Re: PATCH: cdrecord: avoiding scsi device numbering for ide devices

Pascal Schmidt
2004-08-22T13:14:51Z

On Sun, 22 Aug 2004, Joerg Schilling wrote:

> Not checking for Write access permissions at this place is a typical
> mistake made by novice programmers, so I never thought this could be in
> Linux.....

People will find this kind of language inflammatory. ;) However, exactly
because it is such a bad mistake did Linus put out what he deemed a
correct fix *immediately*.

> If Linux still noes not check for write permissions, I would consider
> there is still a bug.

The open question is whether write permission really is meaningful
enough to allow arbitrary SCSI commands. I personally think "being
able to wipe the drive firmware" is too much, and since filtering
of vendor commands is generally impossible to do right, sending SG_IO
should require CAP_SYS_RAWIO capability.

> If there is a list of "aparently safe" SCSI commands that are allowed to
> be executed, then there is another bug in Linux. The only SCSI command
> that could be called safe if Test Unit Ready and even this only if not
> send more then once every few seconds.

Currently (2.6.8.1), there is a list in the kernel. I agree that it
doesn't make sense. I would think read permission means to be able
to read from the device, write means you can write. I would even go
as far as *not* to have that mean "you can also read/write via SG_IO",
because for normal uses of the device, read(2) and write(2) should be
enough.

BTW, there are a number of people on the kernel list who believe a
filter list is bad and generally unmaintainable.

> There are several SCSI commands that look safe but would result in coasters
> if issued while a CD or DVD is written.

Good point.

> The best immediate fix for the problem is to just check for read & write
> permissions on the file descriptor and otherwise revert to how it has been
> before 2.6.8.

I don't think that's going to happen. You already said you'd be okay
with euid==0 being required for burning, if only the transition
period were longer. So if people complain to you that cdrecord is
broken with 2.6.8, you will have to tell them burning requires root
for the moment. Then in your next release, change your startup
code not to drop the CAP_SYS_RAWIO capability when you drop root
privileges.

Alternatively, provide a patch that changes the current code to just
require write permission or CAP_SYS_RAWIO to be able to send
arbitrary commands. Then, after a transition period, submit a patch
that changes it to just CAP_SYS_RAWIO. The patch would look like the
one below (untested).

Jens, since this seems to be your code, what do you think?

--- scsi_ioctl.c 2004-08-14 18:26:17.000000000 +0200
+++ scsi_ioctl.c.new 2004-08-22 15:08:36.000000000 +0200
@@ -105,70 +105,12 @@ static int sg_emulated_host(request_queu
return put_user(1, p);
}

-#define CMD_READ_SAFE 0x01
-#define CMD_WRITE_SAFE 0x02
-#define safe_for_read(cmd) [cmd] = CMD_READ_SAFE
-#define safe_for_write(cmd) [cmd] = CMD_WRITE_SAFE
-
-static int verify_command(struct file *file, unsigned char *cmd)
+static int verify_command(struct file *file)
{
- static const unsigned char cmd_type[256] = {
-
- /* Basic read-only commands */
- safe_for_read(TEST_UNIT_READY),
- safe_for_read(REQUEST_SENSE),
- safe_for_read(READ_6),
- safe_for_read(READ_10),
- safe_for_read(READ_12),
- safe_for_read(READ_16),
- safe_for_read(READ_BUFFER),
- safe_for_read(READ_LONG),
- safe_for_read(INQUIRY),
- safe_for_read(MODE_SENSE),
- safe_for_read(MODE_SENSE_10),
- safe_for_read(START_STOP),
-
- /* Audio CD commands */
- safe_for_read(GPCMD_PLAY_CD),
- safe_for_read(GPCMD_PLAY_AUDIO_10),
- safe_for_read(GPCMD_PLAY_AUDIO_MSF),
- safe_for_read(GPCMD_PLAY_AUDIO_TI),
-
- /* CD/DVD data reading */
- safe_for_read(GPCMD_READ_CD),
- safe_for_read(GPCMD_READ_CD_MSF),
- safe_for_read(GPCMD_READ_DISC_INFO),
- safe_for_read(GPCMD_READ_CDVD_CAPACITY),
- safe_for_read(GPCMD_READ_DVD_STRUCTURE),
- safe_for_read(GPCMD_READ_HEADER),
- safe_for_read(GPCMD_READ_TRACK_RZONE_INFO),
- safe_for_read(GPCMD_READ_SUBCHANNEL),
- safe_for_read(GPCMD_READ_TOC_PMA_ATIP),
- safe_for_read(GPCMD_REPORT_KEY),
- safe_for_read(GPCMD_SCAN),
-
- /* Basic writing commands */
- safe_for_write(WRITE_6),
- safe_for_write(WRITE_10),
- safe_for_write(WRITE_VERIFY),
- safe_for_write(WRITE_12),
- safe_for_write(WRITE_VERIFY_12),
- safe_for_write(WRITE_16),
- safe_for_write(WRITE_BUFFER),
- safe_for_write(WRITE_LONG),
- };
- unsigned char type = cmd_type[cmd[0]];
-
- /* Anybody who can open the device can do a read-safe command */
- if (type & CMD_READ_SAFE)
+ /* write access means being able to send any command (for now) */
+ if (file->f_mode & FMODE_WRITE)
return 0;

- /* Write-safe commands just require a writable open.. */
- if (type & CMD_WRITE_SAFE) {
- if (file->f_mode & FMODE_WRITE)
- return 0;
- }
-
/* And root can do any command.. */
if (capable(CAP_SYS_RAWIO))
return 0;
@@ -181,7 +123,7 @@ static int sg_io(struct file *file, requ
struct gendisk *bd_disk, struct sg_io_hdr *hdr)
{
unsigned long start_time;
- int reading, writing;
+ int reading, writing, res;
struct request *rq;
struct bio *bio;
char sense[SCSI_SENSE_BUFFERSIZE];
@@ -193,8 +135,8 @@ static int sg_io(struct file *file, requ
return -EINVAL;
if (copy_from_user(cmd, hdr->cmdp, hdr->cmd_len))
return -EFAULT;
- if (verify_command(file, cmd))
- return -EPERM;
+ if (res = verify_command(file))
+ return res;

/*
* we'll do that later

--
Ciao,
Pascal