[PATCH v2 net 7/9] net: enetc: fix VF-to-PF message handler unbounded loop DoS

[Wei Fang <wei.fang@nxp.com>]

enetc_msg_task() processes VF-to-PF mailbox messages using an unbounded
for(;;) loop that repeatedly polls ENETC_PSIMSGRR until no MR bits set.

A malicious guest VM can exploit this by immediately sending a new
message as soon as the PF acknowledges the previous one via a w1c write
to ENETC_PSIMSGRR. Since there is no processing budget or yield point,
a VF can keep the MR bit continuously re-asserted, preventing the loop
from ever terminating and starving other tasks on the PF worker thread.

Fix this by replacing the unbounded loop with a single snapshot read of
ENETC_PSIMSGRR at task entry. The task processes only the VFs whose MR
bits were set at that point, clears the corresponding bits in
ENETC_PSIMSGRR and ENETC_PSIIDR, and re-enables the message interrupt
before returning.

No messages are lost: the message interrupt is disabled before
enetc_msg_task() is scheduled (in enetc_msg_psi_msix()), so new messages
arriving during processing do not generate additional CPU interrupts.
When enetc_msg_task() re-enables the interrupts at exit, the hardware
detects any MR bits that were set during execution and generates a new
interrupt, scheduling another task invocation.

This bounds the work per task invocation to at most num_vfs message
processing iterations, regardless of how aggressively VFs send messages.

Fixes: beb74ac878c8 ("enetc: Add vf to pf messaging support")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
---
 .../net/ethernet/freescale/enetc/enetc_hw.h   |  1 +
 .../net/ethernet/freescale/enetc/enetc_msg.c  | 38 +++++++++----------
 2 files changed, 20 insertions(+), 19 deletions(-)

diff --git a/drivers/net/ethernet/freescale/enetc/enetc_hw.h b/drivers/net/ethernet/freescale/enetc/enetc_hw.h
index 662e4fbafb74..39b82faaf041 100644
--- a/drivers/net/ethernet/freescale/enetc/enetc_hw.h
+++ b/drivers/net/ethernet/freescale/enetc/enetc_hw.h
@@ -96,6 +96,7 @@ static inline u32 enetc_vsi_set_msize(u32 size)
 #define ENETC_PSIIER	0xa00
 #define ENETC_PSIIER_MR_MASK	GENMASK(2, 1)
 #define ENETC_PSIIDR	0xa08
+#define ENETC_PSIIDR_MR(n)	BIT((n) + 1) /* n = VSI index */
 #define ENETC_SITXIDR	0xa18
 #define ENETC_SIRXIDR	0xa28
 #define ENETC_SIMSIVR	0xa30
diff --git a/drivers/net/ethernet/freescale/enetc/enetc_msg.c b/drivers/net/ethernet/freescale/enetc/enetc_msg.c
index b4d7457097e6..39f057fe85c7 100644
--- a/drivers/net/ethernet/freescale/enetc/enetc_msg.c
+++ b/drivers/net/ethernet/freescale/enetc/enetc_msg.c
@@ -32,32 +32,32 @@ static void enetc_msg_task(struct work_struct *work)
 {
 	struct enetc_pf *pf = container_of(work, struct enetc_pf, msg_task);
 	struct enetc_hw *hw = &pf->si->hw;
-	unsigned long mr_mask;
+	u32 mr_status;
 	int i;
 
-	for (;;) {
-		mr_mask = enetc_rd(hw, ENETC_PSIMSGRR) & ENETC_PSIMSGRR_MR_MASK;
-		if (!mr_mask) {
-			/* re-arm MR interrupts, w1c the IDR reg */
-			enetc_wr(hw, ENETC_PSIIDR, ENETC_PSIIER_MR_MASK);
-			enetc_msg_enable_mr_int(hw);
-			return;
-		}
+	mr_status = enetc_rd(hw, ENETC_PSIMSGRR) & ENETC_PSIMSGRR_MR_MASK;
+	if (!mr_status)
+		goto out;
 
-		for (i = 0; i < pf->num_vfs; i++) {
-			u32 psimsgrr;
-			u16 msg_code;
+	for (i = 0; i < pf->num_vfs; i++) {
+		u32 psimsgrr;
+		u16 msg_code;
+
+		if (!(ENETC_PSIMSGRR_MR(i) & mr_status))
+			continue;
 
-			if (!(ENETC_PSIMSGRR_MR(i) & mr_mask))
-				continue;
+		enetc_msg_handle_rxmsg(pf, i, &msg_code);
 
-			enetc_msg_handle_rxmsg(pf, i, &msg_code);
+		/* w1c to clear the corresponding VF MR bit */
+		enetc_wr(hw, ENETC_PSIIDR, ENETC_PSIIDR_MR(i));
 
-			psimsgrr = ENETC_SIMSGSR_SET_MC(msg_code);
-			psimsgrr |= ENETC_PSIMSGRR_MR(i); /* w1c */
-			enetc_wr(hw, ENETC_PSIMSGRR, psimsgrr);
-		}
+		psimsgrr = ENETC_SIMSGSR_SET_MC(msg_code);
+		psimsgrr |= ENETC_PSIMSGRR_MR(i); /* w1c */
+		enetc_wr(hw, ENETC_PSIMSGRR, psimsgrr);
 	}
+
+out:
+	enetc_msg_enable_mr_int(hw);
 }
 
 /* Init */
-- 
2.34.1