Re: [PATCH v7] Add udmabuf misc device

From: Gert Wollny
Date: Sun Sep 09 2018 - 07:14:49 EST

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I am by no means a kernel expert, but Tomeu asked whether I could
review this. Generally the patch looks okay, it applied (on top of
4.18.5-gentoo), and compiled without problems.

However, running the experimental qemu branch I get a kernel bug:

BUG: unable to handle kernel NULL pointer dereference at
0000000000000038
[12100.605908] PGD 0 P4D 0
[12100.605918] Oops: 0002 [#1] SMP NOPTI
[12100.605926] CPU: 5 PID: 7247 Comm: qemu-system-x86 Not tainted
4.18.5-gentoo #1
[12100.605930] Hardware name: To be filled by O.E.M. To be filled by
O.E.M./M5A97 R2.0, BIOS 1302 11/14/2012
[12100.605942] RIP: 0010:fput+0x5/0x80
[12100.605945] Code: e4 ff 48 8b 3d b4 23 10 01 48 8b 34 24 48 83 c4 08
e9 6f 6c fd ff 0f 1f 44 00 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00
00 <f0> 48 ff 4f 38 75 51 53 48 89 fb 65 8b 05 e9 fe 9c 5f 65 48 8b 3c
[12100.606003] RSP: 0018:ffffbf00c94ffd90 EFLAGS: 00010246
[12100.606007] RAX: 0000000000000000 RBX: fffffffffffffff8 RCX:
ffff98336e000b00
[12100.606011] RDX: 0000000000000000 RSI: 0000000000004000 RDI:
0000000000000000
[12100.606014] RBP: 0000000000000000 R08: ffff983366504bc0 R09:
00000000ffffffff
[12100.606018] R10: 0000000000000000 R11: 0000000000000000 R12:
ffffffffffffffea
[12100.606021] R13: 0000000000000000 R14: ffff983100c5ca20 R15:
0000000000000000
[12100.606026] FS: 00007fe49ea8fbc0(0000) GS:ffff98338ed40000(0000)
knlGS:0000000000000000
[12100.606029] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[12100.606032] CR2: 0000000000000038 CR3: 00000003df65e000 CR4:
00000000000406e0
[12100.606036] Call Trace:
[12100.606046] udmabuf_create+0x220/0x310
[12100.606057] udmabuf_ioctl+0xf3/0x120
[12100.606064] do_vfs_ioctl+0xb0/0x630
[12100.606071] ? handle_mm_fault+0x108/0x200
[12100.606077] ksys_ioctl+0x92/0xa0
[12100.606082] __x64_sys_ioctl+0x16/0x20
[12100.606089] do_syscall_64+0x4f/0xe0
[12100.606096] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[12100.606103] RIP: 0033:0x7fe4974b43b7

The offending code is in the error exit (0x220 = 544):

0xffffffff816085db <+539>: callq 0xffffffff81240350 <fput>
0xffffffff816085e0 <+544>: mov 0x8(%r14),%rdi
0xffffffff816085e4 <+548>: callq 0xffffffff81217ad0 <kfree>
0xffffffff816085e9 <+553>: mov %r14,%rdi
0xffffffff816085ec <+556>: callq 0xffffffff81217ad0 <kfree>
0xffffffff816085f1 <+561>: mov 0x50(%rsp),%rbx
0xffffffff816085f6 <+566>: xor %gs:0x28,%rbx
0xffffffff816085ff <+575>: mov %r12,%rax
0xffffffff81608602 <+578>: jne 0xffffffff816086c8
<udmabuf_create+776>

err_free_ubuf:
fput(memfd);
kfree(ubuf->pages);
kfree(ubuf);
return ret;

further commenst see below.

