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Date: Fri, 27 Jun 2008 15:52:00 -0500
From: "Serge E. Hallyn" <serue@...ibm.com>
To: "Andrew G. Morgan" <morgan@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
David Howells <dhowells@...hat.com>,
"Serge E. Hallyn" <serue@...ibm.com>,
lkml <linux-kernel@...r.kernel.org>,
Linux Security Modules List
<linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 1/4] security: filesystem capabilities bugfix1
Quoting Andrew G. Morgan (morgan@...nel.org):
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Bugfix for the fragile setuid fixup code in the case that filesystem
> capabilities are supported.
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.2.6 (GNU/Linux)
>
> iD8DBQFIY1eZ+bHCR3gb8jsRAgneAJ4jvnswg0+5Rkr69YFbFYXexK8vNQCgnAS7
> jF5ZqrBAAtU7RNVHia18ODk=
> =cOzB
> -----END PGP SIGNATURE-----
> From a44789119274e6596f08f7d7b967130cf1ae7bb7 Mon Sep 17 00:00:00 2001
> From: Andrew G. Morgan <morgan@...nel.org>
> Date: Wed, 25 Jun 2008 23:12:32 -0700
> Subject: [PATCH] Security fix for experimental filesystem capability code.
>
> This commit includes a bugfix for the fragile setuid fixup code in
> the case that filesystem capabilities are supported (in access()).
> The effect of this fix is gated on filesystem capability support
> because changing securebits is only supported when filesystem
> capabilities support is configured.)
>
> Signed-off-by: Andrew G. Morgan <morgan@...nel.org>
Acked-by: Serge Hallyn <serue@...ibm.com>
> ---
> fs/open.c | 38 +++++++++++++++++++++++---------------
> include/linux/capability.h | 2 ++
> include/linux/securebits.h | 15 ++++++++-------
> kernel/capability.c | 21 +++++++++++++++++++++
> 4 files changed, 54 insertions(+), 22 deletions(-)
>
> diff --git a/fs/open.c b/fs/open.c
> index a145008..3b53948 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -16,6 +16,7 @@
> #include <linux/namei.h>
> #include <linux/backing-dev.h>
> #include <linux/capability.h>
> +#include <linux/securebits.h>
> #include <linux/security.h>
> #include <linux/mount.h>
> #include <linux/vfs.h>
> @@ -425,7 +426,7 @@ asmlinkage long sys_faccessat(int dfd, const char __user *filename, int mode)
> {
> struct nameidata nd;
> int old_fsuid, old_fsgid;
> - kernel_cap_t old_cap;
> + kernel_cap_t uninitialized_var(old_cap); /* !SECURE_NO_SETUID_FIXUP */
> int res;
>
> if (mode & ~S_IRWXO) /* where's F_OK, X_OK, W_OK, R_OK? */
> @@ -433,23 +434,27 @@ asmlinkage long sys_faccessat(int dfd, const char __user *filename, int mode)
>
> old_fsuid = current->fsuid;
> old_fsgid = current->fsgid;
> - old_cap = current->cap_effective;
>
> current->fsuid = current->uid;
> current->fsgid = current->gid;
>
> - /*
> - * Clear the capabilities if we switch to a non-root user
> - *
> - * FIXME: There is a race here against sys_capset. The
> - * capabilities can change yet we will restore the old
> - * value below. We should hold task_capabilities_lock,
> - * but we cannot because user_path_walk can sleep.
> - */
> - if (current->uid)
> - cap_clear(current->cap_effective);
> - else
> - current->cap_effective = current->cap_permitted;
> + if (!issecure(SECURE_NO_SETUID_FIXUP)) {
> + /*
> + * Clear the capabilities if we switch to a non-root user
> + */
> +#ifndef CONFIG_SECURITY_FILE_CAPABILITIES
> + /*
> + * FIXME: There is a race here against sys_capset. The
> + * capabilities can change yet we will restore the old
> + * value below. We should hold task_capabilities_lock,
> + * but we cannot because user_path_walk can sleep.
