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

From: Eric Biggers <[email protected]>

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 1ac0b6dec656 ("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 #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

Reported-by syzbot+d11f321e7f1923157eac80aa990b446596f46439@syzkaller.appspotmail.com

Fixes c8d78c1823f4 ("mm: replace remap_file_pages() syscall with emulation")

Cc [email protected]

Signed-off-by Eric Biggers <[email protected]>

---
 ipc/shm.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

ipc/shm.c +10 -2

index acefe44fefefa..c80c5691a9970 100644
@@ -225,6 +225,12 @@ 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();
 	ipc_update_pid(&shp->shm_lprid, task_tgid(current));
 	shp->shm_nattch++;

@@ -455,8 +461,9 @@ 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)

@@ -480,6 +487,7 @@ 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;

Quoted 45 lines · Eric Biggers

ipc/shm.c +11 -3

index acefe44fefefa..c80c5691a9970 100644
@@ -225,6 +225,12 @@ 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();
 	ipc_update_pid(&shp->shm_lprid, task_tgid(current));
 	shp->shm_nattch++;

@@ -455,8 +461,9 @@ 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)

@@ -480,6 +487,7 @@ 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;

@@ -1432,7 +1440,7 @@ 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;
 

ipc/shm.c long do_shmat(int shmid, char __user *shmaddr, int shmflg, · line 1446

Quoted 1 line

ipc/shm.c +0 -0

 	err = security_mmap_file(file, prot, flags);

Hm. Why do we need sfd->file refcounting now? It's not obvious to me.

Looks like it's either a separate bug or an unneeded change.

It's necessary because if we don't hold a reference to sfd->file, then it can be a stale pointer when we compare it in __shm_open(). In particular, if the new struct file happened to be allocated at the same address as the old one, then 'sfd->file == shp->shm_file' so the mmap would be allowed. But, it will be a different shm segment than was intended. The caller may not even have permissions to map it normally, yet it would be done anyway.

In the end it's just broken to have a pointer to something that can be freed out from under you...

So I don't think the pointer is going anywhere, or am I missing something?

Quoted 9 lines · Eric Biggers

It's necessary because if we don't hold a reference to sfd->file, then it can be a stale pointer when we compare it in __shm_open(). In particular, if the new struct file happened to be allocated at the same address as the old one, then 'sfd->file == shp->shm_file' so the mmap would be allowed. But, it will be a different shm segment than was intended. The caller may not even have permissions to map it normally, yet it would be done anyway.

In the end it's just broken to have a pointer to something that can be freed out from under you...

So this is actually handled by shm_nattch, serialized by the ipc perm->lock. shm_destroy() is called when 0, which in turn does the fput(shm_file). Note that shm_file is given a count of 1 when a new segment is created (deep in get_empty_filp()). So I don't think the pointer is going anywhere, or am I missing something?

Thanks,
Davidlohr

Quoted 18 lines · Davidlohr Bueso

Quoted 9 lines · Eric Biggers

It's necessary because if we don't hold a reference to sfd->file, then it can be a stale pointer when we compare it in __shm_open(). In particular, if the new struct file happened to be allocated at the same address as the old one, then 'sfd->file == shp->shm_file' so the mmap would be allowed. But, it will be a different shm segment than was intended. The caller may not even have permissions to map it normally, yet it would be done anyway.

In the end it's just broken to have a pointer to something that can be freed out from under you...

So this is actually handled by shm_nattch, serialized by the ipc perm->lock. shm_destroy() is called when 0, which in turn does the fput(shm_file). Note that shm_file is given a count of 1 when a new segment is created (deep in get_empty_filp()). So I don't think the pointer is going anywhere, or am I missing something?

Quoted 2 lines

Thanks,
Davidlohr

In the remap_file_pages() case, a reference is taken to the ->vm_file, then the segment is unmapped. If that brings ->shm_nattch to 0, then the underlying shm segment and ID can be removed, which (currently) causes the real shm file to be freed. But, the outer file still exists and will have ->mmap() called on it. That's why the outer file needs to hold a reference to the real shm file.

Quoted 19 lines · Eric Biggers

Quoted 18 lines · Davidlohr Bueso

Quoted 9 lines · Eric Biggers

It's necessary because if we don't hold a reference to sfd->file, then it can be a stale pointer when we compare it in __shm_open(). In particular, if the new struct file happened to be allocated at the same address as the old one, then 'sfd->file == shp->shm_file' so the mmap would be allowed. But, it will be a different shm segment than was intended. The caller may not even have permissions to map it normally, yet it would be done anyway.

In the end it's just broken to have a pointer to something that can be freed out from under you...

So this is actually handled by shm_nattch, serialized by the ipc perm->lock. shm_destroy() is called when 0, which in turn does the fput(shm_file). Note that shm_file is given a count of 1 when a new segment is created (deep in get_empty_filp()). So I don't think the pointer is going anywhere, or am I missing something?

Quoted 8 lines

Quoted 2 lines

Thanks,
Davidlohr

In the remap_file_pages() case, a reference is taken to the ->vm_file, then the segment is unmapped. If that brings ->shm_nattch to 0, then the underlying shm segment and ID can be removed, which (currently) causes the real shm file to be freed. But, the outer file still exists and will have ->mmap() called on it. That's why the outer file needs to hold a reference to the real shm file.

Okay, fair enough. Logic in SysV IPC implementation is often hard to follow. Could you include the description in the commit message?

And feel free to use my

Acked-by Kirill A. Shutemov <[email protected]>

ipc/shm.c +1 -0

@@ -480,6 +487,7 @@ 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;