+
> +static long udmabuf_create(struct udmabuf_create_list *head,
> + struct udmabuf_create_item *list)
> +{
> + DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
> + struct file *memfd = NULL;
> + struct udmabuf *ubuf;
> + struct dma_buf *buf;
> + pgoff_t pgoff, pgcnt, pgidx, pgbuf;
> + struct page *page;
> + int seals, ret = -EINVAL;
> + u32 i, flags;
> +
> + ubuf = kzalloc(sizeof(struct udmabuf), GFP_KERNEL);
> + if (!ubuf)
> + return -ENOMEM;
> +
> + for (i = 0; i < head->count; i++) {
> + if (!IS_ALIGNED(list[i].offset, PAGE_SIZE))
> + goto err_free_ubuf;
> + if (!IS_ALIGNED(list[i].size, PAGE_SIZE))
> + goto err_free_ubuf;
> + ubuf->pagecount += list[i].size >> PAGE_SHIFT;
> + }
> + ubuf->pages = kmalloc_array(ubuf->pagecount, sizeof(struct
> page *),
> + GFP_KERNEL);
> + if (!ubuf->pages) {
> + ret = -ENOMEM;
> + goto err_free_ubuf;
> + }
> +
> + pgbuf = 0;
> + for (i = 0; i < head->count; i++) {
> + memfd = fget(list[i].memfd);
> + if (!memfd)
> + goto err_put_pages;
> + if (!shmem_mapping(file_inode(memfd)->i_mapping))
> + goto err_put_pages;
> + seals = memfd_fcntl(memfd, F_GET_SEALS, 0);
> + if (seals == -EINVAL ||
> + (seals & SEALS_WANTED) != SEALS_WANTED ||
> + (seals & SEALS_DENIED) != 0)
> + goto err_put_pages;
> + pgoff = list[i].offset >> PAGE_SHIFT;
> + pgcnt = list[i].size >> PAGE_SHIFT;
> + for (pgidx = 0; pgidx < pgcnt; pgidx++) {
> + page = shmem_read_mapping_page(
> + file_inode(memfd)->i_mapping, pgoff
> + pgidx);
> + if (IS_ERR(page)) {
> + ret = PTR_ERR(page);
> + goto err_put_pages;
> + }
> + ubuf->pages[pgbuf++] = page;
> + }
> + fput(memfd);
> + }
> + memfd = NULL;
Now memfd is NULL

> +
> + exp_info.ops = &udmabuf_ops;
> + exp_info.size = ubuf->pagecount << PAGE_SHIFT;
> + exp_info.priv = ubuf;
> +
> + buf = dma_buf_export(&exp_info);
> + if (IS_ERR(buf)) {
> + ret = PTR_ERR(buf);
> + goto err_put_pages;
Assume an error occured

> + }
> +
> + flags = 0;
> + if (head->flags & UDMABUF_FLAGS_CLOEXEC)
> + flags |= O_CLOEXEC;
> + return dma_buf_fd(buf, flags);
> +
> +err_put_pages:
> + while (pgbuf > 0)
> + put_page(ubuf->pages[--pgbuf]);
> +err_free_ubuf:
> + fput(memfd);
Now fput it called with NULL, and in fput this is used in

0xffffffff81240350 <+0>: callq 0xffffffff81a015c0 <__fentry__>
0xffffffff81240355 <+5>: lock decq 0x38(%rdi)

where the bug is signalled, so I guess fput doesn't accept a null
pointer.

I'm not sure why you set memfd to NULL at all, because on the following
non-error path it is not used.

The other question is of course, why did dma_buf_export fail for me ...

