Messages in this thread | | | Date | Tue, 25 May 2021 12:19:45 +0200 | From | Christian Brauner <> | Subject | Re: [PATCH] pid: take a reference when initializing `cad_pid` |
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On Mon, May 24, 2021 at 06:22:30PM +0100, Mark Rutland wrote: > 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: > > 9ec52099e4b8678a ("[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: > dump_backtrace+0x0/0x4a8 arch/arm64/kernel/stacktrace.c:105 > show_stack+0x34/0x48 arch/arm64/kernel/stacktrace.c:191 > __dump_stack lib/dump_stack.c:79 [inline] > dump_stack+0x1d4/0x2a0 lib/dump_stack.c:120 > print_address_description.constprop.11+0x60/0x3a8 mm/kasan/report.c:232 > __kasan_report mm/kasan/report.c:399 [inline] > kasan_report+0x1e8/0x200 mm/kasan/report.c:416 > __asan_report_load4_noabort+0x30/0x48 mm/kasan/report_generic.c:308 > 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: > kasan_save_stack+0x28/0x58 mm/kasan/common.c:38 > kasan_set_track mm/kasan/common.c:46 [inline] > set_alloc_info mm/kasan/common.c:427 [inline] > __kasan_slab_alloc+0x88/0xa8 mm/kasan/common.c:460 > kasan_slab_alloc include/linux/kasan.h:223 [inline] > 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: > kasan_save_stack+0x28/0x58 mm/kasan/common.c:38 > kasan_set_track+0x28/0x40 mm/kasan/common.c:46 > kasan_set_free_info+0x28/0x50 mm/kasan/generic.c:357 > ____kasan_slab_free mm/kasan/common.c:360 [inline] > ____kasan_slab_free mm/kasan/common.c:325 [inline] > __kasan_slab_free+0xf4/0x148 mm/kasan/common.c:367 > kasan_slab_free include/linux/kasan.h:199 [inline] > 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 > ================================================================== > > Fixes: 9ec52099e4b8678a ("[PATCH] replace cad_pid by a struct pid") > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Cedric Le Goater <clg@fr.ibm.com> > Cc: Christian Brauner <christian@brauner.io> > Cc: Eric W. Biederman <ebiederm@xmission.com> > Cc: Kees Cook <keescook@chromium.org > Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> > Cc: Paul Mackerras <paulus@samba.org> > Cc: stable@kernel.org > ---
Andrew's already picked this up (I'm down with a proper regular case of a cold right on time for summer.) but fwiw: Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
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