Date:   Thu, 27 Oct 2022 19:55:08 +0800
From:   songyuanzheng <songyuanzheng@...wei.com>
To:     Hugh Dickins <hughd@...gle.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH STABLE 5.10] mm/memory: add non-anonymous page check in
 the copy_present_page()

Hi, Hugh,

On 2022/10/27 5:51, Hugh Dickins wrote:
> On Mon, 24 Oct 2022, Yuanzheng Song wrote:
> 
>> The vma->anon_vma of the child process may be NULL because
>> the entire vma does not contain anonymous pages. In this
>> case, a BUG will occur when the copy_present_page() passes
>> a copy of a non-anonymous page of that vma to the
>> page_add_new_anon_rmap() to set up new anonymous rmap.
>>
>> ------------[ cut here ]------------
>> kernel BUG at mm/rmap.c:1044!
>> Internal error: Oops - BUG: 0 [#1] SMP
>> Modules linked in:
>> CPU: 2 PID: 3617 Comm: test Not tainted 5.10.149 #1
>> Hardware name: linux,dummy-virt (DT)
>> pstate: 80000005 (Nzcv daif -PAN -UAO -TCO BTYPE=--)
>> pc : __page_set_anon_rmap+0xbc/0xf8
>> lr : __page_set_anon_rmap+0xbc/0xf8
>> sp : ffff800014c1b870
>> x29: ffff800014c1b870 x28: 0000000000000001
>> x27: 0000000010100073 x26: ffff1d65c517baa8
>> x25: ffff1d65cab0f000 x24: ffff1d65c416d800
>> x23: ffff1d65cab5f248 x22: 0000000020000000
>> x21: 0000000000000001 x20: 0000000000000000
>> x19: fffffe75970023c0 x18: 0000000000000000
>> x17: 0000000000000000 x16: 0000000000000000
>> x15: 0000000000000000 x14: 0000000000000000
>> x13: 0000000000000000 x12: 0000000000000000
>> x11: 0000000000000000 x10: 0000000000000000
>> x9 : ffffc3096d5fb858 x8 : 0000000000000000
>> x7 : 0000000000000011 x6 : ffff5a5c9089c000
>> x5 : 0000000000020000 x4 : ffff5a5c9089c000
>> x3 : ffffc3096d200000 x2 : ffffc3096e8d0000
>> x1 : ffff1d65ca3da740 x0 : 0000000000000000
>> Call trace:
>>   __page_set_anon_rmap+0xbc/0xf8
>>   page_add_new_anon_rmap+0x1e0/0x390
>>   copy_pte_range+0xd00/0x1248
>>   copy_page_range+0x39c/0x620
>>   dup_mmap+0x2e0/0x5a8
>>   dup_mm+0x78/0x140
>>   copy_process+0x918/0x1a20
>>   kernel_clone+0xac/0x638
>>   __do_sys_clone+0x78/0xb0
>>   __arm64_sys_clone+0x30/0x40
>>   el0_svc_common.constprop.0+0xb0/0x308
>>   do_el0_svc+0x48/0xb8
>>   el0_svc+0x24/0x38
>>   el0_sync_handler+0x160/0x168
>>   el0_sync+0x180/0x1c0
>> Code: 97f8ff85 f9400294 17ffffeb 97f8ff82 (d4210000)
>> ---[ end trace a972347688dc9bd4 ]---
>> Kernel panic - not syncing: Oops - BUG: Fatal exception
>> SMP: stopping secondary CPUs
>> Kernel Offset: 0x43095d200000 from 0xffff800010000000
>> PHYS_OFFSET: 0xffffe29a80000000
>> CPU features: 0x08200022,61806082
>> Memory Limit: none
>> ---[ end Kernel panic - not syncing: Oops - BUG: Fatal exception ]---
>>
>> This problem has been fixed by the fb3d824d1a46
>> ("mm/rmap: split page_dup_rmap() into page_dup_file_rmap() and page_try_dup_anon_rmap()"),
>> but still exists in the linux-5.10.y branch.
>>
>> This patch is not applicable to this version because
>> of the large version differences. Therefore, fix it by
>> adding non-anonymous page check in the copy_present_page().
>>
>> Fixes: 70e806e4e645 ("mm: Do early cow for pinned pages during fork() for ptes")
>> Signed-off-by: Yuanzheng Song <songyuanzheng@...wei.com>
> 
> It's a good point, but this patch should not go into any stable release
> without an explicit Ack from either Peter Xu or David Hildenbrand.
> 
> To my eye, it's simply avoiding the issue, rather than fixing
> it properly; and even if the issue is so rare, and fixing properly
> too difficult or inefficent (a cached anon_vma?), that a workaround
> is good enough, it still looks like the wrong workaround (checking
> dst_vma->anon_vma instead of PageAnon seems more to the point, and
> less lenient).
For an anonymous page that belongs to a vma, the vma->anon_vma is
definitely not NULL, which is why I use PageAnon() instead of directly
judging that dst_vma->anon_vma is not NULL.

