ipc/shm: fix use-after-free of shm file via remap_file_pages()

syzbot reported a use-after-free of shm_file_data(file)->file->f_op in
shm_get_unmapped_area(), called via sys_remap_file_pages().  Unfortunately
it couldn't generate a reproducer, but I found a bug which I think caused
it.  When remap_file_pages() is passed a full System V shared memory
segment, the memory is first unmapped, then a new map is created using the
->vm_file.  Between these steps, the shm ID can be removed and reused for
a new shm segment.  But, shm_mmap() only checks whether the ID is
currently valid before calling the underlying file's ->mmap(); it doesn't
check whether it was reused.  Thus it can use the wrong underlying file,
one that was already freed.

Fix this by making the "outer" shm file (the one that gets put in
->vm_file) hold a reference to the real shm file, and by making
__shm_open() require that the file associated with the shm ID matches the
one associated with the "outer" file.

Commit 1ac0b6d ("ipc/shm: handle removed segments gracefully in
shm_mmap()") almost fixed this bug, but it didn't go far enough because it
didn't consider the case where the shm ID is reused.

The following program usually reproduces this bug:

	#include <stdlib.h>
	#include <sys/shm.h>
	#include <sys/syscall.h>
	#include <unistd.h>

	int main()
	{
		int is_parent = (fork() != 0);
		srand(getpid());
		for (;;) {
			int id = shmget(0xF00F, 4096, IPC_CREAT|0700);
			if (is_parent) {
				void *addr = shmat(id, NULL, 0);
				usleep(rand() % 50);
				while (!syscall(__NR_remap_file_pages, addr, 4096, 0, 0, 0));
			} else {
				usleep(rand() % 50);
				shmctl(id, IPC_RMID, NULL);
			}
		}
	}

It causes the following NULL pointer dereference due to a 'struct file'
being used while it's being freed.  (I couldn't actually get a KASAN
use-after-free splat like in the syzbot report.  But I think it's possible
with this bug; it would just take a more extraordinary race...)

	BUG: unable to handle kernel NULL pointer dereference at 0000000000000058
	PGD 0 P4D 0
	Oops: 0000 [#1] SMP NOPTI
	CPU: 9 PID: 258 Comm: syz_ipc Not tainted 4.16.0-05140-gf8cf2f16a7c95 torvalds#189
	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-20171110_100015-anatol 04/01/2014
	RIP: 0010:d_inode include/linux/dcache.h:519 [inline]
	RIP: 0010:touch_atime+0x25/0xd0 fs/inode.c:1724
	[...]
	Call Trace:
	 file_accessed include/linux/fs.h:2063 [inline]
	 shmem_mmap+0x25/0x40 mm/shmem.c:2149
	 call_mmap include/linux/fs.h:1789 [inline]
	 shm_mmap+0x34/0x80 ipc/shm.c:465
	 call_mmap include/linux/fs.h:1789 [inline]
	 mmap_region+0x309/0x5b0 mm/mmap.c:1712
	 do_mmap+0x294/0x4a0 mm/mmap.c:1483
	 do_mmap_pgoff include/linux/mm.h:2235 [inline]
	 SYSC_remap_file_pages mm/mmap.c:2853 [inline]
	 SyS_remap_file_pages+0x232/0x310 mm/mmap.c:2769
	 do_syscall_64+0x64/0x1a0 arch/x86/entry/common.c:287
	 entry_SYSCALL_64_after_hwframe+0x42/0xb7

Link: http://lkml.kernel.org/r/[email protected]
Reported-by: syzbot+d11f321e7f1923157eac80aa990b446596f46439@syzkaller.appspotmail.com
Fixes: c8d78c1 ("mm: replace remap_file_pages() syscall with emulation")
Signed-off-by: Eric Biggers <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Manfred Spraul <[email protected]>
Cc: "Eric W . Biederman" <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: ebiggers

ipc/shm.c

@@ -203,6 +203,12 @@ static int __shm_open(struct vm_area_struct *vma)
 	if (IS_ERR(shp))
 		return PTR_ERR(shp);
 
+	if (shp->shm_file != sfd->file) {
+		/* ID was reused */
+		shm_unlock(shp);
+		return -EINVAL;
+	}
+
 	shp->shm_atim = ktime_get_real_seconds();
 	shp->shm_lprid = task_tgid_vnr(current);
 	shp->shm_nattch++;
@@ -431,8 +437,9 @@ static int shm_mmap(struct file *file, struct vm_area_struct *vma)
 	int ret;
 
 	/*
-	 * In case of remap_file_pages() emulation, the file can represent
-	 * removed IPC ID: propogate shm_lock() error to caller.
+	 * In case of remap_file_pages() emulation, the file can represent an
+	 * IPC ID that was removed, and possibly even reused by another shm
+	 * segment already.  Propagate this case as an error to caller.
 	 */
 	ret = __shm_open(vma);
 	if (ret)
@@ -456,6 +463,7 @@ static int shm_release(struct inode *ino, struct file *file)
 	struct shm_file_data *sfd = shm_file_data(file);
 
 	put_ipc_ns(sfd->ns);
+	fput(sfd->file);
 	shm_file_data(file) = NULL;
 	kfree(sfd);
 	return 0;
@@ -1415,7 +1423,7 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg,
 	file->f_mapping = shp->shm_file->f_mapping;
 	sfd->id = shp->shm_perm.id;
 	sfd->ns = get_ipc_ns(ns);
-	sfd->file = shp->shm_file;
+	sfd->file = get_file(shp->shm_file);
 	sfd->vm_ops = NULL;
 
 	err = security_mmap_file(file, prot, flags);