Re: [PATCHv5 1/3] syscalls,x86: implement execveat() system call

From: Eric W. Biederman
Date: Wed Oct 22 2014 - 14:08:40 EST

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In reply to: David Drysdale: "[PATCHv5 1/3] syscalls,x86: implement execveat() system call"
Next in thread: David Drysdale: "Re: [PATCHv5 1/3] syscalls,x86: implement execveat() system call"
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David Drysdale <drysdale@xxxxxxxxxx> writes:

> Add a new system execveat(2) syscall. 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.
>
> Only x86-64, i386 and x32 ABIs are supported in this patch.
>
> Based on patches by Meredydd Luff <meredydd@xxxxxxxxxxxxxxx>
>
> Signed-off-by: David Drysdale <drysdale@xxxxxxxxxx>
> ---
> arch/x86/ia32/audit.c | 1 +
> arch/x86/ia32/ia32entry.S | 1 +
> arch/x86/kernel/audit_64.c | 1 +
> arch/x86/kernel/entry_64.S | 28 ++++++++
> arch/x86/syscalls/syscall_32.tbl | 1 +
> arch/x86/syscalls/syscall_64.tbl | 2 +
> arch/x86/um/sys_call_table_64.c | 1 +
> fs/exec.c | 130 ++++++++++++++++++++++++++++++++++----
> fs/namei.c | 2 +-
> include/linux/compat.h | 3 +
> include/linux/fs.h | 1 +
> include/linux/sched.h | 4 ++
> include/linux/syscalls.h | 4 ++
> include/uapi/asm-generic/unistd.h | 4 +-
> kernel/sys_ni.c | 3 +
> lib/audit.c | 3 +
> 16 files changed, 173 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/ia32/audit.c b/arch/x86/ia32/audit.c
> index 5d7b381da692..2eccc8932ae6 100644
> --- a/arch/x86/ia32/audit.c
> +++ b/arch/x86/ia32/audit.c
> @@ -35,6 +35,7 @@ int ia32_classify_syscall(unsigned syscall)
> case __NR_socketcall:
> return 4;
> case __NR_execve:
> + case __NR_execveat:
> return 5;
> default:
> return 1;
> diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
> index 4299eb05023c..2516c09743e0 100644
> --- a/arch/x86/ia32/ia32entry.S
> +++ b/arch/x86/ia32/ia32entry.S
> @@ -464,6 +464,7 @@ GLOBAL(\label)
> PTREGSCALL stub32_rt_sigreturn, sys32_rt_sigreturn
> PTREGSCALL stub32_sigreturn, sys32_sigreturn
> PTREGSCALL stub32_execve, compat_sys_execve
> + PTREGSCALL stub32_execveat, compat_sys_execveat
> PTREGSCALL stub32_fork, sys_fork
> PTREGSCALL stub32_vfork, sys_vfork
>
> diff --git a/arch/x86/kernel/audit_64.c b/arch/x86/kernel/audit_64.c
> index 06d3e5a14d9d..f3672508b249 100644
> --- a/arch/x86/kernel/audit_64.c
> +++ b/arch/x86/kernel/audit_64.c
> @@ -50,6 +50,7 @@ int audit_classify_syscall(int abi, unsigned syscall)
> case __NR_openat:
> return 3;
> case __NR_execve:
> + case __NR_execveat:
> return 5;
> default:
> return 0;
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 2fac1343a90b..00c4526e6ffe 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -665,6 +665,20 @@ ENTRY(stub_execve)
> CFI_ENDPROC
> END(stub_execve)
>
> +ENTRY(stub_execveat)
> + CFI_STARTPROC
> + addq $8, %rsp
> + PARTIAL_FRAME 0
> + SAVE_REST
> + FIXUP_TOP_OF_STACK %r11
> + call sys_execveat
> + RESTORE_TOP_OF_STACK %r11
> + movq %rax,RAX(%rsp)
