CVE-2021-47118: pid: take a reference when initializing `cad_pid`

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

Description
===========

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

pid: take a reference when initializing `cad_pid`

During boot, kernel_init_freeable() initializes `cad_pid` to the init
task's struct pid.  Later on, we may change `cad_pid` via a sysctl, and
when this happens proc_do_cad_pid() will increment the refcount on the
new pid via get_pid(), and will decrement the refcount on the old pid
via put_pid().  As we never called get_pid() when we initialized
`cad_pid`, we decrement a reference we never incremented, can therefore
free the init task's struct pid early.  As there can be dangling
references to the struct pid, we can later encounter a use-after-free
(e.g.  when delivering signals).

This was spotted when fuzzing v5.13-rc3 with Syzkaller, but seems to
have been around since the conversion of `cad_pid` to struct pid in
commit 9ec52099e4b8 ("[PATCH] replace cad_pid by a struct pid") from the
pre-KASAN stone age of v2.6.19.

Fix this by getting a reference to the init task's struct pid when we
assign it to `cad_pid`.

Full KASAN splat below.

   ==================================================================
   BUG: KASAN: use-after-free in ns_of_pid include/linux/pid.h:153 [inline]
   BUG: KASAN: use-after-free in task_active_pid_ns+0xc0/0xc8 kernel/pid.c:509
   Read of size 4 at addr ffff23794dda0004 by task syz-executor.0/273

   CPU: 1 PID: 273 Comm: syz-executor.0 Not tainted 5.12.0-00001-g9aef892b2d15 #1
   Hardware name: linux,dummy-virt (DT)
   Call trace:
    ns_of_pid include/linux/pid.h:153 [inline]
    task_active_pid_ns+0xc0/0xc8 kernel/pid.c:509
    do_notify_parent+0x308/0xe60 kernel/signal.c:1950
    exit_notify kernel/exit.c:682 [inline]
    do_exit+0x2334/0x2bd0 kernel/exit.c:845
    do_group_exit+0x108/0x2c8 kernel/exit.c:922
    get_signal+0x4e4/0x2a88 kernel/signal.c:2781
    do_signal arch/arm64/kernel/signal.c:882 [inline]
    do_notify_resume+0x300/0x970 arch/arm64/kernel/signal.c:936
    work_pending+0xc/0x2dc

   Allocated by task 0:
    slab_post_alloc_hook+0x50/0x5c0 mm/slab.h:516
    slab_alloc_node mm/slub.c:2907 [inline]
    slab_alloc mm/slub.c:2915 [inline]
    kmem_cache_alloc+0x1f4/0x4c0 mm/slub.c:2920
    alloc_pid+0xdc/0xc00 kernel/pid.c:180
    copy_process+0x2794/0x5e18 kernel/fork.c:2129
    kernel_clone+0x194/0x13c8 kernel/fork.c:2500
    kernel_thread+0xd4/0x110 kernel/fork.c:2552
    rest_init+0x44/0x4a0 init/main.c:687
    arch_call_rest_init+0x1c/0x28
    start_kernel+0x520/0x554 init/main.c:1064
    0x0