Best,
Gert

PS: Please CC me since I'm not on lkml,

> + kfree(ubuf->pages);
> + kfree(ubuf);
> + return ret;
> +}
> +
> +static long udmabuf_ioctl_create(struct file *filp, unsigned long
> arg)
> +{
> + struct udmabuf_create create;
> + struct udmabuf_create_list head;
> + struct udmabuf_create_item list;
> +
> + if (copy_from_user(&create, (void __user *)arg,
> + sizeof(struct udmabuf_create)))
> + return -EFAULT;
> +
> + head.flags = create.flags;
> + head.count = 1;
> + list.memfd = create.memfd;
> + list.offset = create.offset;
> + list.size = create.size;
> +
> + return udmabuf_create(&head, &list);
> +}
> +
> +static long udmabuf_ioctl_create_list(struct file *filp, unsigned
> long arg)
> +{
> + struct udmabuf_create_list head;
> + struct udmabuf_create_item *list;
> + int ret = -EINVAL;
> + u32 lsize;
> +
> + if (copy_from_user(&head, (void __user *)arg, sizeof(head)))
> + return -EFAULT;
> + if (head.count > 1024)
> + return -EINVAL;
> + lsize = sizeof(struct udmabuf_create_item) * head.count;
> + list = memdup_user((void __user *)(arg + sizeof(head)),
> lsize);
> + if (IS_ERR(list))
> + return PTR_ERR(list);
> +
> + ret = udmabuf_create(&head, list);
> + kfree(list);
> + return ret;
> +}
> +
> +static long udmabuf_ioctl(struct file *filp, unsigned int ioctl,
> + unsigned long arg)
> +{
> + long ret;
> +
> + switch (ioctl) {
> + case UDMABUF_CREATE:
> + ret = udmabuf_ioctl_create(filp, arg);
> + break;
> + case UDMABUF_CREATE_LIST:
> + ret = udmabuf_ioctl_create_list(filp, arg);
> + break;
> + default:
> + ret = -EINVAL;
> + break;
> + }
> + return ret;
> +}
> +
> +static const struct file_operations udmabuf_fops = {
> + .owner = THIS_MODULE,
> + .unlocked_ioctl = udmabuf_ioctl,
> +};
> +
> +static struct miscdevice udmabuf_misc = {
> + .minor = MISC_DYNAMIC_MINOR,
> + .name = "udmabuf",
> + .fops = &udmabuf_fops,
> +};
> +
> +static int __init udmabuf_dev_init(void)
> +{
> + return misc_register(&udmabuf_misc);
> +}
> +
> +static void __exit udmabuf_dev_exit(void)
> +{
> + misc_deregister(&udmabuf_misc);
> +}
> +
> +module_init(udmabuf_dev_init)
> +module_exit(udmabuf_dev_exit)
> +
> +MODULE_AUTHOR("Gerd Hoffmann <kraxel@xxxxxxxxxx>");
> +MODULE_LICENSE("GPL v2");
> diff --git a/tools/testing/selftests/drivers/dma-buf/udmabuf.c
> b/tools/testing/selftests/drivers/dma-buf/udmabuf.c
> new file mode 100644
> index 0000000000..376b1d6730
> --- /dev/null
> +++ b/tools/testing/selftests/drivers/dma-buf/udmabuf.c
> @@ -0,0 +1,96 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <unistd.h>
> +#include <string.h>
> +#include <errno.h>
> +#include <fcntl.h>
> +#include <malloc.h>
> +
> +#include <sys/ioctl.h>
> +#include <sys/syscall.h>
> +#include <linux/memfd.h>
> +#include <linux/udmabuf.h>
> +
> +#define TEST_PREFIX "drivers/dma-buf/udmabuf"
> +#define NUM_PAGES 4
> +
> +static int memfd_create(const char *name, unsigned int flags)
> +{
> + return syscall(__NR_memfd_create, name, flags);
> +}
> +
> +int main(int argc, char *argv[])
> +{
> + struct udmabuf_create create;
> + int devfd, memfd, buf, ret;
> + off_t size;
> + void *mem;
> +
> + devfd = open("/dev/udmabuf", O_RDWR);
> + if (devfd < 0) {
> + printf("%s: [skip,no-udmabuf]\n", TEST_PREFIX);
> + exit(77);
> + }
> +
> + memfd = memfd_create("udmabuf-test", MFD_CLOEXEC);
> + if (memfd < 0) {
> + printf("%s: [skip,no-memfd]\n", TEST_PREFIX);
> + exit(77);
> + }
> +
> + size = getpagesize() * NUM_PAGES;
> + ret = ftruncate(memfd, size);
> + if (ret == -1) {
> + printf("%s: [FAIL,memfd-truncate]\n", TEST_PREFIX);
> + exit(1);
> + }
> +
> + memset(&create, 0, sizeof(create));
> +
