linux-kernel - re: dmaengine: idxd: fix submission race window

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Message-ID: <92a5510c-f426-1001-5311-6c615df2e7de@canonical.com>
Date:   Thu, 22 Jul 2021 19:56:53 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Dave Jiang <dave.jiang@...el.com>
Cc:     Vinod Koul <vkoul@...nel.org>, dmaengine@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: re: dmaengine: idxd: fix submission race window

Hi,

Static analysis with Coverity on Linux-next has found an issue with the
following commit:

commit 6b4b87f2c31ac1af4f244990a7cbfb50d3f3e33f
Author: Dave Jiang <dave.jiang@...el.com>
Date:   Wed Jul 14 11:50:06 2021 -0700

    dmaengine: idxd: fix submission race window

The analysis is as follows:

180static int irq_process_pending_llist(struct idxd_irq_entry *irq_entry,
181                                     int *processed, u64 data)
182{
183        struct idxd_desc *desc, *t;
184        struct llist_node *head;
185        int queued = 0;
186        unsigned long flags;
187
188        *processed = 0;
189        head = llist_del_all(&irq_entry->pending_llist);
190        if (!head)
191                goto out;
192
193        llist_for_each_entry_safe(desc, t, head, llnode) {

   assignment: Assigning: status = (*desc).completion->status & 0x7f.

194                u8 status = desc->completion->status &
DSA_COMP_STATUS_MASK;
195

   cond_between: Condition status, taking true branch. Now the value of
status is between 1 and 127.
   cond_cannot_single: Condition status, taking true branch. Now the
value of status cannot be equal to 0.

196                if (status) {

   between: At condition status == IDXD_COMP_DESC_ABORT, the value of
status must be between 1 and 127.
   cond_cannot_set: Condition status == IDXD_COMP_DESC_ABORT, taking
false branch. Now the value of status cannot be equal to any of {0, 255}.

   cannot_single: At condition status == IDXD_COMP_DESC_ABORT, the value
of status cannot be equal to 0.
   dead_error_condition: The condition !!(status ==
IDXD_COMP_DESC_ABORT) cannot be true.

197                        if (unlikely(status == IDXD_COMP_DESC_ABORT)) {
   Logically dead code (DEADCODE)
   dead_error_begin: Execution cannot reach this statement:

   complete_desc(desc, IDXD_CO....

198                                complete_desc(desc, IDXD_COMPLETE_ABORT);
199                                (*processed)++;
200                                continue;
201                        }
202
203                        complete_desc(desc, IDXD_COMPLETE_NORMAL);
204                        (*processed)++;

The check (status == IDXD_COMP_DESC_ABORT) is always false since status
was previously masked with 0x7f and IDXD_COMP_DESC_ABORT is 0xff

Colin.