Re: [PATCH v12 2/4] smack: Set the SMACK64TRANSMUTE xattr in smack_inode_init_security()

Casey Schaufler
2023-06-11T16:59:25Z

On 6/10/2023 12:57 AM, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@huawei.com>
>
> With the newly added ability of LSMs to supply multiple xattrs, set
> SMACK64TRASMUTE in smack_inode_init_security(), instead of d_instantiate().
> Do it by incrementing SMACK_INODE_INIT_XATTRS to 2 and by calling
> lsm_get_xattr_slot() a second time, if the transmuting conditions are met.
>
> The LSM infrastructure passes all xattrs provided by LSMs to the
> filesystems through the initxattrs() callback, so that filesystems can
> store xattrs in the disk.
>
> After the change, the SMK_INODE_TRANSMUTE inode flag is always set by
> d_instantiate() after fetching SMACK64TRANSMUTE from the disk. Before it
> was done by smack_inode_post_setxattr() as result of the __vfs_setxattr()
> call.
>
> Removing __vfs_setxattr() also prevents invalidating the EVM HMAC, by
> adding a new xattr without checking and updating the existing HMAC.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>

Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>

> ---
> security/smack/smack.h | 2 +-
> security/smack/smack_lsm.c | 45 ++++++++++++++++++++++++--------------
> 2 files changed, 29 insertions(+), 18 deletions(-)
>
> diff --git a/security/smack/smack.h b/security/smack/smack.h
> index aa15ff56ed6..041688e5a77 100644
> --- a/security/smack/smack.h
> +++ b/security/smack/smack.h
> @@ -128,7 +128,7 @@ struct task_smack {
>
> #define SMK_INODE_INSTANT 0x01 /* inode is instantiated */
> #define SMK_INODE_TRANSMUTE 0x02 /* directory is transmuting */
> -#define SMK_INODE_CHANGED 0x04 /* smack was transmuted */
> +#define SMK_INODE_CHANGED 0x04 /* smack was transmuted (unused) */
> #define SMK_INODE_IMPURE 0x08 /* involved in an impure transaction */
>
> /*
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 25ade3819af..679156601a1 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -52,7 +52,14 @@
> #define SMK_RECEIVING 1
> #define SMK_SENDING 2
>
> -#define SMACK_INODE_INIT_XATTRS 1
> +/*
> + * Smack uses multiple xattrs.
> + * SMACK64 - for access control,
> + * SMACK64TRANSMUTE - label initialization,
> + * Not saved on files - SMACK64IPIN and SMACK64IPOUT,
> + * Must be set explicitly - SMACK64EXEC and SMACK64MMAP
> + */
> +#define SMACK_INODE_INIT_XATTRS 2
>
> #ifdef SMACK_IPV6_PORT_LABELING
> static DEFINE_MUTEX(smack_ipv6_lock);
> @@ -935,7 +942,6 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
> struct xattr *xattrs, int *xattr_count)
> {
> struct task_smack *tsp = smack_cred(current_cred());
> - struct inode_smack *issp = smack_inode(inode);
> struct smack_known *skp = smk_of_task(tsp);
> struct smack_known *isp = smk_of_inode(inode);
> struct smack_known *dsp = smk_of_inode(dir);
> @@ -963,6 +969,8 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
> if ((tsp->smk_task == tsp->smk_transmuted) ||
> (may > 0 && ((may & MAY_TRANSMUTE) != 0) &&
> smk_inode_transmutable(dir))) {
> + struct xattr *xattr_transmute;
> +
> /*
> * The caller of smack_dentry_create_files_as()
> * should have overridden the current cred, so the
> @@ -971,7 +979,18 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
> */
> if (tsp->smk_task != tsp->smk_transmuted)
> isp = dsp;
> - issp->smk_flags |= SMK_INODE_CHANGED;
> + xattr_transmute = lsm_get_xattr_slot(xattrs,
> + xattr_count);
> + if (xattr_transmute) {
> + xattr_transmute->value = kmemdup(TRANS_TRUE,
> + TRANS_TRUE_SIZE,
> + GFP_NOFS);
> + if (!xattr_transmute->value)
> + return -ENOMEM;
> +
> + xattr_transmute->value_len = TRANS_TRUE_SIZE;
> + xattr_transmute->name = XATTR_SMACK_TRANSMUTE;
> + }
> }
>
> xattr->value = kstrdup(isp->smk_known, GFP_NOFS);
> @@ -3518,20 +3537,12 @@ static void smack_d_instantiate(struct dentry *opt_dentry, struct inode *inode)
> * If there is a transmute attribute on the
> * directory mark the inode.
> */
> - if (isp->smk_flags & SMK_INODE_CHANGED) {
> - isp->smk_flags &= ~SMK_INODE_CHANGED;
> - rc = __vfs_setxattr(&nop_mnt_idmap, dp, inode,
> - XATTR_NAME_SMACKTRANSMUTE,
> - TRANS_TRUE, TRANS_TRUE_SIZE,
> - 0);
> - } else {
> - rc = __vfs_getxattr(dp, inode,
> - XATTR_NAME_SMACKTRANSMUTE, trattr,
> - TRANS_TRUE_SIZE);
> - if (rc >= 0 && strncmp(trattr, TRANS_TRUE,
> - TRANS_TRUE_SIZE) != 0)
> - rc = -EINVAL;
> - }
> + rc = __vfs_getxattr(dp, inode,
> + XATTR_NAME_SMACKTRANSMUTE, trattr,
> + TRANS_TRUE_SIZE);
> + if (rc >= 0 && strncmp(trattr, TRANS_TRUE,
> + TRANS_TRUE_SIZE) != 0)
> + rc = -EINVAL;
> if (rc >= 0)
> transflag = SMK_INODE_TRANSMUTE;
> }