The patch titled Subject: ipc/shm: fix use-after-free of shm file via remap_file_pages() has been added to the -mm tree. Its filename is ipc-shm-fix-use-after-free-of-shm-file-via-remap_file_pages.patch
This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/ipc-shm-fix-use-after-free-of-shm-f... and later at http://ozlabs.org/~akpm/mmotm/broken-out/ipc-shm-fix-use-after-free-of-shm-f...
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------------------------------------------------------ From: Eric Biggers ebiggers@google.com Subject: 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 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
Link: http://lkml.kernel.org/r/20180409043039.28915-1-ebiggers3@gmail.com Reported-by: syzbot+d11f321e7f1923157eac80aa990b446596f46439@syzkaller.appspotmail.com Fixes: c8d78c1823f4 ("mm: replace remap_file_pages() syscall with emulation") Signed-off-by: Eric Biggers ebiggers@google.com Cc: Davidlohr Bueso dave@stgolabs.net Cc: Manfred Spraul manfred@colorfullife.com Cc: "Eric W . Biederman" ebiederm@xmission.com Cc: "Kirill A. Shutemov" kirill.shutemov@linux.intel.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
diff -puN ipc/shm.c~ipc-shm-fix-use-after-free-of-shm-file-via-remap_file_pages ipc/shm.c --- a/ipc/shm.c~ipc-shm-fix-use-after-free-of-shm-file-via-remap_file_pages +++ a/ipc/shm.c @@ -225,6 +225,12 @@ static int __shm_open(struct vm_area_str 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, s 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 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 *sh 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); _
Patches currently in -mm which might be from ebiggers@google.com are
ipc-shm-fix-use-after-free-of-shm-file-via-remap_file_pages.patch