Comment 0 for bug 1895132

SRU Justification

Impact: On kernels prior to 5.8 when a task is in traced state (due to audit, ptrace, or seccomp) s390x and a syscall is issued that the kernel doesn't know about s390x will not return ENOSYS in r2 but instead will return the syscall number. This breaks userspace all over the place. The following program compiled on s390x will output 500 instead of -ENOSYS:

root@test:~# cat test.c
#define _GNU_SOURCE
#include <libgen.h>
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

static inline int dummy_inline_asm(void)
{
        register long r1 asm("r1") = 500;
        register long r2 asm("r2") = -1;
        register long r3 asm("r3") = -1;
        register long r4 asm("r4") = -1;
        register long r5 asm("r5") = -1;
        register long __res_r2 asm("r2");
        asm volatile(
            "svc 0\n\t"
             : "=d"(__res_r2)
             : "d"(r1), "0"(r2), "d"(r3), "d"(r4), "d"(r5)
             : "memory");
        return (int) __res_r2;
}

static inline int dummy_syscall(void)
{
        return syscall(500, -1, -1, -1, -1);
}

int main(int argc, char *argv[])
{
        printf("Uhm: %d\n", dummy_inline_asm());
        printf("Uhm: %d\n", dummy_syscall());

        exit(EXIT_SUCCESS);
}

Fix: Backport
commit cd29fa798001075a554b978df3a64e6656c25794
Author: Sven Schnelle <email address hidden>
Date: Fri Mar 6 13:18:31 2020 +0100

    s390/ptrace: return -ENOSYS when invalid syscall is supplied

    The current code returns the syscall number which an invalid
    syscall number is supplied and tracing is enabled. This makes
    the strace testsuite fail.

    Signed-off-by: Sven Schnelle <email address hidden>
    Signed-off-by: Vasily Gorbik <email address hidden>

which got released with 5.8. The commit missed to Cc stable and although I've asked Sven to include it in stable I'm not sure when or if it will show up there.

Regression Potential: Limited to s390x.

Test Case: The reproducer given above needs to output -ENOSYS instead of 500.