mm/mempolicy: fix uninit-value in mpol_rebind_policy()

In the Linux kernel, the following vulnerability has been resolved:

mm/mempolicy: fix uninit-value in mpol_rebind_policy()

mpol_set_nodemask()(mm/mempolicy.c) does not set up nodemask when

pol->mode is MPOL_LOCAL. Check pol->mode before access

pol->w.cpuset_mems_allowed in mpol_rebind_policy()(mm/mempolicy.c).

BUG: KMSAN: uninit-value in mpol_rebind_policy mm/mempolicy.c:352 [inline]

BUG: KMSAN: uninit-value in mpol_rebind_task+0x2ac/0x2c0 mm/mempolicy.c:368

mpol_rebind_policy mm/mempolicy.c:352 [inline]

mpol_rebind_task+0x2ac/0x2c0 mm/mempolicy.c:368

cpuset_change_task_nodemask kernel/cgroup/cpuset.c:1711 [inline]

cpuset_attach+0x787/0x15e0 kernel/cgroup/cpuset.c:2278

cgroup_migrate_execute+0x1023/0x1d20 kernel/cgroup/cgroup.c:2515

cgroup_migrate kernel/cgroup/cgroup.c:2771 [inline]

cgroup_attach_task+0x540/0x8b0 kernel/cgroup/cgroup.c:2804

__cgroup1_procs_write+0x5cc/0x7a0 kernel/cgroup/cgroup-v1.c:520

cgroup1_tasks_write+0x94/0xb0 kernel/cgroup/cgroup-v1.c:539

cgroup_file_write+0x4c2/0x9e0 kernel/cgroup/cgroup.c:3852

kernfs_fop_write_iter+0x66a/0x9f0 fs/kernfs/file.c:296

call_write_iter include/linux/fs.h:2162 [inline]

new_sync_write fs/read_write.c:503 [inline]

vfs_write+0x1318/0x2030 fs/read_write.c:590

ksys_write+0x28b/0x510 fs/read_write.c:643

__do_sys_write fs/read_write.c:655 [inline]

__se_sys_write fs/read_write.c:652 [inline]

__x64_sys_write+0xdb/0x120 fs/read_write.c:652

do_syscall_x64 arch/x86/entry/common.c:51 [inline]

do_syscall_64+0x54/0xd0 arch/x86/entry/common.c:82

entry_SYSCALL_64_after_hwframe+0x44/0xae

Uninit was created at:

slab_post_alloc_hook mm/slab.h:524 [inline]

slab_alloc_node mm/slub.c:3251 [inline]

slab_alloc mm/slub.c:3259 [inline]

kmem_cache_alloc+0x902/0x11c0 mm/slub.c:3264

mpol_new mm/mempolicy.c:293 [inline]

do_set_mempolicy+0x421/0xb70 mm/mempolicy.c:853

kernel_set_mempolicy mm/mempolicy.c:1504 [inline]

__do_sys_set_mempolicy mm/mempolicy.c:1510 [inline]

__se_sys_set_mempolicy+0x44c/0xb60 mm/mempolicy.c:1507

__x64_sys_set_mempolicy+0xd8/0x110 mm/mempolicy.c:1507

do_syscall_x64 arch/x86/entry/common.c:51 [inline]

do_syscall_64+0x54/0xd0 arch/x86/entry/common.c:82

entry_SYSCALL_64_after_hwframe+0x44/0xae

KMSAN: uninit-value in mpol_rebind_task (2)

https://syzkaller.appspot.com/bug?id=d6eb90f952c2a5de9ea718a1b873c55cb13b59dc

This patch seems to fix below bug too.

KMSAN: uninit-value in mpol_rebind_mm (2)

https://syzkaller.appspot.com/bug?id=f2fecd0d7013f54ec4162f60743a2b28df40926b

The uninit-value is pol->w.cpuset_mems_allowed in mpol_rebind_policy().

When syzkaller reproducer runs to the beginning of mpol_new(),

mpol_new() mm/mempolicy.c

do_mbind() mm/mempolicy.c

kernel_mbind() mm/mempolicy.c

`mode` is 1(MPOL_PREFERRED), nodes_empty(*nodes) is `true` and `flags`

is 0. Then

mode = MPOL_LOCAL;

...

policy->mode = mode;

policy->flags = flags;

will be executed. So in mpol_set_nodemask(),

mpol_set_nodemask() mm/mempolicy.c

do_mbind()

kernel_mbind()

pol->mode is 4 (MPOL_LOCAL), that `nodemask` in `pol` is not initialized,

which will be accessed in mpol_rebind_policy().