> + */
> +#endif /* ndef CONFIG_SECURITY_FILE_CAPABILITIES */
> + if (current->uid)
> + old_cap = cap_set_effective(__cap_empty_set);
> + else
> + old_cap = cap_set_effective(current->cap_permitted);
> + }
>
> res = __user_walk_fd(dfd, filename, LOOKUP_FOLLOW|LOOKUP_ACCESS, &nd);
> if (res)
> @@ -478,7 +483,10 @@ out_path_release:
> out:
> current->fsuid = old_fsuid;
> current->fsgid = old_fsgid;
> - current->cap_effective = old_cap;
> +
> + if (!issecure(SECURE_NO_SETUID_FIXUP)) {
> + (void) cap_set_effective(old_cap);
> + }
>
> return res;
> }
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index fa830f8..0267384 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -501,6 +501,8 @@ extern const kernel_cap_t __cap_empty_set;
> extern const kernel_cap_t __cap_full_set;
> extern const kernel_cap_t __cap_init_eff_set;
>
> +kernel_cap_t cap_set_effective(const kernel_cap_t pE_new);
> +
> int capable(int cap);
> int __capable(struct task_struct *t, int cap);
>
> diff --git a/include/linux/securebits.h b/include/linux/securebits.h
> index c1f19db..92f09bd 100644
> --- a/include/linux/securebits.h
> +++ b/include/linux/securebits.h
> @@ -7,14 +7,15 @@
> inheritance of root-permissions and suid-root executable under
> compatibility mode. We raise the effective and inheritable bitmasks
> *of the executable file* if the effective uid of the new process is
> - 0. If the real uid is 0, we raise the inheritable bitmask of the
> + 0. If the real uid is 0, we raise the effective (legacy) bit of the
> executable file. */
> #define SECURE_NOROOT 0
> #define SECURE_NOROOT_LOCKED 1 /* make bit-0 immutable */
>
> -/* When set, setuid to/from uid 0 does not trigger capability-"fixes"
> - to be compatible with old programs relying on set*uid to loose
> - privileges. When unset, setuid doesn't change privileges. */
> +/* 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
> + capabilities to be gained/lost. */
> #define SECURE_NO_SETUID_FIXUP 2
> #define SECURE_NO_SETUID_FIXUP_LOCKED 3 /* make bit-2 immutable */
>
> @@ -26,10 +27,10 @@
> #define SECURE_KEEP_CAPS 4
> #define SECURE_KEEP_CAPS_LOCKED 5 /* make bit-4 immutable */
>
> -/* Each securesetting is implemented using two bits. One bit specify
> +/* 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 fixed or not. A setting which is fixed cannot be changed
> - from user-level. */
> + 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->securebits)
>
> diff --git a/kernel/capability.c b/kernel/capability.c
> index cfbe442..901e0fd 100644
> --- a/kernel/capability.c
> +++ b/kernel/capability.c
> @@ -121,6 +121,27 @@ static int cap_validate_magic(cap_user_header_t header, unsigned *tocopy)
> * uninteresting and/or not to be changed.
> */
>
> +/*
> + * Atomically modify the effective capabilities returning the original
> + * value. No permission check is performed here - it is assumed that the
> + * caller is permitted to set the desired effective capabilities.
> + */
> +kernel_cap_t cap_set_effective(const kernel_cap_t pE_new)
> +{
> + kernel_cap_t pE_old;
> +
> + spin_lock(&task_capability_lock);
> +
> + pE_old = current->cap_effective;
> + current->cap_effective = pE_new;
> +
> + spin_unlock(&task_capability_lock);
> +
> + return pE_old;
> +}
> +
> +EXPORT_SYMBOL(cap_set_effective);
> +
> /**
> * sys_capget - get the capabilities of a given process.
> * @header: pointer to struct that contains capability version and
> --
> 1.5.3.7
>
--
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