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syscalls: implement execveat() system call

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This patchset adds execveat(2) for x86, and is derived from Meredydd
Luff's patch from Sept 2012 (https://lkml.org/lkml/2012/9/11/528).

The primary aim of adding an execveat syscall is to allow an
implementation of fexecve(3) that does not rely on the /proc filesystem,
at least for executables (rather than scripts).  The current glibc version
of fexecve(3) is implemented via /proc, which causes problems in sandboxed
or otherwise restricted environments.

Given the desire for a /proc-free fexecve() implementation, HPA suggested
(https://lkml.org/lkml/2006/7/11/556) that an execveat(2) syscall would be
an appropriate generalization.

Also, having a new syscall means that it can take a flags argument without
back-compatibility concerns.  The current implementation just defines the
AT_EMPTY_PATH and AT_SYMLINK_NOFOLLOW flags, but other flags could be
added in future -- for example, flags for new namespaces (as suggested at
https://lkml.org/lkml/2006/7/11/474).

Related history:
 - https://lkml.org/lkml/2006/12/27/123 is an example of someone
   realizing that fexecve() is likely to fail in a chroot environment.
 - http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=514043 covered
   documenting the /proc requirement of fexecve(3) in its manpage, to
   "prevent other people from wasting their time".
 - https://bugzilla.redhat.com/show_bug.cgi?id=241609 described a
   problem where a process that did setuid() could not fexecve()
   because it no longer had access to /proc/self/fd; this has since
   been fixed.

This patch (of 4):

Add a new execveat(2) system call.  execveat() is to execve() as openat()
is to open(): it takes a file descriptor that refers to a directory, and
resolves the filename relative to that.

In addition, if the filename is empty and AT_EMPTY_PATH is specified,
execveat() executes the file to which the file descriptor refers.  This
replicates the functionality of fexecve(), which is a system call in other
UNIXen, but in Linux glibc it depends on opening "/proc/self/fd/<fd>" (and
so relies on /proc being mounted).

The filename fed to the executed program as argv[0] (or the name of the
script fed to a script interpreter) will be of the form "/dev/fd/<fd>"
(for an empty filename) or "/dev/fd/<fd>/<filename>", effectively
reflecting how the executable was found.  This does however mean that
execution of a script in a /proc-less environment won't work; also, script
execution via an O_CLOEXEC file descriptor fails (as the file will not be
accessible after exec).

Based on patches by Meredydd Luff.

Signed-off-by: David Drysdale <drysdale@google.com>
Cc: Meredydd Luff <meredydd@senatehouse.org>
Cc: Shuah Khan <shuah.kh@samsung.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rich Felker <dalias@aerifal.cx>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
David Drysdale 2014-12-12 16:57:29 -08:00 • committed by Linus Torvalds
parent c0ef0cc9d2
commit 51f39a1f0c
13 changed files with 145 additions and 15 deletions
Download patch file Download diff file Expand all files Collapse all files