> + /* should fail (offset not page aligned) */
> + create.memfd = memfd;
> + create.offset = getpagesize()/2;
> + create.size = getpagesize();
> + buf = ioctl(devfd, UDMABUF_CREATE, &create);
> + if (buf >= 0) {
> + printf("%s: [FAIL,test-1]\n", TEST_PREFIX);
> + exit(1);
> + }
> +
> + /* should fail (size not multiple of page) */
> + create.memfd = memfd;
> + create.offset = 0;
> + create.size = getpagesize()/2;
> + buf = ioctl(devfd, UDMABUF_CREATE, &create);
> + if (buf >= 0) {
> + printf("%s: [FAIL,test-2]\n", TEST_PREFIX);
> + exit(1);
> + }
> +
> + /* should fail (not memfd) */
> + create.memfd = 0; /* stdin */
> + create.offset = 0;
> + create.size = size;
> + buf = ioctl(devfd, UDMABUF_CREATE, &create);
> + if (buf >= 0) {
> + printf("%s: [FAIL,test-3]\n", TEST_PREFIX);
> + exit(1);
> + }
> +
> + /* should work */
> + create.memfd = memfd;
> + create.offset = 0;
> + create.size = size;
> + buf = ioctl(devfd, UDMABUF_CREATE, &create);
> + if (buf < 0) {
> + printf("%s: [FAIL,test-4]\n", TEST_PREFIX);
> + exit(1);
> + }
> +
> + fprintf(stderr, "%s: ok\n", TEST_PREFIX);
> + close(buf);
> + close(memfd);
> + close(devfd);
> + return 0;
> +}
> diff --git a/MAINTAINERS b/MAINTAINERS
> index a5b256b259..11a9b04277 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -14934,6 +14934,14 @@ S: Maintained
> F: Documentation/filesystems/udf.txt
> F: fs/udf/
>
> +UDMABUF DRIVER
> +M: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> +L: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> +S: Maintained
> +F: drivers/dma-buf/udmabuf.c
> +F: include/uapi/linux/udmabuf.h
> +F: tools/testing/selftests/drivers/dma-buf/udmabuf.c
> +
> UDRAW TABLET
> M: Bastien Nocera <hadess@xxxxxxxxxx>
> L: linux-input@xxxxxxxxxxxxxxx
> @@ -15343,6 +15351,14 @@ F: arch/x86/um/
> F: fs/hostfs/
> F: fs/hppfs/
>
> +USERSPACE DMA BUFFER DRIVER
> +M: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> +S: Maintained
> +L: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> +F: drivers/dma-buf/udmabuf.c
> +F: include/uapi/linux/udmabuf.h
> +T: git git://anongit.freedesktop.org/drm/drm-misc
> +
> USERSPACE I/O (UIO)
> M: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> S: Maintained
> diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
> index ed3b785bae..338129eb12 100644
> --- a/drivers/dma-buf/Kconfig
> +++ b/drivers/dma-buf/Kconfig
> @@ -30,4 +30,12 @@ config SW_SYNC
> WARNING: improper use of this can result in deadlocking
> kernel
> drivers from userspace. Intended for test and debug only.
>
> +config UDMABUF
> + bool "userspace dmabuf misc driver"
> + default n
> + depends on DMA_SHARED_BUFFER
> + help
> + A driver to let userspace turn memfd regions into dma-
> bufs.
> + Qemu can use this to create host dmabufs for guest
> framebuffers.
> +
> endmenu
> diff --git a/drivers/dma-buf/Makefile b/drivers/dma-buf/Makefile
> index c33bf88631..0913a6ccab 100644
> --- a/drivers/dma-buf/Makefile
> +++ b/drivers/dma-buf/Makefile
> @@ -1,3 +1,4 @@
> obj-y := dma-buf.o dma-fence.o dma-fence-array.o reservation.o
> seqno-fence.o
> obj-$(CONFIG_SYNC_FILE) += sync_file.o
> obj-$(CONFIG_SW_SYNC) += sw_sync.o sync_debug.o
> +obj-$(CONFIG_UDMABUF) += udmabuf.o
> diff --git a/tools/testing/selftests/drivers/dma-buf/Makefile
> b/tools/testing/selftests/drivers/dma-buf/Makefile
> new file mode 100644
> index 0000000000..4154c3d7aa
> --- /dev/null
> +++ b/tools/testing/selftests/drivers/dma-buf/Makefile
> @@ -0,0 +1,5 @@
> +CFLAGS += -I../../../../../usr/include/
> +
> +TEST_GEN_PROGS := udmabuf
> +
> +include ../../lib.mk
> --
> 2.9.3

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