds include/linux/securebits to the installed headers.
> Then perhaps it can be included by glibc's sys/prctl.h.
>
> Changelog:
>        Oct 14: (Suggestions by Michael Kerrisk):
>                1. spell out SETUID in SECBIT_NO_SETUID*
>                2. SECBIT_X_LOCKED does not imply SECBIT_X
>                3. add definitions for keepcaps

Thanks for these changes.

>        Oct 14: As suggested by Michael Kerrisk, don't
>                use SB_* as that convention is already in
>                use.  Use SECBIT_ prefix instead.
>
> Signed-off-by: Serge E. Hallyn <serue@xxxxxxxxxx>
> Acked-by: Andrew G. Morgan <morgan@xxxxxxxxxx>

Acked-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>

Cheers,

Michael



> Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> Cc: Ulrich Drepper <drepper@xxxxxxxxxx>
> ---
>  include/linux/Kbuild       |    1 +
>  include/linux/securebits.h |   22 ++++++++++++++++------
>  2 files changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/Kbuild b/include/linux/Kbuild
> index 3e8bd18..94fe9f7 100644
> --- a/include/linux/Kbuild
> +++ b/include/linux/Kbuild
> @@ -328,6 +328,7 @@ unifdef-y += scc.h
>  unifdef-y += sched.h
>  unifdef-y += screen_info.h
>  unifdef-y += sdla.h
> +unifdef-y += securebits.h
>  unifdef-y += selinux_netlink.h
>  unifdef-y += sem.h
>  unifdef-y += serial_core.h
> diff --git a/include/linux/securebits.h b/include/linux/securebits.h
> index d2c5ed8..9ad109e 100644
> --- a/include/linux/securebits.h
> +++ b/include/linux/securebits.h
> @@ -1,6 +1,13 @@
>  #ifndef _LINUX_SECUREBITS_H
>  #define _LINUX_SECUREBITS_H 1
>
> +/* Each securesetting is implemented using two bits. One bit specifies
> +   whether the setting is on or off. The other bit specify whether the
> +   setting is locked or not. A setting which is locked cannot be
> +   changed from user-level. */
> +#define issecure_mask(X)       (1 << (X))
> +#define issecure(X)            (issecure_mask(X) & current_cred_xxx(securebits))
> +
>  #define SECUREBITS_DEFAULT 0x00000000
>
>  /* When set UID 0 has no special privileges. When unset, we support
> @@ -12,6 +19,9 @@
>  #define SECURE_NOROOT                  0
>  #define SECURE_NOROOT_LOCKED           1  /* make bit-0 immutable */
>
> +#define SECBIT_NOROOT          (issecure_mask(SECURE_NOROOT))
> +#define SECBIT_NOROOT_LOCKED   (issecure_mask(SECURE_NOROOT_LOCKED))
> +
>  /* When set, setuid to/from uid 0 does not trigger capability-"fixup".
>    When unset, to provide compatiblility with old programs relying on
>    set*uid to gain/lose privilege, transitions to/from uid 0 cause
> @@ -19,6 +29,10 @@
>  #define SECURE_NO_SETUID_FIXUP         2
>  #define SECURE_NO_SETUID_FIXUP_LOCKED  3  /* make bit-2 immutable */
>
> +#define SECBIT_NO_SETUID_FIXUP (issecure_mask(SECURE_NO_SETUID_FIXUP))
> +#define SECBIT_NO_SETUID_FIXUP_LOCKED \
> +                       (issecure_mask(SECURE_NO_SETUID_FIXUP_LOCKED))
> +
>  /* When set, a process can retain its capabilities even after
>    transitioning to a non-root user (the set-uid fixup suppressed by
>    bit 2). Bit-4 is cleared when a process calls exec(); setting both
> @@ -27,12 +41,8 @@
>  #define SECURE_KEEP_CAPS               4
>  #define SECURE_KEEP_CAPS_LOCKED                5  /* make bit-4 immutable */
>
> -/* Each securesetting is implemented using two bits. One bit specifies
> -   whether the setting is on or off. The other bit specify whether the
> -   setting is locked or not. A setting which is locked cannot be
> -   changed from user-level. */
> -#define issecure_mask(X)       (1 << (X))
> -#define issecure(X)            (issecure_mask(X) & current_cred_xxx(securebits))
> +#define SECBIT_KEEP_CAPS       (issecure_mask(SECURE_KEEP_CAPS))
> +#define SECBIT_KEEP_CAPS_LOCKED (issecure_mask(SECURE_KEEP_CAPS_LOCKED))
>
>  #define SECURE_ALL_BITS                (issecure_mask(SECURE_NOROOT) | \
>                                 issecure_mask(SECURE_NO_SETUID_FIXUP) | \
> --
> 1.6.1
>
>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface" http://blog.man7.org/
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