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exec: Compute file based creds only once

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Move the computation of creds from prepare_binfmt into begin_new_exec
so that the creds need only be computed once.  This is just code
reorganization no semantic changes of any kind are made.

Moving the computation is safe.  I have looked through the kernel and
verified none of the binfmts look at bprm->cred directly, and that
there are no helpers that look at bprm->cred indirectly.  Which means
that it is not a problem to compute the bprm->cred later in the
execution flow as it is not used until it becomes current->cred.

A new function bprm_creds_from_file is added to contain the work that
needs to be done.  bprm_creds_from_file first computes which file
bprm->executable or most likely bprm->file that the bprm->creds
will be computed from.

The funciton bprm_fill_uid is updated to receive the file instead of
accessing bprm->file.  The now unnecessary work needed to reset the
bprm->cred->euid, and bprm->cred->egid is removed from brpm_fill_uid.
A small comment to document that bprm_fill_uid now only deals with the
work to handle suid and sgid files.  The default case is already
heandled by prepare_exec_creds.

The function security_bprm_repopulate_creds is renamed
security_bprm_creds_from_file and now is explicitly passed the file
from which to compute the creds.  The documentation of the
bprm_creds_from_file security hook is updated to explain when the hook
is called and what it needs to do.  The file is passed from
cap_bprm_creds_from_file into get_file_caps so that the caps are
computed for the appropriate file.  The now unnecessary work in
cap_bprm_creds_from_file to reset the ambient capabilites has been
removed.  A small comment to document that the work of
cap_bprm_creds_from_file is to read capabilities from the files
secureity attribute and derive capabilities from the fact the
user had uid 0 has been added.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman
2020-05-29 22:00:54 -05:00
parent a7868323c2
commit 56305aa9b6
8 changed files with 61 additions and 79 deletions
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24
security/commoncap.c
View File

@@ -647,7 +647,8 @@ int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data
* its xattrs and, if present, apply them to the proposed credentials being
* constructed by execve().
*/
static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_fcap)
static int get_file_caps(struct linux_binprm *bprm, struct file *file,
bool *effective, bool *has_fcap)
{
int rc = 0;
struct cpu_vfs_cap_data vcaps;
@@ -657,7 +658,7 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_f
if (!file_caps_enabled)
return 0;
if (!mnt_may_suid(bprm->file->f_path.mnt))
if (!mnt_may_suid(file->f_path.mnt))
return 0;
/*
@@ -665,10 +666,10 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_f
* explicit that capability bits are limited to s_user_ns and its
* descendants.
*/
if (!current_in_userns(bprm->file->f_path.mnt->mnt_sb->s_user_ns))
if (!current_in_userns(file->f_path.mnt->mnt_sb->s_user_ns))
return 0;
rc = get_vfs_caps_from_disk(bprm->file->f_path.dentry, &vcaps);
rc = get_vfs_caps_from_disk(file->f_path.dentry, &vcaps);
if (rc < 0) {
if (rc == -EINVAL)
printk(KERN_NOTICE "Invalid argument reading file caps for %s\n",
@@ -797,26 +798,27 @@ static inline bool nonroot_raised_pE(struct cred *new, const struct cred *old,
}
/**
* cap_bprm_repopulate_creds - Set up the proposed credentials for execve().
* cap_bprm_creds_from_file - Set up the proposed credentials for execve().
* @bprm: The execution parameters, including the proposed creds
* @file: The file to pull the credentials from
*
* Set up the proposed credentials for a new execution context being
* constructed by execve(). The proposed creds in @bprm->cred is altered,
* which won't take effect immediately. Returns 0 if successful, -ve on error.
*/
int cap_bprm_repopulate_creds(struct linux_binprm *bprm)
int cap_bprm_creds_from_file(struct linux_binprm *bprm, struct file *file)
{
/* Process setpcap binaries and capabilities for uid 0 */
const struct cred *old = current_cred();
struct cred *new = bprm->cred;
bool effective = false, has_fcap = false, is_setid;
int ret;
kuid_t root_uid;
new->cap_ambient = old->cap_ambient;
if (WARN_ON(!cap_ambient_invariant_ok(old)))
return -EPERM;
ret = get_file_caps(bprm, &effective, &has_fcap);
ret = get_file_caps(bprm, file, &effective, &has_fcap);
if (ret < 0)
return ret;
@@ -826,7 +828,7 @@ int cap_bprm_repopulate_creds(struct linux_binprm *bprm)
/* if we have fs caps, clear dangerous personality flags */
if (__cap_gained(permitted, new, old))
bprm->pf_per_clear |= PER_CLEAR_ON_SETID;
bprm->per_clear |= PER_CLEAR_ON_SETID;
/* Don't let someone trace a set[ug]id/setpcap binary with the revised
* credentials unless they have the appropriate permit.
@@ -889,7 +891,7 @@ int cap_bprm_repopulate_creds(struct linux_binprm *bprm)
(!__is_real(root_uid, new) &&
(effective ||
__cap_grew(permitted, ambient, new))))
bprm->active_secureexec = 1;
bprm->secureexec = 1;
return 0;
}
@@ -1346,7 +1348,7 @@ static struct security_hook_list capability_hooks[] __lsm_ro_after_init = {
LSM_HOOK_INIT(ptrace_traceme, cap_ptrace_traceme),
LSM_HOOK_INIT(capget, cap_capget),
LSM_HOOK_INIT(capset, cap_capset),
LSM_HOOK_INIT(bprm_repopulate_creds, cap_bprm_repopulate_creds),
LSM_HOOK_INIT(bprm_creds_from_file, cap_bprm_creds_from_file),
LSM_HOOK_INIT(inode_need_killpriv, cap_inode_need_killpriv),
LSM_HOOK_INIT(inode_killpriv, cap_inode_killpriv),
LSM_HOOK_INIT(inode_getsecurity, cap_inode_getsecurity),