[PATCH 3.16 264/305] block: fix use-after-free in sys_ioprio_get()

Ben Hutchings
2016-08-14T11:01:44Z

From: Omar Sandoval <osandov@fb.com>

commit 8ba8682107ee2ca3347354e018865d8e1967c5f4 upstream.

get_task_ioprio() accesses the task->io_context without holding the task
lock and thus can race with exit_io_context(), leading to a
use-after-free. The reproducer below hits this within a few seconds on
my 4-core QEMU VM:

#define _GNU_SOURCE
#include <assert.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <sys/wait.h>

int main(int argc, char **argv)
{
pid_t pid, child;
long nproc, i;

/* ioprio_set(IOPRIO_WHO_PROCESS, 0, IOPRIO_PRIO_VALUE(IOPRIO_CLASS_IDLE, 0)); */
syscall(SYS_ioprio_set, 1, 0, 0x6000);

nproc = sysconf(_SC_NPROCESSORS_ONLN);

for (i = 0; i < nproc; i++) {
pid = fork();
assert(pid != -1);
if (pid == 0) {
for (;;) {
pid = fork();
assert(pid != -1);
if (pid == 0) {
_exit(0);
} else {
child = wait(NULL);
assert(child == pid);
}
}
}

pid = fork();
assert(pid != -1);
if (pid == 0) {
for (;;) {
/* ioprio_get(IOPRIO_WHO_PGRP, 0); */
syscall(SYS_ioprio_get, 2, 0);
}
}
}

for (;;) {
/* ioprio_get(IOPRIO_WHO_PGRP, 0); */
syscall(SYS_ioprio_get, 2, 0);
}

return 0;
}

This gets us KASAN dumps like this:

[ 35.526914] ==================================================================
[ 35.530009] BUG: KASAN: out-of-bounds in get_task_ioprio+0x7b/0x90 at addr ffff880066f34e6c
[ 35.530009] Read of size 2 by task ioprio-gpf/363
[ 35.530009] =============================================================================
[ 35.530009] BUG blkdev_ioc (Not tainted): kasan: bad access detected
[ 35.530009] -----------------------------------------------------------------------------

[ 35.530009] Disabling lock debugging due to kernel taint
[ 35.530009] INFO: Allocated in create_task_io_context+0x2b/0x370 age=0 cpu=0 pid=360
[ 35.530009] ___slab_alloc+0x55d/0x5a0
[ 35.530009] __slab_alloc.isra.20+0x2b/0x40
[ 35.530009] kmem_cache_alloc_node+0x84/0x200
[ 35.530009] create_task_io_context+0x2b/0x370
[ 35.530009] get_task_io_context+0x92/0xb0
[ 35.530009] copy_process.part.8+0x5029/0x5660
[ 35.530009] _do_fork+0x155/0x7e0
[ 35.530009] SyS_clone+0x19/0x20
[ 35.530009] do_syscall_64+0x195/0x3a0
[ 35.530009] return_from_SYSCALL_64+0x0/0x6a
[ 35.530009] INFO: Freed in put_io_context+0xe7/0x120 age=0 cpu=0 pid=1060
[ 35.530009] __slab_free+0x27b/0x3d0
[ 35.530009] kmem_cache_free+0x1fb/0x220
[ 35.530009] put_io_context+0xe7/0x120
[ 35.530009] put_io_context_active+0x238/0x380
[ 35.530009] exit_io_context+0x66/0x80
[ 35.530009] do_exit+0x158e/0x2b90
[ 35.530009] do_group_exit+0xe5/0x2b0
[ 35.530009] SyS_exit_group+0x1d/0x20
[ 35.530009] entry_SYSCALL_64_fastpath+0x1a/0xa4
[ 35.530009] INFO: Slab 0xffffea00019bcd00 objects=20 used=4 fp=0xffff880066f34ff0 flags=0x1fffe0000004080
[ 35.530009] INFO: Object 0xffff880066f34e58 @offset=3672 fp=0x0000000000000001
[ 35.530009] ==================================================================

Fix it by grabbing the task lock while we poke at the io_context.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
block/ioprio.c | 2 ++
1 file changed, 2 insertions(+)

--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -149,8 +149,10 @@ static int get_task_ioprio(struct task_s
if (ret)
goto out;
ret = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_NONE, IOPRIO_NORM);
+ task_lock(p);
if (p->io_context)
ret = p->io_context->ioprio;
+ task_unlock(p);
out:
return ret;
}