Re: [PATCH 58/59] LSM: Specify which LSM to display with /proc/self/attr/display

[Stephen Smalley <stephen.smalley@gmail.com>]

On Wed, Apr 10, 2019 at 8:43 AM Stephen Smalley
<stephen.smalley@gmail.com> wrote:
>
> On Tue, Apr 9, 2019 at 5:42 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> >
> > Create a new entry "display" in /proc/.../attr for controlling
> > which LSM security information is displayed for a process.
> > The name of an active LSM that supplies hooks for human readable
> > data may be written to "display" to set the value. The name of
> > the LSM currently in use can be read from "display".
> >
> > Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> > ---
> >  fs/proc/base.c      |   1 +
> >  security/security.c | 123 ++++++++++++++++++++++++++++++++++++++++++--
> >  2 files changed, 121 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index ddef482f1334..7bf70e041315 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -2618,6 +2618,7 @@ static const struct pid_entry attr_dir_stuff[] = {
> >         ATTR(NULL, "fscreate",          0666),
> >         ATTR(NULL, "keycreate",         0666),
> >         ATTR(NULL, "sockcreate",        0666),
> > +       ATTR(NULL, "display",           0666),
> >  #ifdef CONFIG_SECURITY_SMACK
> >         DIR("smack",                    0555,
> >             proc_smack_attr_dir_inode_ops, proc_smack_attr_dir_ops),
> > diff --git a/security/security.c b/security/security.c
> > index 29149db3f78a..6e304aa796f9 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -47,9 +47,13 @@ static struct kmem_cache *lsm_inode_cache;
> >
> >  char *lsm_names;
> >
> > -/* Socket blobs include infrastructure managed data */
> > +/*
> > + *     Socket blobs include infrastructure managed data
> > + *     Cred blobs include context display instructions
> > + */
> >  static struct lsm_blob_sizes blob_sizes __lsm_ro_after_init = {
> >         .lbs_sock = sizeof(struct lsm_export),
> > +       .lbs_cred = sizeof(struct lsm_one_hooks),
> >  };
> >
> >  /**
> > @@ -751,7 +755,10 @@ int lsm_superblock_alloc(struct super_block *sb)
> >
> >  #define call_one_int_hook(FUNC, IRC, ...) ({                   \
> >         int RC = IRC;                                           \
> > -       if (lsm_base_one.FUNC.FUNC)                             \
> > +       struct lsm_one_hooks *LOH = current_cred()->security;   \
> > +       if (LOH->FUNC.FUNC)                                     \
> > +               RC = LOH->FUNC.FUNC(__VA_ARGS__);               \
> > +       else if (LOH->lsm == NULL && lsm_base_one.FUNC.FUNC)    \
> >                 RC = lsm_base_one.FUNC.FUNC(__VA_ARGS__);       \
> >         RC;                                                     \
> >  })
> > @@ -1617,6 +1624,7 @@ int security_cred_alloc_blank(struct cred *cred, gfp_t gfp)
> >
> >  void security_cred_free(struct cred *cred)
> >  {
> > +       struct lsm_one_hooks *loh;
> >         /*
> >          * There is a failure case in prepare_creds() that
> >          * may result in a call here with ->security being NULL.
> > @@ -1626,26 +1634,44 @@ void security_cred_free(struct cred *cred)
> >
> >         call_void_hook(cred_free, cred);
> >
> > +       loh = cred->security;
> > +       kfree(loh->lsm);
> >         kfree(cred->security);
> >         cred->security = NULL;
> >  }
> >
> > +static int copy_loh(struct lsm_one_hooks *new, struct lsm_one_hooks *old,
> > +                   gfp_t gfp)
> > +{
> > +       *new = *old;
> > +       if (old->lsm) {
> > +               new->lsm = kstrdup(old->lsm, gfp);
> > +               if (unlikely(new->lsm == NULL))
> > +                       return -ENOMEM;
> > +       }
> > +       return 0;
> > +}
> > +
> >  int security_prepare_creds(struct cred *new, const struct cred *old, gfp_t gfp)
> >  {
> >         int rc = lsm_cred_alloc(new, gfp);
> >
> > -       if (rc)
> > +       if (unlikely(rc))
> >                 return rc;
> >
> >         rc = call_int_hook(cred_prepare, 0, new, old, gfp);
> >         if (unlikely(rc))
> >                 security_cred_free(new);
> > +       else
> > +               rc = copy_loh(new->security, old->security, gfp);
> > +
> >         return rc;
> >  }
> >
> >  void security_transfer_creds(struct cred *new, const struct cred *old)
> >  {
> >         call_void_hook(cred_transfer, new, old);
> > +       WARN_ON(copy_loh(new->security, old->security, GFP_KERNEL));
> >  }
> >
> >  void security_cred_getsecid(const struct cred *c, struct lsm_export *l)
> > @@ -1960,10 +1986,28 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
> >                                 char **value)
> >  {
> >         struct security_hook_list *hp;
> > +       struct lsm_one_hooks *loh = current_cred()->security;
> > +       char *s;
> > +
> > +       if (!strcmp(name, "display")) {
> > +               if (loh->lsm)
> > +                       s = loh->lsm;