You are right. Maybe I didn't really solve the problem. Instead, I
simply avoided the issue. You can use the following test case to
reproduce the problem and find a better solution to the problem.
If you find the right solution, please inform me. Thanks.

---------------------------------------------------------------
#define _GNU_SOURCE

#include <dirent.h>
#include <endian.h>
#include <errno.h>
#include <fcntl.h>
#include <pthread.h>
#include <signal.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <time.h>
#include <unistd.h>

#include <linux/futex.h>

#ifndef __NR_clone
#define __NR_clone 220
#endif
#ifndef __NR_exit
#define __NR_exit 93
#endif
#ifndef __NR_gettid
#define __NR_gettid 178
#endif
#ifndef __NR_mmap
#define __NR_mmap 222
#endif
#ifndef __NR_process_vm_readv
#define __NR_process_vm_readv 270
#endif
#ifndef __NR_setsockopt
#define __NR_setsockopt 208
#endif
#ifndef __NR_socket
#define __NR_socket 198
#endif

static unsigned long long procid;

static void sleep_ms(uint64_t ms)
{
	usleep(ms * 1000);
}

static uint64_t current_time_ms(void)
{
	struct timespec ts;
	if (clock_gettime(CLOCK_MONOTONIC, &ts))
	exit(1);
	return (uint64_t)ts.tv_sec * 1000 + (uint64_t)ts.tv_nsec / 1000000;
}

static void thread_start(void* (*fn)(void*), void* arg)
{
	pthread_t th;
	pthread_attr_t attr;
	pthread_attr_init(&attr);
	pthread_attr_setstacksize(&attr, 128 << 10);
	int i = 0;
	for (; i < 100; i++) {
		if (pthread_create(&th, &attr, fn, arg) == 0) {
			pthread_attr_destroy(&attr);
			return;
		}
		if (errno == EAGAIN) {
			usleep(50);
			continue;
		}
		break;
	}
	exit(1);
}

typedef struct {
	int state;
} event_t;

static void event_init(event_t* ev)
{
	ev->state = 0;
}

static void event_reset(event_t* ev)
{
	ev->state = 0;
}

static void event_set(event_t* ev)
{
	if (ev->state)
	exit(1);
	__atomic_store_n(&ev->state, 1, __ATOMIC_RELEASE);
	syscall(SYS_futex, &ev->state, FUTEX_WAKE | FUTEX_PRIVATE_FLAG, 1000000);
}

static void event_wait(event_t* ev)
{
	while (!__atomic_load_n(&ev->state, __ATOMIC_ACQUIRE))
		syscall(SYS_futex, &ev->state, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0, 0);
}

static int event_isset(event_t* ev)
{
	return __atomic_load_n(&ev->state, __ATOMIC_ACQUIRE);
}

static int event_timedwait(event_t* ev, uint64_t timeout)
{
	uint64_t start = current_time_ms();
	uint64_t now = start;
	for (;;) {
		uint64_t remain = timeout - (now - start);
		struct timespec ts;
		ts.tv_sec = remain / 1000;
		ts.tv_nsec = (remain % 1000) * 1000 * 1000;
		syscall(SYS_futex, &ev->state, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0, &ts);
		if (__atomic_load_n(&ev->state, __ATOMIC_ACQUIRE))
			return 1;
		now = current_time_ms();
		if (now - start > timeout)
			return 0;
	}
}

