[PATCH v16 34/45] sg: protect multiple receivers

Douglas Gilbert
2021-02-20T02:07:22Z

If two threads call ioctl(SG_IORECEIVE) [or read()] on the same
file descriptor there is a potential race on the same request
response. Use atomic bit operations to make sure only one thread
gets each request response. [The other thread will either get
another request response or nothing.]

Also make sfp cleanup a bit more robust and report if the
number of submitted requests (which are decremented when
completed) is other than the expected value of zero.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
---
drivers/scsi/sg.c | 54 +++++++++++++++++++++++++++++++++--------------
1 file changed, 38 insertions(+), 16 deletions(-)

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index e049628bd575..d992c71a98ca 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -109,6 +109,7 @@ enum sg_rq_state { /* N.B. sg_rq_state_arr assumes SG_RS_AWAIT_RCV==2 */
#define SG_FRQ_SYNC_INVOC 2 /* synchronous (blocking) invocation */
#define SG_FRQ_NO_US_XFER 3 /* no user space transfer of data */
#define SG_FRQ_DEACT_ORPHAN 6 /* not keeping orphan so de-activate */
+#define SG_FRQ_RECEIVING 7 /* guard against multiple receivers */
#define SG_FRQ_BLK_PUT_REQ 8 /* set when blk_put_request() called */

/* Bit positions (flags) for sg_fd::ffd_bm bitmask follow */
@@ -1286,6 +1287,7 @@ sg_ctl_ioreceive(struct file *filp, struct sg_fd *sfp, void __user *p)
SG_LOG(3, sfp, "%s: non_block(+IMMED)=%d\n", __func__, non_block);
/* read in part of v3 or v4 header for pack_id or tag based find */
id = pack_id;
+try_again:
srp = sg_find_srp_by_id(sfp, id);
if (!srp) { /* nothing available so wait on packet or */
if (unlikely(SG_IS_DETACHING(sdp)))
@@ -1300,6 +1302,10 @@ sg_ctl_ioreceive(struct file *filp, struct sg_fd *sfp, void __user *p)
if (res)
return res; /* signal --> -ERESTARTSYS */
} /* now srp should be valid */
+ if (test_and_set_bit(SG_FRQ_RECEIVING, srp->frq_bm)) {
+ cpu_relax();
+ goto try_again;
+ }
return sg_receive_v4(sfp, srp, p, h4p);
}

@@ -1336,7 +1342,7 @@ sg_ctl_ioreceive_v3(struct file *filp, struct sg_fd *sfp, void __user *p)

if (test_bit(SG_FFD_FORCE_PACKID, sfp->ffd_bm))
pack_id = h3p->pack_id;
-
+try_again:
srp = sg_find_srp_by_id(sfp, pack_id);
if (!srp) { /* nothing available so wait on packet or */
if (unlikely(SG_IS_DETACHING(sdp)))
@@ -1351,6 +1357,10 @@ sg_ctl_ioreceive_v3(struct file *filp, struct sg_fd *sfp, void __user *p)
if (unlikely(res))
return res; /* signal --> -ERESTARTSYS */
} /* now srp should be valid */
+ if (test_and_set_bit(SG_FRQ_RECEIVING, srp->frq_bm)) {
+ cpu_relax();
+ goto try_again;
+ }
return sg_receive_v3(sfp, srp, SZ_SG_IO_HDR, p);
}

@@ -1503,6 +1513,7 @@ sg_read(struct file *filp, char __user *p, size_t count, loff_t *ppos)
want_id = h2p->pack_id;
}
}
+try_again:
srp = sg_find_srp_by_id(sfp, want_id);
if (!srp) { /* nothing available so wait on packet to arrive or */
if (unlikely(SG_IS_DETACHING(sdp)))
@@ -1518,6 +1529,10 @@ sg_read(struct file *filp, char __user *p, size_t count, loff_t *ppos)
return ret;
/* otherwise srp should be valid */
}
+ if (test_and_set_bit(SG_FRQ_RECEIVING, srp->frq_bm)) {
+ cpu_relax();
+ goto try_again;
+ }
if (srp->s_hdr3.interface_id == '\0')
ret = sg_read_v1v2(p, (int)count, sfp, srp);
else
@@ -3044,32 +3059,33 @@ sg_finish_scsi_blk_rq(struct sg_request *srp)
atomic_dec(&sfp->submitted);
atomic_dec(&sfp->waiting);
}
- if (srp->bio) {
- bool us_xfer = !test_bit(SG_FRQ_NO_US_XFER, srp->frq_bm);
-
- if (us_xfer) {
- ret = blk_rq_unmap_user(srp->bio);
- if (ret) { /* -EINTR (-4) can be ignored */
- SG_LOG(6, sfp,
- "%s: blk_rq_unmap_user() --> %d\n",
- __func__, ret);
- }
- }
- srp->bio = NULL;
- }
- /* In worst case READ data returned to user space by this point */

/* Expect blk_put_request(rq) already called in sg_rq_end_io() */
if (unlikely(!test_and_set_bit(SG_FRQ_BLK_PUT_REQ, srp->frq_bm))) {
struct request *rq = srp->rq;

+ srp->rq = NULL;
if (rq) { /* blk_get_request() may have failed */
if (scsi_req(rq))
scsi_req_free_cmd(scsi_req(rq));
- srp->rq = NULL;
blk_put_request(rq);
}
}
+ if (srp->bio) {
+ bool us_xfer = !test_bit(SG_FRQ_NO_US_XFER, srp->frq_bm);
+ struct bio *bio = srp->bio;
+
+ srp->bio = NULL;
+ if (us_xfer && bio) {
+ ret = blk_rq_unmap_user(bio);
+ if (ret) { /* -EINTR (-4) can be ignored */
+ SG_LOG(6, sfp,
+ "%s: blk_rq_unmap_user() --> %d\n",
+ __func__, ret);
+ }
+ }
+ }
+ /* In worst case, READ data returned to user space by this point */
}

static int
@@ -3499,6 +3515,7 @@ sg_deact_request(struct sg_fd *sfp, struct sg_request *srp)
return;
sbp = srp->sense_bp;
srp->sense_bp = NULL;
+ srp->frq_bm[0] = 0;
sg_rq_state_chg(srp, 0, SG_RS_INACTIVE, true /* force */, __func__);
/* maybe orphaned req, thus never read */
if (sbp)
@@ -3631,6 +3648,7 @@ static void
sg_remove_sfp_usercontext(struct work_struct *work)
{
__maybe_unused int o_count;
+ int subm;
unsigned long idx, iflags;
struct sg_device *sdp;
struct sg_fd *sfp = container_of(work, struct sg_fd, ew_fd.work);
@@ -3668,6 +3686,10 @@ sg_remove_sfp_usercontext(struct work_struct *work)
SG_LOG(6, sfp, "%s: kfree: srp=%pK --\n", __func__, srp);
kfree(srp);
}
+ subm = atomic_read(&sfp->submitted);
+ if (subm != 0)
+ SG_LOG(1, sfp, "%s: expected submitted=0 got %d\n",
+ __func__, subm);
xa_destroy(xafp);
xa_lock_irqsave(xadp, iflags);
e_sfp = __xa_erase(xadp, sfp->idx);
--
2.25.1