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Subject[PATCH 7/7] exec: Implement kernel_execve

To allow the kernel not to play games with set_fs to call exec
implement kernel_execve. The function kernel_execve takes pointers
into kernel memory and copies the values pointed to onto the new
userspace stack.

The calls with arguments from kernel space of do_execve are replaced
with calls to kernel_execve.

The calls do_execve and do_execveat are made static as there are now
no callers outside of exec.

The comments that mention do_execve are updated to refer to
kernel_execve or execve depending on the circumstances. In addition
to correcting the comments, this makes it easy to grep for do_execve
and verify it is not used.

Inspired-by: https://lkml.kernel.org/r/20200627072704.2447163-1-hch@lst.de
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
arch/x86/entry/entry_32.S | 2 +-
arch/x86/entry/entry_64.S | 2 +-
arch/x86/kernel/unwind_frame.c | 2 +-
fs/exec.c | 88 +++++++++++++++++++++++++++++++++-
include/linux/binfmts.h | 9 +---
init/main.c | 4 +-
kernel/umh.c | 6 +--
security/tomoyo/common.h | 2 +-
security/tomoyo/domain.c | 4 +-
security/tomoyo/tomoyo.c | 4 +-
10 files changed, 100 insertions(+), 23 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 024d7d276cd4..8f4e085ee06d 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -854,7 +854,7 @@ SYM_CODE_START(ret_from_fork)
CALL_NOSPEC ebx
/*
* A kernel thread is allowed to return here after successfully
- * calling do_execve(). Exit to userspace to complete the execve()
+ * calling kernel_execve(). Exit to userspace to complete the execve()
* syscall.
*/
movl $0, PT_EAX(%esp)
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index d2a00c97e53f..73c7e255256b 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -293,7 +293,7 @@ SYM_CODE_START(ret_from_fork)
CALL_NOSPEC rbx
/*
* A kernel thread is allowed to return here after successfully
- * calling do_execve(). Exit to userspace to complete the execve()
+ * calling kernel_execve(). Exit to userspace to complete the execve()
* syscall.
*/
movq $0, RAX(%rsp)
diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
index 722a85f3b2dd..e40b4942157f 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -275,7 +275,7 @@ bool unwind_next_frame(struct unwind_state *state)
* This user_mode() check is slightly broader than a PF_KTHREAD
* check because it also catches the awkward situation where a
* newly forked kthread transitions into a user task by calling
- * do_execve(), which eventually clears PF_KTHREAD.
+ * kernel_execve(), which eventually clears PF_KTHREAD.
*/
if (!user_mode(regs))
goto the_end;
diff --git a/fs/exec.c b/fs/exec.c
index f8135dc149b3..3698252719a3 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -448,6 +448,23 @@ static int count(struct user_arg_ptr argv, int max)
return i;
}

+static int count_strings_kernel(const char *const *argv)
+{
+ int i;
+
+ if (!argv)
+ return 0;
+
+ for (i = 0; argv[i]; ++i) {
+ if (i >= MAX_ARG_STRINGS)
+ return -E2BIG;
+ if (fatal_signal_pending(current))
+ return -ERESTARTNOHAND;
+ cond_resched();
+ }
+ return i;
+}
+
static int bprm_stack_limits(struct linux_binprm *bprm)
{
unsigned long limit, ptr_size;
@@ -624,6 +641,20 @@ int copy_string_kernel(const char *arg, struct linux_binprm *bprm)
}
EXPORT_SYMBOL(copy_string_kernel);

+static int copy_strings_kernel(int argc, const char *const *argv,
+ struct linux_binprm *bprm)
+{
+ while (argc-- > 0) {
+ int ret = copy_string_kernel(argv[argc], bprm);
+ if (ret < 0)
+ return ret;
+ if (fatal_signal_pending(current))
+ return -ERESTARTNOHAND;
+ cond_resched();
+ }
+ return 0;
+}
+
#ifdef CONFIG_MMU

/*
@@ -1991,7 +2022,60 @@ static int do_execveat_common(int fd, struct filename *filename,
return retval;
}

-int do_execve(struct filename *filename,
+int kernel_execve(const char *kernel_filename,
+ const char *const *argv, const char *const *envp)
+{
+ struct filename *filename;
+ struct linux_binprm *bprm;
+ int fd = AT_FDCWD;
+ int retval;
+
+ filename = getname_kernel(kernel_filename);
+ if (IS_ERR(filename))
+ return PTR_ERR(filename);
+
+ bprm = alloc_bprm(fd, filename);
+ if (IS_ERR(bprm)) {
+ retval = PTR_ERR(bprm);
+ goto out_ret;
+ }
+
+ retval = count_strings_kernel(argv);
+ if (retval < 0)
+ goto out_free;
+ bprm->argc = retval;
+
+ retval = count_strings_kernel(envp);
+ if (retval < 0)
+ goto out_free;
+ bprm->envc = retval;
+
+ retval = bprm_stack_limits(bprm);
+ if (retval < 0)
+ goto out_free;
+
+ retval = copy_string_kernel(bprm->filename, bprm);
+ if (retval < 0)
+ goto out_free;
+ bprm->exec = bprm->p;
+
+ retval = copy_strings_kernel(bprm->envc, envp, bprm);
+ if (retval < 0)
+ goto out_free;
+
+ retval = copy_strings_kernel(bprm->argc, argv, bprm);
+ if (retval < 0)
+ goto out_free;
+
+ retval = bprm_execve(bprm, fd, filename, 0);
+out_free:
+ free_bprm(bprm);
+out_ret:
+ putname(filename);
+ return retval;
+}
+
+static int do_execve(struct filename *filename,
const char __user *const __user *__argv,
const char __user *const __user *__envp)
{
@@ -2000,7 +2084,7 @@ int do_execve(struct filename *filename,
return do_execveat_common(AT_FDCWD, filename, argv, envp, 0);
}

