linux-kernel - [PATCH 0/1] *** Fix kill(-1,s) returning 0 on 0 kills ***

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Date:   Tue, 22 Nov 2022 17:12:40 +0100
From:   Petr Skocik <pskocik@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Oleg Nesterov <oleg@...hat.com>, Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>, linux-kernel@...r.kernel.org,
        Petr Skocik <pskocik@...il.com>
Subject: [PATCH 0/1] *** Fix kill(-1,s) returning 0 on 0 kills ***

Hi. I've never sent a kernel patch before but this one seemed trivial,
so I thought I'd give it a shot.

My issue: kill(-1,s) on Linux doesn't return -ESCHR when it has nothing
to kill.

The code sample below demonstrates the problem, which gets fixed by the
patch:

    #define _GNU_SOURCE
    #include <assert.h>
    #include <errno.h>
    #include <signal.h>
    #include <stdio.h>
    #include <sys/wait.h>
    #include <unistd.h>
    #define VICTIM_UID 4200 //check these are safe to use on your system!
    #define UNUSED_UID 4300
    int main(){
        uid_t r,e,s;
        if(geteuid()) return 1; //requires root privileges

        //pipe to let the parent know when the child has changed ids
        int fds[2]; if(0>pipe(fds)) return 1;
        pid_t pid;
        if(0>(pid=fork())) return 1;
        else if(0==pid){
            setreuid(VICTIM_UID,VICTIM_UID);
            getresuid(&r,&e,&s); printf("child: %u %u %u\n", r,e,s);
            close(fds[0]); close(fds[1]); //let the parent continue
            for(;;) pause();
        }
        close(fds[1]);
        read(fds[0],&(char){0},1); //wait for uid change in the child

        #if 1
        setreuid(VICTIM_UID,(uid_t)-1); seteuid(VICTIM_UID);
        #else
        setresuid(UNUSED_UID,VICTIM_UID,0);
        #endif
        getresuid(&r,&e,&s); printf("parent: %u %u %u\n", r,e,s); //4200 4200 0

        int err = kill(-1,-111); (void)err; //test -EINVAL
        assert(err < 0 &&  errno == EINVAL);

        int rc = kill(-1,SIGTERM); //test 0
        if(rc>=0) wait(0);
        int rc2 = kill(-1,SIGTERM); //test -ESCHR
        printf("1st kill ok==%d; 2nd kill ESRCH==%d\n", rc==0, rc2<0&& errno==ESRCH);
    }

Thank you for considering the patch.

Best regards,
Petr S.


Petr Skocik (1):
  Fix kill(-1,s) returning 0 on 0 kills

 kernel/signal.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

-- 
2.25.1