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authorDavid Drysdale <drysdale@google.com>2014-12-12 16:57:29 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2014-12-13 12:42:51 -0800
commit51f39a1f0cea1cacf8c787f652f26dfee9611874 (patch)
tree4b9199e785bdd9e8c0c55a0ec94ce8d268885bc5 /fs/exec.c
parentc0ef0cc9d277f0f2a83b5a287a816b3916d9f026 (diff)
downloadlwn-51f39a1f0cea1cacf8c787f652f26dfee9611874.tar.gz
lwn-51f39a1f0cea1cacf8c787f652f26dfee9611874.zip
syscalls: implement execveat() system call
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>
Diffstat (limited to 'fs/exec.c')
-rw-r--r--fs/exec.c113
1 files changed, 100 insertions, 13 deletions
diff --git a/fs/exec.c b/fs/exec.c
index 01aebe300200..ad8798e26be9 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -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