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

From: David Drysdale
Date: Thu Oct 23 2014 - 02:41:15 EST

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On Wed, Oct 22, 2014 at 7:07 PM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> 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.

Just using do_filp_open() throughout looks mostly plausible on a quick
experiment, but my initial version appears to make O_PATH fds unexpectedly
fexecve()-able (I'm glad I had a test case for that).

I'll look for a way around that, hopefully without an explicit special case.

>> + 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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