113
fs/exec.c
View file

@ -748,18 +748,25 @@ EXPORT_SYMBOL(setup_arg_pages);
#endif /* CONFIG_MMU */
static struct file *do_open_exec(struct filename *name)
static struct file *do_open_execat(int fd, struct filename *name, int flags)
{
struct file *file;
int err;
static const struct open_flags open_exec_flags = {
struct open_flags open_exec_flags = {
.open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
.acc_mode = MAY_EXEC | MAY_OPEN,
.intent = LOOKUP_OPEN,
.lookup_flags = LOOKUP_FOLLOW,
};
file = do_filp_open(AT_FDCWD, name, &open_exec_flags);
if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
return ERR_PTR(-EINVAL);
if (flags & AT_SYMLINK_NOFOLLOW)
open_exec_flags.lookup_flags &= ~LOOKUP_FOLLOW;
if (flags & AT_EMPTY_PATH)
open_exec_flags.lookup_flags |= LOOKUP_EMPTY;
file = do_filp_open(fd, name, &open_exec_flags);
if (IS_ERR(file))
goto out;
@ -770,12 +777,13 @@ static struct file *do_open_exec(struct filename *name)
if (file->f_path.mnt->mnt_flags & MNT_NOEXEC)
goto exit;
fsnotify_open(file);
err = deny_write_access(file);
if (err)
goto exit;
if (name->name[0] != '\0')
fsnotify_open(file);
out:
return file;
@ -787,7 +795,7 @@ exit:
struct file *open_exec(const char *name)
{
struct filename tmp = { .name = name };
return do_open_exec(&tmp);
return do_open_execat(AT_FDCWD, &tmp, 0);
}
EXPORT_SYMBOL(open_exec);
@ -1428,10 +1436,12 @@ static int exec_binprm(struct linux_binprm *bprm)
/*
* sys_execve() executes a new program.
*/
static int do_execve_common(struct filename *filename,
struct user_arg_ptr argv,
struct user_arg_ptr envp)
static int do_execveat_common(int fd, struct filename *filename,
struct user_arg_ptr argv,
struct user_arg_ptr envp,
int flags)
{
char *pathbuf = NULL;
struct linux_binprm *bprm;
struct file *file;
struct files_struct *displaced;
@ -1472,7 +1482,7 @@ static int do_execve_common(struct filename *filename,
check_unsafe_exec(bprm);
current->in_execve = 1;
file = do_open_exec(filename);
file = do_open_execat(fd, filename, flags);
retval = PTR_ERR(file);
if (IS_ERR(file))
goto out_unmark;
@ -1480,7 +1490,28 @@ static int do_execve_common(struct filename *filename,
sched_exec();
bprm->file = file;
bprm->filename = bprm->interp = filename->name;
if (fd == AT_FDCWD || filename->name[0] == '/') {
bprm->filename = filename->name;
} else {
if (filename->name[0] == '\0')
pathbuf = kasprintf(GFP_TEMPORARY, "/dev/fd/%d", fd);
else
pathbuf = kasprintf(GFP_TEMPORARY, "/dev/fd/%d/%s",
fd, filename->name);
if (!pathbuf) {
retval = -ENOMEM;
goto out_unmark;
}
/*
* Record that a name derived from an O_CLOEXEC fd will be
* inaccessible after exec. Relies on having exclusive access to
* current->files (due to unshare_files above).
*/
if (close_on_exec(fd, rcu_dereference_raw(current->files->fdt)))
bprm->interp_flags |= BINPRM_FLAGS_PATH_INACCESSIBLE;
bprm->filename = pathbuf;
}
bprm->interp = bprm->filename;
retval = bprm_mm_init(bprm);
if (retval)
@ -1521,6 +1552,7 @@ static int do_execve_common(struct filename *filename,
acct_update_integrals(current);
task_numa_free(current);
free_bprm(bprm);
kfree(pathbuf);
putname(filename);
if (displaced)
put_files_struct(displaced);
@ -1538,6 +1570,7 @@ out_unmark:
out_free:
free_bprm(bprm);
kfree(pathbuf);
out_files:
if (displaced)
@ -1553,7 +1586,18 @@ int do_execve(struct filename *filename,
{
struct user_arg_ptr argv = { .ptr.native = __argv };
struct user_arg_ptr envp = { .ptr.native = __envp };
return do_execve_common(filename, argv, envp);
return do_execveat_common(AT_FDCWD, filename, argv, envp, 0);
}
int do_execveat(int fd, struct filename *filename,
const char __user *const __user *__argv,
const char __user *const __user *__envp,
int flags)
{
struct user_arg_ptr argv = { .ptr.native = __argv };
struct user_arg_ptr envp = { .ptr.native = __envp };
return do_execveat_common(fd, filename, argv, envp, flags);
}
#ifdef CONFIG_COMPAT
@ -1569,7 +1613,23 @@ static int compat_do_execve(struct filename *filename,
.is_compat = true,
.ptr.compat = __envp,
};
return do_execve_common(filename, argv, envp);
return do_execveat_common(AT_FDCWD, filename, argv, envp, 0);
}
static int compat_do_execveat(int fd, struct filename *filename,
const compat_uptr_t __user *__argv,
const compat_uptr_t __user *__envp,
int flags)
{
struct user_arg_ptr argv = {
.is_compat = true,
.ptr.compat = __argv,
};
struct user_arg_ptr envp = {
.is_compat = true,
.ptr.compat = __envp,
};
return do_execveat_common(fd, filename, argv, envp, flags);
}
#endif
@ -1609,6 +1669,20 @@ SYSCALL_DEFINE3(execve,
{
return do_execve(getname(filename), argv, envp);
}
SYSCALL_DEFINE5(execveat,
int, fd, const char __user *, filename,
const char __user *const __user *, argv,
const char __user *const __user *, envp,
int, flags)
{
int lookup_flags = (flags & AT_EMPTY_PATH) ? LOOKUP_EMPTY : 0;
return do_execveat(fd,
getname_flags(filename, lookup_flags, NULL),
argv, envp, flags);
}
#ifdef CONFIG_COMPAT
COMPAT_SYSCALL_DEFINE3(execve, const char __user *, filename,
const compat_uptr_t __user *, argv,
@ -1616,4 +1690,17 @@ COMPAT_SYSCALL_DEFINE3(execve, const char __user *, filename,
{
return compat_do_execve(getname(filename), argv, envp);
}
COMPAT_SYSCALL_DEFINE5(execveat, int, fd,
const char __user *, filename,
const compat_uptr_t __user *, argv,
const compat_uptr_t __user *, envp,
int, flags)
{
int lookup_flags = (flags & AT_EMPTY_PATH) ? LOOKUP_EMPTY : 0;
return compat_do_execveat(fd,
getname_flags(filename, lookup_flags, NULL),
argv, envp, flags);
}
#endif