> + RESTORE_REST
> + jmp int_ret_from_sys_call
> + CFI_ENDPROC
> +END(stub_execveat)
> +
> /*
> * sigreturn is special because it needs to restore all registers on return.
> * This cannot be done with SYSRET, so use the IRET return path instead.
> @@ -710,6 +724,20 @@ ENTRY(stub_x32_execve)
> CFI_ENDPROC
> END(stub_x32_execve)
>
> +ENTRY(stub_x32_execveat)
> + CFI_STARTPROC
> + addq $8, %rsp
> + PARTIAL_FRAME 0
> + SAVE_REST
> + FIXUP_TOP_OF_STACK %r11
> + call compat_sys_execveat
> + RESTORE_TOP_OF_STACK %r11
> + movq %rax,RAX(%rsp)
> + RESTORE_REST
> + jmp int_ret_from_sys_call
> + CFI_ENDPROC
> +END(stub_x32_execveat)
> +
> #endif
>
> /*
> diff --git a/arch/x86/syscalls/syscall_32.tbl b/arch/x86/syscalls/syscall_32.tbl
> index 028b78168d85..2633e3195455 100644
> --- a/arch/x86/syscalls/syscall_32.tbl
> +++ b/arch/x86/syscalls/syscall_32.tbl
> @@ -363,3 +363,4 @@
> 354 i386 seccomp sys_seccomp
> 355 i386 getrandom sys_getrandom
> 356 i386 memfd_create sys_memfd_create
> +357 i386 execveat sys_execveat stub32_execveat
> diff --git a/arch/x86/syscalls/syscall_64.tbl b/arch/x86/syscalls/syscall_64.tbl
> index 35dd922727b9..1af5badd159c 100644
> --- a/arch/x86/syscalls/syscall_64.tbl
> +++ b/arch/x86/syscalls/syscall_64.tbl
> @@ -327,6 +327,7 @@
> 318 common getrandom sys_getrandom
> 319 common memfd_create sys_memfd_create
> 320 common kexec_file_load sys_kexec_file_load
> +321 64 execveat stub_execveat
>
> #
> # x32-specific system call numbers start at 512 to avoid cache impact
> @@ -365,3 +366,4 @@
> 542 x32 getsockopt compat_sys_getsockopt
> 543 x32 io_setup compat_sys_io_setup
> 544 x32 io_submit compat_sys_io_submit
> +545 x32 execveat stub_x32_execveat
> diff --git a/arch/x86/um/sys_call_table_64.c b/arch/x86/um/sys_call_table_64.c
> index f2f0723070ca..20c3649d0691 100644
> --- a/arch/x86/um/sys_call_table_64.c
> +++ b/arch/x86/um/sys_call_table_64.c
> @@ -31,6 +31,7 @@
> #define stub_fork sys_fork
> #define stub_vfork sys_vfork
> #define stub_execve sys_execve
> +#define stub_execveat sys_execveat
> #define stub_rt_sigreturn sys_rt_sigreturn
>
> #define __SYSCALL_COMMON(nr, sym, compat) __SYSCALL_64(nr, sym, compat)
> diff --git a/fs/exec.c b/fs/exec.c
> index a2b42a98c743..92a6e14f096a 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -747,7 +747,7 @@ 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;
> @@ -757,10 +757,34 @@ static struct file *do_open_exec(struct filename *name)
> .intent = LOOKUP_OPEN,
> .lookup_flags = LOOKUP_FOLLOW,
> };
> + static const struct open_flags open_exec_nofollow_flags = {
> + .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
> + .acc_mode = MAY_EXEC | MAY_OPEN,
> + .intent = LOOKUP_OPEN,
> + .lookup_flags = 0,
> + };
>
> - file = do_filp_open(AT_FDCWD, name, &open_exec_flags);
> - if (IS_ERR(file))
> - goto out;
> + if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
> + return ERR_PTR(-EINVAL);
> +
> + if (name->name[0] != '\0') {

