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SubjectRe: [PATCH 02/24] compat: scsi: sg: fix v3 compat read/write interface
On Wed, Dec 11, 2019 at 09:42:36PM +0100, Arnd Bergmann wrote:
> In the v5.4 merge window, a cleanup patch from Al Viro conflicted
> with my rework of the compat handling for sg.c read(). Linus Torvalds
> did a correct merge but pointed out that the resulting code is still
> unsatisfactory.
>
> I later noticed that the sg_new_read() function still gets the compat
> mode wrong, when the 'count' argument is large enough to pass a
> compat_sg_io_hdr object, but not a nativ sg_io_hdr.
>
> To address both of these, move the definition of compat_sg_io_hdr
> into a scsi/sg.h to make it visible to sg.c and rewrite the logic
> for reading req_pack_id as well as the size check to a simpler
> version that gets the expected results.
>
> Fixes: c35a5cfb4150 ("scsi: sg: sg_read(): simplify reading ->pack_id of userland sg_io_hdr_t")
> Fixes: 98aaaec4a150 ("compat_ioctl: reimplement SG_IO handling")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> block/scsi_ioctl.c | 29 +----------
> drivers/scsi/sg.c | 125 +++++++++++++++++++++------------------------
> include/scsi/sg.h | 30 +++++++++++
> 3 files changed, 89 insertions(+), 95 deletions(-)
>
> diff --git a/block/scsi_ioctl.c b/block/scsi_ioctl.c
> index 650bade5ea5a..b61dbf4d8443 100644
> --- a/block/scsi_ioctl.c
> +++ b/block/scsi_ioctl.c
> @@ -20,6 +20,7 @@
> #include <scsi/scsi.h>
> #include <scsi/scsi_ioctl.h>
> #include <scsi/scsi_cmnd.h>
> +#include <scsi/sg.h>
>
> struct blk_cmd_filter {
> unsigned long read_ok[BLK_SCSI_CMD_PER_LONG];
> @@ -550,34 +551,6 @@ static inline int blk_send_start_stop(struct request_queue *q,
> return __blk_send_generic(q, bd_disk, GPCMD_START_STOP_UNIT, data);
> }
>
> -#ifdef CONFIG_COMPAT
> -struct compat_sg_io_hdr {
> - compat_int_t interface_id; /* [i] 'S' for SCSI generic (required) */
> - compat_int_t dxfer_direction; /* [i] data transfer direction */
> - unsigned char cmd_len; /* [i] SCSI command length ( <= 16 bytes) */
> - unsigned char mx_sb_len; /* [i] max length to write to sbp */
> - unsigned short iovec_count; /* [i] 0 implies no scatter gather */
> - compat_uint_t dxfer_len; /* [i] byte count of data transfer */
> - compat_uint_t dxferp; /* [i], [*io] points to data transfer memory
> - or scatter gather list */
> - compat_uptr_t cmdp; /* [i], [*i] points to command to perform */
> - compat_uptr_t sbp; /* [i], [*o] points to sense_buffer memory */
> - compat_uint_t timeout; /* [i] MAX_UINT->no timeout (unit: millisec) */
> - compat_uint_t flags; /* [i] 0 -> default, see SG_FLAG... */
> - compat_int_t pack_id; /* [i->o] unused internally (normally) */
> - compat_uptr_t usr_ptr; /* [i->o] unused internally */
> - unsigned char status; /* [o] scsi status */
> - unsigned char masked_status; /* [o] shifted, masked scsi status */
> - unsigned char msg_status; /* [o] messaging level data (optional) */
> - unsigned char sb_len_wr; /* [o] byte count actually written to sbp */
> - unsigned short host_status; /* [o] errors from host adapter */
> - unsigned short driver_status; /* [o] errors from software driver */
> - compat_int_t resid; /* [o] dxfer_len - actual_transferred */
> - compat_uint_t duration; /* [o] time taken by cmd (unit: millisec) */
> - compat_uint_t info; /* [o] auxiliary information */
> -};
> -#endif
> -
> int put_sg_io_hdr(const struct sg_io_hdr *hdr, void __user *argp)
> {
> #ifdef CONFIG_COMPAT
> diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
> index 160748ad9c0f..985546aac236 100644
> --- a/drivers/scsi/sg.c
> +++ b/drivers/scsi/sg.c
> @@ -198,6 +198,7 @@ static void sg_device_destroy(struct kref *kref);
>
> #define SZ_SG_HEADER sizeof(struct sg_header)
> #define SZ_SG_IO_HDR sizeof(sg_io_hdr_t)
> +#define SZ_COMPAT_SG_IO_HDR sizeof(struct compat_sg_io_hdr)

I'd rather not add more defines like this. The raw sizeof is
much more readable and obvious.

>
> + if (count < SZ_SG_HEADER)
> + goto unknown_id;
> +
> + /* negative reply_len means v3 format, otherwise v1/v2 */
> + if (get_user(reply_len, &old_hdr->reply_len))
> + return -EFAULT;
> + if (reply_len >= 0)
> + return get_user(*pack_id, &old_hdr->pack_id);
> +
> + if (in_compat_syscall() && count >= SZ_COMPAT_SG_IO_HDR) {
> + struct compat_sg_io_hdr __user *hp = buf;
> + return get_user(*pack_id, &hp->pack_id);
> + }
> +
> + if (count >= SZ_SG_IO_HDR) {
> + struct sg_io_hdr __user *hp = buf;
> + return get_user(*pack_id, &hp->pack_id);
> + }
> +
> +unknown_id:
> + /* no valid header was passed, so ignore the pack_id */
> + *pack_id = -1;
> + return 0;
> +}

I find the structure here a little confusing, as it doesn't follow
the normal flow. What do you think of:

if (count >= SZ_SG_HEADER) {
if (get_user(reply_len, &old_hdr->reply_len))
return -EFAULT;

/* negative reply_len means v3 format, otherwise v1/v2 */
if (reply_len >= 0)
return get_user(*pack_id, &old_hdr->pack_id);

if (in_compat_syscall() {
if (count >= SZ_COMPAT_SG_IO_HDR) {
struct compat_sg_io_hdr __user *hp = buf;

return get_user(*pack_id, &hp->pack_id);
}
} else {
if (count >= SZ_SG_IO_HDR) {
struct sg_io_hdr __user *hp = buf;

return get_user(*pack_id, &hp->pack_id);
}
}
}

/* no valid header was passed, so ignore the pack_id */
*pack_id = -1;
return 0;
}

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