static bool write_file(const char* file, const char* what, ...)
{
	char buf[1024];
	va_list args;
	va_start(args, what);
	vsnprintf(buf, sizeof(buf), what, args);
	va_end(args);
	buf[sizeof(buf) - 1] = 0;
	int len = strlen(buf);
	int fd = open(file, O_WRONLY | O_CLOEXEC);
	if (fd == -1)
		return false;
	if (write(fd, buf, len) != len) {
		int err = errno;
		close(fd);
		errno = err;
		return false;
	}
	close(fd);
	return true;
}

static void kill_and_wait(int pid, int* status)
{
	kill(-pid, SIGKILL);
	kill(pid, SIGKILL);
	for (int i = 0; i < 100; i++) {
		if (waitpid(-1, status, WNOHANG | __WALL) == pid)
			return;
		usleep(1000);
	}
	DIR* dir = opendir("/sys/fs/fuse/connections");
	if (dir) {
		for (;;) {
			struct dirent* ent = readdir(dir);
			if (!ent)
				break;
			if (strcmp(ent->d_name, ".") == 0 || strcmp(ent->d_name, "..") == 0)
				continue;
			char abort[300];
			snprintf(abort, sizeof(abort), "/sys/fs/fuse/connections/%s/abort", 
ent->d_name);
			int fd = open(abort, O_WRONLY);
			if (fd == -1) {
				continue;
			}
			if (write(fd, abort, 1) < 0) {
			}
			close(fd);
		}
		closedir(dir);
	} else {
	}
	while (waitpid(-1, status, __WALL) != pid) {
	}
}

static void setup_test()
{
	prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
	setpgrp();
	write_file("/proc/self/oom_score_adj", "1000");
}

#define USLEEP_FORKED_CHILD (3 * 50 *1000)

static long handle_clone_ret(long ret)
{
	if (ret != 0) {
		return ret;
	}
	usleep(USLEEP_FORKED_CHILD);
	syscall(__NR_exit, 0);
	while (1) {
	}
}

static long syz_clone(volatile long flags, volatile long stack, volatile 
long stack_len,
		      volatile long ptid, volatile long ctid, volatile long tls)
{
	long sp = (stack + stack_len) & ~15;
	long ret = (long)syscall(__NR_clone, flags & ~CLONE_VM, sp, ptid, ctid, 
tls);
	return handle_clone_ret(ret);
}

struct thread_t {
	int created, call;
	event_t ready, done;
};

static struct thread_t threads[16];
static void execute_call(int call);
static int running;

static void* thr(void* arg)
{
	struct thread_t* th = (struct thread_t*)arg;
	for (;;) {
		event_wait(&th->ready);
		event_reset(&th->ready);
		execute_call(th->call);
		__atomic_fetch_sub(&running, 1, __ATOMIC_RELAXED);
		event_set(&th->done);
	}
	return 0;
}

static void execute_one(void)
{
	int i, call, thread;
	for (call = 0; call < 6; call++) {
		for (thread = 0; thread < (int)(sizeof(threads) / sizeof(threads[0])); 
thread++) {
			struct thread_t* th = &threads[thread];
			if (!th->created) {
				th->created = 1;
				event_init(&th->ready);
				event_init(&th->done);
				event_set(&th->done);
				thread_start(thr, th);
			}
			if (!event_isset(&th->done))
				continue;
			event_reset(&th->done);
			th->call = call;
			__atomic_fetch_add(&running, 1, __ATOMIC_RELAXED);
			event_set(&th->ready);
			if (call == 3 || call == 4)
				break;
			event_timedwait(&th->done, 50);
			break;
		}
	}
	for (i = 0; i < 100 && __atomic_load_n(&running, __ATOMIC_RELAXED); i++)
		sleep_ms(1);
}

static void execute_one(void);

#define WAIT_FLAGS __WALL

static void loop(void)
{
	int iter = 0;
	for (;; iter++) {
		int pid = fork();
		if (pid < 0)
	exit(1);
		if (pid == 0) {
			setup_test();
			execute_one();
			exit(0);
		}
		int status = 0;
		uint64_t start = current_time_ms();
		for (;;) {
			if (waitpid(-1, &status, WNOHANG | WAIT_FLAGS) == pid)
				break;
			sleep_ms(1);
			if (current_time_ms() - start < 5000)
				continue;
			kill_and_wait(pid, &status);
			break;
		}
	}
}

uint64_t r[2] = {0xffffffffffffffff, 0x0};

