Re: [syzbot] BUG: unable to handle kernel paging request in truncate_inode_partial_folio

[Eric Biggers]

On Tue, Jun 28, 2022 at 03:59:26PM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    941e3e791269 Merge tag 'for_linus' of git://git.kernel.org..
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=1670ded4080000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=833001d0819ddbc9
> dashboard link: https://syzkaller.appspot.com/bug?extid=9bd2b7adbd34b30b87e4
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=140f9ba8080000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15495188080000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+9bd2b7adbd34b30b87e4@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> BUG: unable to handle page fault for address: ffff888021f7e005
> #PF: supervisor write access in kernel mode
> #PF: error_code(0x0002) - not-present page
> PGD 11401067 P4D 11401067 PUD 11402067 PMD 21f7d063 PTE 800fffffde081060
> Oops: 0002 [#1] PREEMPT SMP KASAN
> CPU: 0 PID: 3761 Comm: syz-executor281 Not tainted 5.19.0-rc4-syzkaller-00014-g941e3e791269 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:memset_erms+0x9/0x10 arch/x86/lib/memset_64.S:64
> Code: c1 e9 03 40 0f b6 f6 48 b8 01 01 01 01 01 01 01 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 90 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 90 49 89 fa 40 0f b6 ce 48 b8 01 01 01 01 01 01
> RSP: 0018:ffffc9000329fa90 EFLAGS: 00010202
> RAX: 0000000000000000 RBX: 0000000000001000 RCX: 0000000000000ffb
> RDX: 0000000000000ffb RSI: 0000000000000000 RDI: ffff888021f7e005
> RBP: ffffea000087df80 R08: 0000000000000001 R09: ffff888021f7e005
> R10: ffffed10043efdff R11: 0000000000000000 R12: 0000000000000005
> R13: 0000000000000000 R14: 0000000000001000 R15: 0000000000000ffb
> FS:  00007fb29d8b2700(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: ffff888021f7e005 CR3: 0000000026e7b000 CR4: 00000000003506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  zero_user_segments include/linux/highmem.h:272 [inline]
>  folio_zero_range include/linux/highmem.h:428 [inline]
>  truncate_inode_partial_folio+0x76a/0xdf0 mm/truncate.c:237
>  truncate_inode_pages_range+0x83b/0x1530 mm/truncate.c:381
>  truncate_inode_pages mm/truncate.c:452 [inline]
>  truncate_pagecache+0x63/0x90 mm/truncate.c:753
>  simple_setattr+0xed/0x110 fs/libfs.c:535
>  secretmem_setattr+0xae/0xf0 mm/secretmem.c:170
>  notify_change+0xb8c/0x12b0 fs/attr.c:424
>  do_truncate+0x13c/0x200 fs/open.c:65
>  do_sys_ftruncate+0x536/0x730 fs/open.c:193
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x46/0xb0
> RIP: 0033:0x7fb29d900899
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007fb29d8b2318 EFLAGS: 00000246 ORIG_RAX: 000000000000004d
> RAX: ffffffffffffffda RBX: 00007fb29d988408 RCX: 00007fb29d900899
> RDX: 00007fb29d900899 RSI: 0000000000000005 RDI: 0000000000000003
> RBP: 00007fb29d988400 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007fb29d98840c
> R13: 00007ffca01a23bf R14: 00007fb29d8b2400 R15: 0000000000022000
>  </TASK>
> Modules linked in:
> CR2: ffff888021f7e005
> ---[ end trace 0000000000000000 ]---

I think this is a bug in memfd_secret.  secretmem_setattr() can race with a page
being faulted in by secretmem_fault().  Specifically, a page can be faulted in
after secretmem_setattr() has set i_size but before it zeroes out the partial
page past i_size.  memfd_secret pages aren't mapped in the kernel direct map, so
the crash occurs when the kernel tries to zero out the partial page.

I don't know what the best solution is -- maybe a rw_semaphore protecting
secretmem_fault() and secretmem_setattr()?  Or perhaps secretmem_setattr()
should avoid the call to truncate_setsize() by not using simple_setattr(), given
that secretmem_setattr() only supports the size going from zero to nonzero.

The following commit tried to fix a similar bug, but it wasn't enough:

	commit f9b141f93659e09a52e28791ccbaf69c273b8e92
	Author: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
	Date:   Thu Apr 14 19:13:31 2022 -0700

	    mm/secretmem: fix panic when growing a memfd_secret


Here's a simplified reproducer.  Note, for memfd_secret to be supported, the
kernel config must contain CONFIG_SECRETMEM=y and the kernel command line must
contain secretmem.enable=1.

#include <pthread.h>
#include <setjmp.h>
#include <signal.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <unistd.h>

static volatile int fd;
static jmp_buf jump_buf;

static void *truncate_thread(void *arg)
{
	for (;;)
		ftruncate(fd, 1000);
}

static void handle_sigbus(int sig)
{
	longjmp(jump_buf, 1);
}

int main(void)
{
	struct sigaction act = {
		.sa_handler = handle_sigbus,
		.sa_flags = SA_NODEFER,
	};
	pthread_t t;
	void *addr;

	sigaction(SIGBUS, &act, NULL);

	pthread_create(&t, NULL, truncate_thread, NULL);
	for (;;) {
		fd = syscall(__NR_memfd_secret, 0);
		addr = mmap(NULL, 8192, PROT_WRITE, MAP_SHARED, fd, 0);
		if (setjmp(jump_buf) == 0)
			*(unsigned int *)addr = 0;
		munmap(addr, 8192);
		close(fd);
	}
}