Re: [PATCH] x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task

[Jann Horn]

On Fri, Aug 14, 2020 at 9:03 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > On Aug 14, 2020, at 11:16 AM, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > syzbot found its way in 86_fsgsbase_read_task() [1]
> >
> > Fix is to make sure ldt pointer is not NULL
>
> Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>
>
> Maybe add something like this to the changelog:
>
> This can happen if ptrace() or sigreturn() pokes an LDT selector into FS or GS for a task with no LDT and something tries to read the base before a return to usermode notices the bad selector and fixes it.
>
> I’ll see if I can whip up a test case too.

This is the reproducer I used to test this on 4.20.17:

#include <stdio.h>
#include <unistd.h>
#include <err.h>
#include <sys/wait.h>
#include <sys/ptrace.h>
#include <sys/user.h>
#include <sys/prctl.h>
#include <signal.h>
#include <stddef.h>
#include <errno.h>

#define SEL_LDT 0x4
#define USER_RPL 0x3
#define LDT_SELECTOR(idx) (((idx)<<3) | SEL_LDT | USER_RPL)

int main(void) {
  pid_t child = fork();
  if (child == -1) err(1, "fork");
  if (child == 0) {
    prctl(PR_SET_PDEATHSIG, SIGKILL);
    while (1) pause();
  }
  if (ptrace(PTRACE_ATTACH, child, NULL, NULL)) err(1, "PTRACE_ATTACH");
  int status;
  if (waitpid(child, &status, __WALL) != child) err(1, "waitpid");
  if (ptrace( PTRACE_POKEUSER, child, (void*)offsetof(struct
user_regs_struct, fs), (void*)LDT_SELECTOR(0) ))
    err(1, "PTRACE_POKEUSER");
  errno = 0;
  unsigned long val = ptrace( PTRACE_PEEKUSER, child,
(void*)offsetof(struct user_regs_struct, fs_base), NULL );
  printf("PTRACE_PEEKUSER returns user_regs_struct.fs_base = 0x%lx
(%m)\n", val);
}