On 12-01-12 10:01 AM, Paolo Bonzini wrote:
Partition block devices or LVM volumes can be sent SCSI commands via
SG_IO, which are then passed down to the underlying device; it's
been this way forever, it was mentioned in 2004 in LKML at
https://lkml.org/lkml/2004/8/12/218 and it is even documented in the
sg_dd man page:
blk_sgio=1
when set to 0, block devices (e.g. /dev/sda) are treated
like normal files (i.e. read(2) and write(2) are used for
IO). When set to 1, block devices are assumed to accept the
SG_IO ioctl and SCSI commands are issued for IO. [...]
If the input or output device is a block device partition
(e.g. /dev/sda3) then setting this option causes the
partition information to be ignored (since access is
directly to the underlying device).
The ability to use the SG_IO ioctl on a block device was added at
the start of the lk 2.6 series. It should have been restricted to
non-partition block device nodes (e.g. allowed on /dev/sda,
disallowed on /dev/sda3).
The successor to sg_dd is called ddpt which will abort a copy when
the pass-through (requested by "iflag=pt") is used on a partition
node:
# ddpt if=/dev/sda3 iflag=pt bs=512 of=/dev/null count=1
>> warning: Size of input block device is different from pt size.
>> Pass-through on block partition can give unexpected offsets.
>> Abort copy, use iflag=force to override.
ddpt is ported to FreeBSD and Win32. The ability to call a pass-through
on a partition node is a Linux specific problem.
This is problematic because "safe" SCSI commands, including READ or WRITE,
can be sent to the disk without any particular capability. All that is
required is having a file descriptor for the block device, and permission
to send a ioctl. However, when a user lets a program access /dev/sda2,
it still should not be able to read/write /dev/sda outside the boundaries
of that partition.
Encryption on the host is a mitigating factor, but it does not provide
a full solution. In particular it doesn't protect against DoS (write
random data), replay attacks (reinstate old ciphertext sectors), or
writes to unencrypted areas including the MBR, the partition table, or
/boot.
The patches implement a simple global whitelist for both partitions
and partial disk mappings. Patch 1 refactors the code to prepare for
introduction of the whitelist, while patch 2 actually implements it for
the SCSI ioctls. Logical volumes are also affected if they have only one
target, and this target can pass ioctls to the underlying block device.
Patch 3 thus adds the whitelist to logical volumes as well.
This should be entirely independent of capabilities. Continuing the
previous example, if the same user gives CAP_SYS_RAWIO to the program and
write access to /dev/sdb, the program should be able to send arbitrary
SCSI commands to /dev/sdb, but still should not be able to access /dev/sda
outside the boundaries of /dev/sda2. However, for now when the program
has CAP_SYS_RAWIO the ioctls are let through (while still being logged
to dmesg).
drivers/ide/ has several ioctls that should only be restricted to the full
block device (for example HDIO_SET_*, HDIO_DRIVE_CMD, HDIO_DRIVE_TASK,
HDIO_DRIVE_RESET). However, all of them require either CAP_SYS_ADMIN
or CAP_SYS_RAWIO, so they do not need any change given the above interim
measure.
Tested on top of 3.2 + Linus's patch to sanitize ioctl return values.
Is that a fixed version of patch at the end of this post:
http://marc.info/?l=linux-kernel&m=132578310403616&w=2
The fix being
s/ENOIOCTLCMD/-ENOIOCTLCMD/
in is_unrecognized_ioctl() ?
If not could you post the patch you are referring to the linux-scsi
list. Also could you post "PATCH v2 3/3 ..." to this list as well so
we have a complete set?
Doug Gilbert
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