CVE-2021-47118

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 tasks 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 tasks 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 tasks 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 [ff —truncated—

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/0711f0d7050b9e07c44bc159bbc64ac0a1022c7f
• https://git.kernel.org/stable/c/2cd6eedfa6344f5ef5c3dac3aee57a39b5b46dff
• https://git.kernel.org/stable/c/4dbd8808a591b49b717862e6e0081bcf14a87788
• https://git.kernel.org/stable/c/7178be006d495ffb741c329012da289b62dddfe6
• https://git.kernel.org/stable/c/764c2e892d1fe895392aff62fb353fdce43bb529
• https://git.kernel.org/stable/c/b8ff869f20152fbe66b6c2e2715d26a2f9897cca
• https://git.kernel.org/stable/c/d106f05432e60f9f62d456ef017687f5c73cb414
• https://git.kernel.org/stable/c/f86c80515a8a3703e0ca2e56deb50fc2879c5ea4
• https://git.kernel.org/stable/c/0711f0d7050b9e07c44bc159bbc64ac0a1022c7f
• https://git.kernel.org/stable/c/2cd6eedfa6344f5ef5c3dac3aee57a39b5b46dff
• https://git.kernel.org/stable/c/4dbd8808a591b49b717862e6e0081bcf14a87788
• https://git.kernel.org/stable/c/7178be006d495ffb741c329012da289b62dddfe6
• https://git.kernel.org/stable/c/764c2e892d1fe895392aff62fb353fdce43bb529
• https://git.kernel.org/stable/c/b8ff869f20152fbe66b6c2e2715d26a2f9897cca
• https://git.kernel.org/stable/c/d106f05432e60f9f62d456ef017687f5c73cb414
• https://git.kernel.org/stable/c/f86c80515a8a3703e0ca2e56deb50fc2879c5ea4