Ubuntu Manpage: setfsuid - set user identity used for filesystem checks

NAME

       setfsuid - set user identity used for filesystem checks

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/fsuid.h>

       int setfsuid(uid_t fsuid);

DESCRIPTION

On Linux, a process has both a filesystem user ID and an effective user ID. The (Linux-
specific) filesystem user ID is used for permissions checking when accessing filesystem
objects, while the effective user ID is used for various other kinds of permissions checks
(see credentials(7)).

Normally, the value of the process's filesystem user ID is the same as the value of its
effective user ID. This is so, because whenever a process's effective user ID is changed,
the kernel also changes the filesystem user ID to be the same as the new value of the
effective user ID. A process can cause the value of its filesystem user ID to diverge
from its effective user ID by using setfsuid() to change its filesystem user ID to the
value given in fsuid.

Explicit calls to setfsuid() and setfsgid(2) are (were) usually used only by programs such
as the Linux NFS server that need to change what user and group ID is used for file access
without a corresponding change in the real and effective user and group IDs. A change in
the normal user IDs for a program such as the NFS server is (was) a security hole that can
expose it to unwanted signals. (However, this issue is historical; see below.)

setfsuid() will succeed only if the caller is the superuser or if fsuid matches either the
caller's real user ID, effective user ID, saved set-user-ID, or current filesystem user
ID.

RETURN VALUE

       On  both  success  and  failure,  this call returns the previous filesystem user ID of the
       caller.

VERSIONS

       This system call is present since Linux 1.2.

STANDARDS

       setfsuid() is Linux-specific and should not be used in programs intended to be portable.

NOTES

At the time when this system call was introduced, one process could send a signal to
another process with the same effective user ID. This meant that if a privileged process
changed its effective user ID for the purpose of file permission checking, then it could
become vulnerable to receiving signals sent by another (unprivileged) process with the
same user ID. The filesystem user ID attribute was thus added to allow a process to
change its user ID for the purposes of file permission checking without at the same time
becoming vulnerable to receiving unwanted signals. Since Linux 2.0, signal permission
handling is different (see kill(2)), with the result that a process can change its
effective user ID without being vulnerable to receiving signals from unwanted processes.
Thus, setfsuid() is nowadays unneeded and should be avoided in new applications (likewise
for setfsgid(2)).

The original Linux setfsuid() system call supported only 16-bit user IDs. Subsequently,
Linux 2.4 added setfsuid32() supporting 32-bit IDs. The glibc setfsuid() wrapper function
transparently deals with the variation across kernel versions.

C library/kernel differences
In glibc 2.15 and earlier, when the wrapper for this system call determines that the
argument can't be passed to the kernel without integer truncation (because the kernel is
old and does not support 32-bit user IDs), it will return -1 and set errno to EINVAL
without attempting the system call.

BUGS

       No  error  indications  of  any  kind  are  returned to the caller, and the fact that both
       successful and unsuccessful calls return the same value makes it  impossible  to  directly
       determine  whether  the  call  succeeded  or  failed.   Instead, the caller must resort to
       looking at the return value from a further call such as setfsuid(-1)  (which  will  always
       fail), in order to determine if a preceding call to setfsuid() changed the filesystem user
       ID.  At the very least, EPERM should be returned when the call fails (because  the  caller
       lacks the CAP_SETUID capability).