   Freed by task 270:
    slab_free_hook mm/slub.c:1562 [inline]
    slab_free_freelist_hook+0x98/0x260 mm/slub.c:1600
    slab_free mm/slub.c:3161 [inline]
    kmem_cache_free+0x224/0x8e0 mm/slub.c:3177
    put_pid.part.4+0xe0/0x1a8 kernel/pid.c:114
    put_pid+0x30/0x48 kernel/pid.c:109
    proc_do_cad_pid+0x190/0x1b0 kernel/sysctl.c:1401
    proc_sys_call_handler+0x338/0x4b0 fs/proc/proc_sysctl.c:591
    proc_sys_write+0x34/0x48 fs/proc/proc_sysctl.c:617
    call_write_iter include/linux/fs.h:1977 [inline]
    new_sync_write+0x3ac/0x510 fs/read_write.c:518
    vfs_write fs/read_write.c:605 [inline]
    vfs_write+0x9c4/0x1018 fs/read_write.c:585
    ksys_write+0x124/0x240 fs/read_write.c:658
    __do_sys_write fs/read_write.c:670 [inline]
    __se_sys_write fs/read_write.c:667 [inline]
    __arm64_sys_write+0x78/0xb0 fs/read_write.c:667
    __invoke_syscall arch/arm64/kernel/syscall.c:37 [inline]
    invoke_syscall arch/arm64/kernel/syscall.c:49 [inline]
    el0_svc_common.constprop.1+0x16c/0x388 arch/arm64/kernel/syscall.c:129
    do_el0_svc+0xf8/0x150 arch/arm64/kernel/syscall.c:168
    el0_svc+0x28/0x38 arch/arm64/kernel/entry-common.c:416
    el0_sync_handler+0x134/0x180 arch/arm64/kernel/entry-common.c:432
    el0_sync+0x154/0x180 arch/arm64/kernel/entry.S:701

   The buggy address belongs to the object at ffff23794dda0000
    which belongs to the cache pid of size 224
   The buggy address is located 4 bytes inside of
    224-byte region [ffff23794dda0000, ffff23794dda00e0)
   The buggy address belongs to the page:
   page:(____ptrval____) refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4dda0
   head:(____ptrval____) order:1 compound_mapcount:0
   flags: 0x3fffc0000010200(slab|head)
   raw: 03fffc0000010200 dead000000000100 dead000000000122 ffff23794d40d080
   raw: 0000000000000000 0000000000190019 00000001ffffffff 0000000000000000
   page dumped because: kasan: bad access detected

   Memory state around the buggy address:
    ffff23794dd9ff00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
    ffff23794dd9ff80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
   >ffff23794dda0000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                      ^
    ffff23794dda0080: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
    ffff23794dda0100: fc fc fc fc fc fc fc fc 00 00 00 00 00 00 00 00
   ==================================================================

The Linux kernel CVE team has assigned CVE-2021-47118 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 2.6.19 with commit 9ec52099e4b8678a and fixed in 4.4.272 with commit 764c2e892d1f
	Issue introduced in 2.6.19 with commit 9ec52099e4b8678a and fixed in 4.9.272 with commit f86c80515a8a
	Issue introduced in 2.6.19 with commit 9ec52099e4b8678a and fixed in 4.14.236 with commit 4dbd8808a591
	Issue introduced in 2.6.19 with commit 9ec52099e4b8678a and fixed in 4.19.194 with commit d106f05432e6
	Issue introduced in 2.6.19 with commit 9ec52099e4b8678a and fixed in 5.4.125 with commit 2cd6eedfa634
	Issue introduced in 2.6.19 with commit 9ec52099e4b8678a and fixed in 5.10.43 with commit 7178be006d49
	Issue introduced in 2.6.19 with commit 9ec52099e4b8678a and fixed in 5.12.10 with commit b8ff869f2015
	Issue introduced in 2.6.19 with commit 9ec52099e4b8678a and fixed in 5.13 with commit 0711f0d7050b

Please see https://www.kernel.org or a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-47118
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	init/main.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/764c2e892d1fe895392aff62fb353fdce43bb529
	https://git.kernel.org/stable/c/f86c80515a8a3703e0ca2e56deb50fc2879c5ea4
	https://git.kernel.org/stable/c/4dbd8808a591b49b717862e6e0081bcf14a87788
	https://git.kernel.org/stable/c/d106f05432e60f9f62d456ef017687f5c73cb414
	https://git.kernel.org/stable/c/2cd6eedfa6344f5ef5c3dac3aee57a39b5b46dff
	https://git.kernel.org/stable/c/7178be006d495ffb741c329012da289b62dddfe6
	https://git.kernel.org/stable/c/b8ff869f20152fbe66b6c2e2715d26a2f9897cca
	https://git.kernel.org/stable/c/0711f0d7050b9e07c44bc159bbc64ac0a1022c7f