Re: [PATCH] sg, bsg: mitigate read/write abuse, block uaccess in release

Jann Horn
2018-06-15T17:03:00Z

On Fri, Jun 15, 2018 at 6:58 PM Jann Horn <jannh@google.com> wrote:
>
> On Fri, Jun 15, 2018 at 6:49 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > On Fri, Jun 15, 2018 at 05:23:35PM +0200, Jann Horn wrote:
> > > As Al Viro noted in commit 128394eff343 ("sg_write()/bsg_write() is not fit
> > > to be called under KERNEL_DS"), sg and bsg improperly access userspace
> > > memory outside the provided buffer, permitting kernel memory corruption via
> > > splice().
> > > But they don't just do it on ->write(), also on ->read() and (in the case
> > > of bsg) even on ->release().
> > >
> > > As a band-aid, make sure that the ->read() and ->write() handlers can not
> > > be called in weird contexts (kernel context or credentials different from
> > > file opener), like for ib_safe_file_access().
> > > Also, completely prevent user memory accesses from ->release().
> >
> > Band-aid it is, and a bloody awful one, at that. What the hell is going on
> > in bsg_put_device() and can it _ever_ hit that call chain? I.e.
> > bsg_release()
> > bsg_put_device()
> > blk_complete_sgv4_hdr_rq()
> > ->complete_rq()
> > copy_to_user()
> > If it can, the whole thing is FUBAR by design - ->release() may bloody well
> > be called in a context that has no userspace at all.
> >
> > This is completely insane; what's going on there?
>
> Perhaps I should have split my patch into two parts; it consists of
> two somewhat related changes.
>
> The first change is that ->read() and ->write() violate the normal
> contract and, as a band-aid, should not be called in uaccess_kernel()
> context or with changed creds.
>
> The second change is an actual fix: AFAICS ->release() accidentally
> accessed userspace, which I've fixed using the added "cleaning_up"
> parameter.

FWIW, the demo code I'm using to test this in a QEMU VM:

$ cat test.c
#define _GNU_SOURCE
#include <fcntl.h>
#include <unistd.h>
#include <linux/bsg.h>
#include <string.h>
#include <err.h>
#include <stdio.h>

int main(void) {
int fd = open("/dev/bsg/0:0:0:0", O_RDWR);
if (fd == -1)
err(1, "foo");
__u8 buf1[255];
__u8 request[10] = {
[0] = 0x5a, // MODE_SENSE_10
[2] = 0x41,
[8] = 0x10
};
__u8 sense[32];
memset(sense, 'A', sizeof(sense));
memset(buf1, 'A', sizeof(buf1));
struct sg_io_v4 req = {
.guard = 'Q',
.protocol = BSG_PROTOCOL_SCSI,
.subprotocol = BSG_SUB_PROTOCOL_SCSI_CMD,
.request_len = sizeof(request),
.request = (__u64)request,
.max_response_len = sizeof(sense),
.response = (__u64)sense,
.din_xfer_len = sizeof(buf1),
.din_xferp = (__u64)buf1,
.timeout = 1000
};
if (write(fd, &req, sizeof(req)) != sizeof(req))
err(1, "write");
printf("sense[0] after write: 0x%02hhx\n", sense[0]);

/*
struct sg_io_v4 resp;
if (splice(fd, NULL, pipe_fds[1], NULL, sizeof(struct sg_io_v4), 0)
!= sizeof(struct sg_io_v4))
err(1, "splice");
*/

sleep(1);
printf("sense[0] after sleep: 0x%02hhx\n", sense[0]);
close(fd);
printf("sense[0] after close: 0x%02hhx\n", sense[0]);
}
$ gcc -o test test.c -Wall && sudo ./test
sense[0] after write: 0x41
sense[0] after sleep: 0x41
sense[0] after close: 0xf0
$ uname -a
Linux debian 4.17.0+ #10 SMP Fri Jun 15 14:48:42 CEST 2018 x86_64 GNU/Linux