void execute_call(int call)
{
		intptr_t res = 0;
	switch (call) {
	case 0:
		res = syscall(__NR_socket, 0x11ul, 3ul, 0x300);
		if (res != -1)
				r[0] = res;
		break;
	case 1:
*(uint32_t*)0x20000100 = 0x10000;
*(uint32_t*)0x20000104 = 7;
*(uint32_t*)0x20000108 = 0x1f0;
*(uint32_t*)0x2000010c = 0x39c;
*(uint32_t*)0x20000110 = 0;
*(uint32_t*)0x20000114 = 0;
*(uint32_t*)0x20000118 = 0;
		syscall(__NR_setsockopt, r[0], 0x107, 0xd, 0x20000100ul, 0x1cul);
		break;
	case 2:
		res = syscall(__NR_gettid);
		if (res != -1)
				r[1] = res;
		break;
	case 3:
		syscall(__NR_mmap, 0x20000000ul, 0x70000ul, 0x13ul, 0x12ul, r[0], 0ul);
		break;
	case 4:
syz_clone(0, 0, 0, 0, 0, 0);
		break;
	case 5:
*(uint64_t*)0x20000440 = 0x20000000;
*(uint64_t*)0x20000448 = 0xf1;
*(uint64_t*)0x20000450 = 0;
*(uint64_t*)0x20000458 = 0;
*(uint64_t*)0x20000460 = 0;
*(uint64_t*)0x20000468 = 0;
*(uint64_t*)0x20000470 = 0;
*(uint64_t*)0x20000478 = 0;
*(uint64_t*)0x20000480 = 0;
*(uint64_t*)0x20000488 = 0;
*(uint64_t*)0x20000490 = 0;
*(uint64_t*)0x20000498 = 0;
*(uint64_t*)0x200004a0 = 0;
*(uint64_t*)0x200004a8 = 0;
*(uint64_t*)0x200004b0 = 0;
*(uint64_t*)0x200004b8 = 0;
*(uint64_t*)0x200004c0 = 0;
*(uint64_t*)0x200004c8 = 0;
*(uint64_t*)0x20001700 = 0x20000500;
*(uint64_t*)0x20001708 = 0x15;
*(uint64_t*)0x20001710 = 0;
*(uint64_t*)0x20001718 = 0;
*(uint64_t*)0x20001720 = 0;
*(uint64_t*)0x20001728 = 0;
*(uint64_t*)0x20001730 = 0;
*(uint64_t*)0x20001738 = 0;
*(uint64_t*)0x20001740 = 0;
*(uint64_t*)0x20001748 = 0;
		syscall(__NR_process_vm_readv, r[1], 0x20000440ul, 9ul, 0x20001700ul, 
5ul, 0ul);
		break;
	}

}
int main(void)
{
		syscall(__NR_mmap, 0x1ffff000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
	syscall(__NR_mmap, 0x20000000ul, 0x1000000ul, 7ul, 0x32ul, -1, 0ul);
	syscall(__NR_mmap, 0x21000000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
	for (procid = 0; procid < 4; procid++) {
		if (fork() == 0) {
			loop();
		}
	}
	sleep(1000000);
	return 0;
}
---------------------------------------------------------------
> But my eye on COW is very poor nowadays, and I may be plain wrong.
> 
> Hugh
> 
>> ---
>>   mm/memory.c | 11 +++++++++++
>>   1 file changed, 11 insertions(+)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index cc50fa0f4590..45973fd97be8 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -823,6 +823,17 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
>>   	if (likely(!page_maybe_dma_pinned(page)))
>>   		return 1;
>>   
>> +	/*
>> +	 * The vma->anon_vma of the child process may be NULL
>> +	 * because the entire vma does not contain anonymous pages.
>> +	 * A BUG will occur when the copy_present_page() passes
>> +	 * a copy of a non-anonymous page of that vma to the
>> +	 * page_add_new_anon_rmap() to set up new anonymous rmap.
>> +	 * Return 1 if the page is not an anonymous page.
>> +	*/
>> +	if (!PageAnon(page))
>> +		return 1;
>> +
>>   	new_page = *prealloc;
>>   	if (!new_page)
>>   		return -EAGAIN;
>> -- 
>> 2.25.1
> .
> 
Thanks,

Yuanzheng
.

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