-int do_execveat(int fd, struct filename *filename,
+static int do_execveat(int fd, struct filename *filename,
const char __user *const __user *__argv,
const char __user *const __user *__envp,
int flags)
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 8e9e1b0c8eb8..0571701ab1c5 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -135,12 +135,7 @@ int copy_string_kernel(const char *arg, struct linux_binprm *bprm);
extern void set_binfmt(struct linux_binfmt *new);
extern ssize_t read_code(struct file *, unsigned long, loff_t, size_t);

-extern int do_execve(struct filename *,
- const char __user * const __user *,
- const char __user * const __user *);
-extern int do_execveat(int, struct filename *,
- const char __user * const __user *,
- const char __user * const __user *,
- int);
+int kernel_execve(const char *filename,
+ const char *const *argv, const char *const *envp);

#endif /* _LINUX_BINFMTS_H */
diff --git a/init/main.c b/init/main.c
index 0ead83e86b5a..78ccec5c28f3 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1329,9 +1329,7 @@ static int run_init_process(const char *init_filename)
pr_debug(" with environment:\n");
for (p = envp_init; *p; p++)
pr_debug(" %s\n", *p);
- return do_execve(getname_kernel(init_filename),
- (const char __user *const __user *)argv_init,
- (const char __user *const __user *)envp_init);
+ return kernel_execve(init_filename, argv_init, envp_init);
}

static int try_to_run_init_process(const char *init_filename)
diff --git a/kernel/umh.c b/kernel/umh.c
index 6ca2096298b9..a25433f9cd9a 100644
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -98,9 +98,9 @@ static int call_usermodehelper_exec_async(void *data)

commit_creds(new);

- retval = do_execve(getname_kernel(sub_info->path),
- (const char __user *const __user *)sub_info->argv,
- (const char __user *const __user *)sub_info->envp);
+ retval = kernel_execve(sub_info->path,
+ (const char *const *)sub_info->argv,
+ (const char *const *)sub_info->envp);
out:
sub_info->retval = retval;
/*
diff --git a/security/tomoyo/common.h b/security/tomoyo/common.h
index 050473df5809..85246b9df7ca 100644
--- a/security/tomoyo/common.h
+++ b/security/tomoyo/common.h
@@ -425,7 +425,7 @@ struct tomoyo_request_info {
struct tomoyo_obj_info *obj;
/*
* For holding parameters specific to execve() request.
- * NULL if not dealing do_execve().
+ * NULL if not dealing execve().
*/
struct tomoyo_execve *ee;
struct tomoyo_domain_info *domain;
diff --git a/security/tomoyo/domain.c b/security/tomoyo/domain.c
index 7869d6a9980b..53b3e1f5f227 100644
--- a/security/tomoyo/domain.c
+++ b/security/tomoyo/domain.c
@@ -767,7 +767,7 @@ int tomoyo_find_next_domain(struct linux_binprm *bprm)

/*
* Check for domain transition preference if "file execute" matched.
- * If preference is given, make do_execve() fail if domain transition
+ * If preference is given, make execve() fail if domain transition
* has failed, for domain transition preference should be used with
* destination domain defined.
*/
@@ -810,7 +810,7 @@ int tomoyo_find_next_domain(struct linux_binprm *bprm)
snprintf(ee->tmp, TOMOYO_EXEC_TMPSIZE - 1, "<%s>",
candidate->name);
/*
- * Make do_execve() fail if domain transition across namespaces
+ * Make execve() fail if domain transition across namespaces
* has failed.
*/
reject_on_transition_failure = true;
diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
index f9adddc42ac8..1f3cd432d830 100644
--- a/security/tomoyo/tomoyo.c
+++ b/security/tomoyo/tomoyo.c
@@ -93,7 +93,7 @@ static int tomoyo_bprm_check_security(struct linux_binprm *bprm)
struct tomoyo_task *s = tomoyo_task(current);

/*
- * Execute permission is checked against pathname passed to do_execve()
+ * Execute permission is checked against pathname passed to execve()
* using current domain.
*/
if (!s->old_domain_info) {
@@ -307,7 +307,7 @@ static int tomoyo_file_fcntl(struct file *file, unsigned int cmd,
*/
static int tomoyo_file_open(struct file *f)
{
- /* Don't check read permission here if called from do_execve(). */
+ /* Don't check read permission here if called from execve(). */
if (current->in_execve)
return 0;
return tomoyo_check_open_permission(tomoyo_domain(), &f->f_path,
--
2.25.0
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