> > +               else if (lsm_base_one.lsm)
> > +                       s = lsm_base_one.lsm;
> > +               else
> > +                       return -EINVAL;
> > +
> > +               *value = kstrdup(s, GFP_KERNEL);
> > +               if (*value)
> > +                       return strlen(s);
> > +               return -ENOMEM;
> > +       }
> >
> >         hlist_for_each_entry(hp, &security_hook_heads.getprocattr, list) {
> >                 if (lsm != NULL && strcmp(lsm, hp->lsm))
> >                         continue;
> > +               if (lsm == NULL && loh->lsm && strcmp(loh->lsm, hp->lsm))
> > +                       continue;
> >                 return hp->hook.getprocattr(p, name, value);
> >         }
> >         return -EINVAL;
> > @@ -1973,10 +2017,83 @@ int security_setprocattr(const char *lsm, const char *name, void *value,
> >                          size_t size)
> >  {
> >         struct security_hook_list *hp;
> > +       struct lsm_one_hooks *loh = current_cred()->security;
> > +       bool found = false;
> > +       char *s;
> > +
> > +       /*
> > +        * End the passed name at a newline.
> > +        */
> > +       s = strnchr(value, size, '\n');
> > +       if (s)
> > +               *s = '\0';
> > +
> > +       if (!strcmp(name, "display")) {
> > +               union security_list_options secid_to_secctx;
> > +               union security_list_options secctx_to_secid;
> > +               union security_list_options socket_getpeersec_stream;
> > +
> > +               if (size == 0 || size >= 100)
> > +                       return -EINVAL;
> > +
> > +               secid_to_secctx.secid_to_secctx = NULL;
> > +               hlist_for_each_entry(hp, &security_hook_heads.secid_to_secctx,
> > +                                    list) {
> > +                       if (size >= strlen(hp->lsm) &&
> > +                           !strncmp(value, hp->lsm, size)) {
> > +                               secid_to_secctx = hp->hook;
> > +                               found = true;
> > +                               break;
> > +                       }
> > +               }
> > +               secctx_to_secid.secctx_to_secid = NULL;
> > +               hlist_for_each_entry(hp, &security_hook_heads.secctx_to_secid,
> > +                                    list) {
> > +                       if (size >= strlen(hp->lsm) &&
> > +                           !strncmp(value, hp->lsm, size)) {
> > +                               secctx_to_secid = hp->hook;
> > +                               found = true;
> > +                               break;
> > +                       }
> > +               }
> > +               socket_getpeersec_stream.socket_getpeersec_stream = NULL;
> > +               hlist_for_each_entry(hp,
> > +                               &security_hook_heads.socket_getpeersec_stream,
> > +                                    list) {
> > +                       if (size >= strlen(hp->lsm) &&
> > +                           !strncmp(value, hp->lsm, size)) {
> > +                               socket_getpeersec_stream = hp->hook;
> > +                               found = true;
> > +                               break;
> > +                       }
> > +               }
> > +               if (!found)
> > +                       return -EINVAL;
> > +
> > +               /*
> > +                * The named lsm is active and supplies one or more
> > +                * of the relevant hooks. Switch to it.
> > +                */
> > +               s = kmemdup(value, size + 1, GFP_KERNEL);
> > +               if (s == NULL)
> > +                       return -ENOMEM;
> > +               s[size] = '\0';
> > +
> > +               if (loh->lsm)
> > +                       kfree(loh->lsm);
> > +               loh->lsm = s;
> > +               loh->secid_to_secctx = secid_to_secctx;
> > +               loh->secctx_to_secid = secctx_to_secid;
> > +               loh->socket_getpeersec_stream = socket_getpeersec_stream;
>
> You can't just write to the cred security blob like this; it is a
> shared data structure, not per-task.

To be clear, you either need to perform a new = prepare_creds(); /*
modify new->security as desired */; commit_creds(new); sequence here,
or use the task security blob instead of the cred security blob.  The
latter seems more amenable to your needs.
>
> > +
> > +               return size;
> > +       }
> >
> >         hlist_for_each_entry(hp, &security_hook_heads.setprocattr, list) {
> >                 if (lsm != NULL && strcmp(lsm, hp->lsm))
> >                         continue;
> > +               if (lsm == NULL && loh->lsm && strcmp(loh->lsm, hp->lsm))
> > +                       continue;
> >                 return hp->hook.setprocattr(name, value, size);
> >         }
> >         return -EINVAL;
> > --
> > 2.19.1
> >