block: GPF in get_task_ioprio

[Dmitry Vyukov]

Hello,

The following program triggers GPF in get_task_ioprio if run in a parallel loop:

// autogenerated by syzkaller (http://github.com/google/syzkaller)
#include <stdint.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>

int main()
{
  syscall(SYS_ioprio_set, 0x2ul, 0x0ul, 0x7ffful, 0, 0, 0);
  syscall(SYS_ioprio_get, 0x3ul, 0x0ul, 0, 0, 0, 0);
  return 0;
}

This patch also seems to help to trigger it:

 int set_task_ioprio(struct task_struct *task, int ioprio)
 {
@@ -150,8 +151,10 @@ static int get_task_ioprio(struct task_struct *p)
        if (ret)
                goto out;
        ret = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_NONE, IOPRIO_NORM);
-       if (p->io_context)
+       if (p->io_context) {
+               ndelay(10000);
                ret = p->io_context->ioprio;
+       }
 out:
        return ret;
 }

kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN
Modules linked in:
CPU: 2 PID: 8978 Comm: a.out Not tainted 4.7.0-rc5+ #22
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff8800612397c0 ti: ffff880062d08000 task.ti: ffff880062d08000
RIP: 0010:[<ffffffff82c7fa2a>]
  [<ffffffff82c7fa2a>] get_task_ioprio+0x9a/0xe0 block/ioprio.c:156
RSP: 0018:ffff880062d0feb8  EFLAGS: 00010206
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000007605
RDX: 0000000000000009 RSI: 0000002004a58c71 RDI: 0000000000000048
RBP: ffff880062d0fed0 R08: 0000000000000002 R09: 0000000000000002
R10: 0000000000000000 R11: 0000000000000001 R12: ffff8800647817c0
R13: ffff8800647827b0 R14: ffffffff82c80ac2 R15: dffffc0000000000
FS:  0000000001127880(0000) GS:ffff88006d400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004b20c0 CR3: 0000000061894000 CR4: 00000000000006e0
Stack:
 ffff8800647817c0 0000000000000000 ffffffff8813a660 ffff880062d0ff48
 ffffffff82c80ec5 ffffffff82c80ac2 ffffffff880f8240 0000000362d0ff48
 0000000000000000 000040037fff4003 fffffbfff101f048 ffffffff880f81e0
Call Trace:
 [<     inline     >] SYSC_ioprio_get block/ioprio.c:230
 [<ffffffff82c80ec5>] SyS_ioprio_get+0x795/0x9c0 block/ioprio.c:182
 [<ffffffff86a9b380>] entry_SYSCALL_64_fastpath+0x23/0xc1
arch/x86/entry/entry_64.S:207
Code: 00 fc ff df 48 c1 ea 03 80 3c 02 00 75 4f 49 8b 9c 24 f0 0f 00
00 48 b8 00 00 00 00 00 fc ff df 48 8d 7b 48 48 89 fa 48 c1 ea 03 <0f>
b6 04 02 84 c0 74 04 3c 01 7e 12 0f b7 5b 48 e8 51 6f 91 fe
RIP  [<ffffffff82c7fa2a>] get_task_ioprio+0x9a/0xe0 block/ioprio.c:156
 RSP <ffff880062d0feb8>
---[ end trace 8b400ca760ff21a5 ]---

On commit 00bf377d19ad3d80cbc7a036521279a86e397bfb (Jun 29).