Is it really necessary to special case AT_EMPTY_PATH here. I would
have thought the existing logic in namei.c would have been fine
assuning we passed LOOKUP_EMPTY.

> + const struct open_flags *oflags = ((flags & AT_SYMLINK_NOFOLLOW)
> + ? &open_exec_nofollow_flags
> + : &open_exec_flags);
> +
> + file = do_filp_open(fd, name, oflags);
> + if (IS_ERR(file))
> + goto out;
> + } else {
> + file = fget(fd);
> + if (!file)
> + return ERR_PTR(-EBADF);
> +
> + err = inode_permission(file->f_path.dentry->d_inode,
> + open_exec_flags.acc_mode);
> + if (err)
> + goto exit;
> + }
>
> err = -EACCES;
> if (!S_ISREG(file_inode(file)->i_mode))
> @@ -769,12 +793,13 @@ 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;
>
> @@ -786,7 +811,7 @@ 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);
>
> @@ -1422,10 +1447,12 @@ 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;
> @@ -1466,7 +1493,7 @@ 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;
> @@ -1474,7 +1501,27 @@ 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 {
> + /*
> + * Build a pathname that reflects how we got to the file,
> + * either "/dev/fd/<fd>" (for an empty filename) or
> + * "/dev/fd/<fd>/<filename>".
> + */
> + pathbuf = kmalloc(PATH_MAX, GFP_TEMPORARY);
> + if (!pathbuf) {
> + retval = -ENOMEM;
> + goto out_unmark;
> + }
> + bprm->filename = pathbuf;
> + if (filename->name[0] == '\0')
> + sprintf(pathbuf, "/dev/fd/%d", fd);
> + else
> + snprintf(pathbuf, PATH_MAX,
> + "/dev/fd/%d/%s", fd, filename->name);
> + }
> + bprm->interp = bprm->filename;
>
> retval = bprm_mm_init(bprm);
> if (retval)
> @@ -1532,6 +1579,7 @@ out_unmark:
>
> out_free:
> free_bprm(bprm);
> + kfree(pathbuf);
>
> out_files:
> if (displaced)
> @@ -1547,7 +1595,18 @@ 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
> @@ -1563,7 +1622,23 @@ 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
>
> @@ -1603,6 +1678,20 @@ 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,
> @@ -1610,4 +1699,17 @@ 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
> diff --git a/fs/namei.c b/fs/namei.c
> index a7b05bf82d31..553c84d3e0cc 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -130,7 +130,7 @@ void final_putname(struct filename *name)
>
> #define EMBEDDED_NAME_MAX (PATH_MAX - sizeof(struct filename))
>
> -static struct filename *
> +struct filename *
> getname_flags(const char __user *filename, int flags, int *empty)
> {
> struct filename *result, *err;
> diff --git a/include/linux/compat.h b/include/linux/compat.h
> index e6494261eaff..7450ca2ac1fc 100644
> --- a/include/linux/compat.h
> +++ b/include/linux/compat.h
> @@ -357,6 +357,9 @@ asmlinkage long compat_sys_lseek(unsigned int, compat_off_t, unsigned int);
>
> asmlinkage long compat_sys_execve(const char __user *filename, const compat_uptr_t __user *argv,
> const compat_uptr_t __user *envp);
> +asmlinkage long compat_sys_execveat(int dfd, const char __user *filename,
> + const compat_uptr_t __user *argv,
> + const compat_uptr_t __user *envp, int flags);
>
> asmlinkage long compat_sys_select(int n, compat_ulong_t __user *inp,
> compat_ulong_t __user *outp, compat_ulong_t __user *exp,
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 94187721ad41..e9818574d738 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2060,6 +2060,7 @@ extern struct file *file_open_root(struct dentry *, struct vfsmount *,
> extern struct file * dentry_open(const struct path *, int, const struct cred *);
> extern int filp_close(struct file *, fl_owner_t id);
>
> +extern struct filename *getname_flags(const char __user *, int, int *);
> extern struct filename *getname(const char __user *);
> extern struct filename *getname_kernel(const char *);
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index b867a4dab38a..33e056da7d33 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2430,6 +2430,10 @@ extern void do_group_exit(int);
> 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);
> extern long do_fork(unsigned long, unsigned long, unsigned long, int __user *, int __user *);
> struct task_struct *fork_idle(int);
> extern pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags);
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 0f86d85a9ce4..df5422294deb 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -876,4 +876,8 @@ asmlinkage long sys_seccomp(unsigned int op, unsigned int flags,
> asmlinkage long sys_getrandom(char __user *buf, size_t count,
> unsigned int flags);
>
> +asmlinkage long sys_execveat(int dfd, const char __user *filename,
> + const char __user *const __user *argv,
> + const char __user *const __user *envp, int flags);
> +
> #endif
> diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index 11d11bc5c78f..feef07d29663 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -705,9 +705,11 @@ __SYSCALL(__NR_seccomp, sys_seccomp)
> __SYSCALL(__NR_getrandom, sys_getrandom)
> #define __NR_memfd_create 279
> __SYSCALL(__NR_memfd_create, sys_memfd_create)
> +#define __NR_execveat 280
> +__SC_COMP(__NR_execveat, sys_execveat, compat_sys_execveat)
>
> #undef __NR_syscalls
> -#define __NR_syscalls 280
> +#define __NR_syscalls 281
>
> /*
> * All syscalls below here should go away really,
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 391d4ddb6f4b..efb06058ad3e 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -218,3 +218,6 @@ cond_syscall(sys_kcmp);
>
> /* operate on Secure Computing state */
> cond_syscall(sys_seccomp);
> +
> +/* execveat */
> +cond_syscall(sys_execveat);
> diff --git a/lib/audit.c b/lib/audit.c
> index 1d726a22565b..b8fb5ee81e26 100644
> --- a/lib/audit.c
> +++ b/lib/audit.c
> @@ -54,6 +54,9 @@ int audit_classify_syscall(int abi, unsigned syscall)
> case __NR_socketcall:
> return 4;
> #endif
> +#ifdef __NR_execveat
> + case __NR_execveat:
> +#endif
> case __NR_execve:
> return 5;
> default:
--
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