From: Tomasz Jeznach <tjeznach@rivosinc.com>

The IMSIC state variable eistate[] is modified by CSR instructions
within a range dedicated to the local CPU and by MMIO writes from any CPU.
Access to eistate from MMIO accessors is protected by the BQL, but
read-modify-write (RMW) sequences from CSRRW do not acquire the BQL,
making the RMW sequence vulnerable to a race condition with MMIO access
from a remote CPU.

This race can manifest as missing IPI or MSI in multi-CPU systems, eg:

[   43.008092] watchdog: BUG: soft lockup - CPU#2 stuck for 27s! [kworker/u19:1:52]
[   43.011723] CPU: 2 UID: 0 PID: 52 Comm: kworker/u19:1 Not tainted 6.11.0-rc6
[   43.013070] Workqueue: events_unbound deferred_probe_work_func
[   43.018776] [<ffffffff800b4a86>] smp_call_function_many_cond+0x190/0x5c2
[   43.019205] [<ffffffff800b4f28>] on_each_cpu_cond_mask+0x20/0x32
[   43.019447] [<ffffffff8001069a>] __flush_tlb_range+0xf2/0x190
[   43.019683] [<ffffffff80010914>] flush_tlb_kernel_range+0x20/0x28

The interrupt line raise/lower sequence was changed to prevent a race
between the evaluation of the eistate and the execution of the qemu_irq
raise/lower, ensuring that the interrupt line is not incorrectly
deactivated based on a stale topei check result. To avoid holding BQL
all modifications of eistate are converted to atomic operations.

Signed-off-by: Tomasz Jeznach <tjeznach@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <a7604e4d61068ca4d384ae2a1377e1521d4d0235.1725651699.git.tjeznach@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
---
 hw/intc/riscv_imsic.c | 50 +++++++++++++++++++++++++++----------------
 1 file changed, 32 insertions(+), 18 deletions(-)

diff --git a/hw/intc/riscv_imsic.c b/hw/intc/riscv_imsic.c
index b90f0d731d..9ef65d4012 100644
--- a/hw/intc/riscv_imsic.c
+++ b/hw/intc/riscv_imsic.c
@@ -55,7 +55,7 @@ static uint32_t riscv_imsic_topei(RISCVIMSICState *imsic, uint32_t page)
                (imsic->eithreshold[page] <= imsic->num_irqs)) ?
                imsic->eithreshold[page] : imsic->num_irqs;
     for (i = 1; i < max_irq; i++) {
-        if ((imsic->eistate[base + i] & IMSIC_EISTATE_ENPEND) ==
+        if ((qatomic_read(&imsic->eistate[base + i]) & IMSIC_EISTATE_ENPEND) ==
                 IMSIC_EISTATE_ENPEND) {
             return (i << IMSIC_TOPEI_IID_SHIFT) | i;
         }
@@ -66,10 +66,24 @@ static uint32_t riscv_imsic_topei(RISCVIMSICState *imsic, uint32_t page)
 
 static void riscv_imsic_update(RISCVIMSICState *imsic, uint32_t page)
 {
+    uint32_t base = page * imsic->num_irqs;
+
+    /*
+     * Lower the interrupt line if necessary, then evaluate the current
+     * IMSIC state.
+     * This sequence ensures that any race between evaluating the eistate and
+     * updating the interrupt line will not result in an incorrectly
+     * deactivated connected CPU IRQ line.
+     * If multiple interrupts are pending, this sequence functions identically
+     * to qemu_irq_pulse.
+     */
+
+    if (qatomic_fetch_and(&imsic->eistate[base], ~IMSIC_EISTATE_ENPEND)) {
+        qemu_irq_lower(imsic->external_irqs[page]);
+    }
     if (imsic->eidelivery[page] && riscv_imsic_topei(imsic, page)) {
         qemu_irq_raise(imsic->external_irqs[page]);
-    } else {
-        qemu_irq_lower(imsic->external_irqs[page]);
+        qatomic_or(&imsic->eistate[base], IMSIC_EISTATE_ENPEND);
     }
 }
 
@@ -125,12 +139,11 @@ static int riscv_imsic_topei_rmw(RISCVIMSICState *imsic, uint32_t page,
         topei >>= IMSIC_TOPEI_IID_SHIFT;
         base = page * imsic->num_irqs;
         if (topei) {
-            imsic->eistate[base + topei] &= ~IMSIC_EISTATE_PENDING;
+            qatomic_and(&imsic->eistate[base + topei], ~IMSIC_EISTATE_PENDING);
         }
-
-        riscv_imsic_update(imsic, page);
     }
 
+    riscv_imsic_update(imsic, page);
     return 0;
 }
 
@@ -139,7 +152,7 @@ static int riscv_imsic_eix_rmw(RISCVIMSICState *imsic,
                                uint32_t num, bool pend, target_ulong *val,
                                target_ulong new_val, target_ulong wr_mask)
 {
-    uint32_t i, base;
+    uint32_t i, base, prev;
     target_ulong mask;
     uint32_t state = (pend) ? IMSIC_EISTATE_PENDING : IMSIC_EISTATE_ENABLED;
 
@@ -157,10 +170,6 @@ static int riscv_imsic_eix_rmw(RISCVIMSICState *imsic,
 
     if (val) {
         *val = 0;
-        for (i = 0; i < xlen; i++) {
-            mask = (target_ulong)1 << i;
-            *val |= (imsic->eistate[base + i] & state) ? mask : 0;
-        }
     }
 
     for (i = 0; i < xlen; i++) {
@@ -172,10 +181,15 @@ static int riscv_imsic_eix_rmw(RISCVIMSICState *imsic,
         mask = (target_ulong)1 << i;
         if (wr_mask & mask) {
             if (new_val & mask) {
-                imsic->eistate[base + i] |= state;
+                prev = qatomic_fetch_or(&imsic->eistate[base + i], state);
             } else {
-                imsic->eistate[base + i] &= ~state;
+                prev = qatomic_fetch_and(&imsic->eistate[base + i], ~state);
             }
+        } else {
+            prev = qatomic_read(&imsic->eistate[base + i]);
+        }
+        if (val && (prev & state)) {
+            *val |= mask;
         }
     }
 
@@ -302,14 +316,14 @@ static void riscv_imsic_write(void *opaque, hwaddr addr, uint64_t value,
     page = addr >> IMSIC_MMIO_PAGE_SHIFT;
     if ((addr & (IMSIC_MMIO_PAGE_SZ - 1)) == IMSIC_MMIO_PAGE_LE) {
         if (value && (value < imsic->num_irqs)) {
-            imsic->eistate[(page * imsic->num_irqs) + value] |=
-                                                    IMSIC_EISTATE_PENDING;
+            qatomic_or(&imsic->eistate[(page * imsic->num_irqs) + value],
+                       IMSIC_EISTATE_PENDING);
+
+            /* Update CPU external interrupt status */
+            riscv_imsic_update(imsic, page);
         }
     }
 
-    /* Update CPU external interrupt status */
-    riscv_imsic_update(imsic, page);
-
     return;
 
 err:
-- 
2.46.0