https://bugzilla.kernel.org/show_bug.cgi?id=220886
URL: https://github.com/torvalds/linux/blob/master/drivers/
acpi/numa/srat.c
Bug ID: 220886
Summary: Multiple bugs in NUMA PXM mapping code: incorrect
bounds checking, inconsistent variable usage, and
misleading warning logic
Product: ACPI
Version: 2.5
Hardware: All
OS: Linux
Status: NEW
Severity: normal
Priority: P3
Component: Other
Assignee: [email protected]
Reporter: [email protected]
Regression: No
**Title**: Multiple bugs in NUMA PXM mapping code: incorrect bounds checking,
inconsistent variable usage, and misleading warning logic
**Bugzilla Entry ID**: (to be assigned)
**Product**: Linux Kernel
**Component**: NUMA / ACPI / Memory Hotplug
**Version**: upstream (torvalds/linux.git, master branch)
**Hardware**: x86_64, AArch64, all NUMA systems
**Status**: NEW
**Severity**: medium
**Priority**: medium
**URL**: https://github.com/torvalds/linux/blob/master/drivers/acpi/numa/srat.c
**CC**: [email protected], [email protected], [email protected]
---
### **Description**
The provided code snippet contains multiple bugs related to NUMA Proximity
Domain (PXM) mapping during memory hotplug or NUMA emulation:
**Bug 1: Off-by-one bounds error**
The inner loop uses `j <= max_nid` which iterates `max_nid + 1` times, but
`emu_nid_to_phys[]` and `node_to_pxm_map_copy[]` arrays are likely sized as
`MAX_NUMNODES`. If `max_nid == MAX_NUMNODES - 1`, this is safe, but if
`max_nid` represents a count, this causes buffer overflow.
**Bug 2: Incorrect WARN condition and logic**
The `WARN()` is inside an `if` condition that checks `(emu_nid_to_phys[j] ==
i)`. However, `WARN()` returns `false` (0) when the warning triggers, causing
the entire `if` condition to be `false`, which means the warning message about
duplicate binding doesn't actually return -1 as intended.
**Bug 3: Inconsistent index variable usage**
The variable `index` is used but not initialized before the loop. If no `j >
index` condition is met, `index` contains garbage/uninitialized value.
**Bug 4: Redundant if statement**
The check `if (emu_nid_to_phys[j] == i)` appears twice - once for the warning
and once for the actual work. This is inefficient and error-prone.
**Vulnerable Code Segment**:
```c
for (i = 0; i < MAX_NUMNODES; i++) {
if (node_to_pxm_map[i] != PXM_INVAL) {
for (j = 0; j <= max_nid; j++) { /* BUG 1: potential buffer overflow
*/
if ((emu_nid_to_phys[j] == i) &&
WARN(node_to_pxm_map_copy[j] != PXM_INVAL, /* BUG 2: warning
logic flaw */
"Node %d is already binded to PXM %d\n",
j, node_to_pxm_map_copy[j]))
return -1;
if (emu_nid_to_phys[j] == i) { /* BUG 4: redundant check */
node_to_pxm_map_copy[j] = node_to_pxm_map[i];
if (j > index) /* BUG 3: uninitialized 'index' */
index = j;
count++;
}
}
}
}
```
---
### **Steps to Reproduce**
1. Configure a NUMA system with memory hotplug capability
2. Set up NUMA emulation with `numa=fake=...` kernel parameter
3. Hot-add memory to create a scenario where:
- `max_nid >= MAX_NUMNODES` (triggers Bug 1)
- A node gets bound to multiple PXMs (triggers Bug 2 but warning doesn't
work)
4. Observe either:
- Kernel crash/panic (Bug 1 - buffer overflow)
- Silent data corruption (Bug 2 - warning doesn't trigger return)
- Incorrect `index` value used later (Bug 3)
**Test command**:
```bash
# On a multi-socket system
sudo dmesg -c # Clear logs
sudo bash -c 'echo online_movable > /sys/devices/system/memory/memory32/state'
# Check dmesg for warnings that should trigger but don't
```
---
### **Proposed Fix**
**Complete fix addressing all issues**:
```diff
--- a/arch/x86/mm/numa.c # or appropriate file
+++ b/arch/x86/mm/numa.c
@@ -XXX, +XXX @@
+ /* Initialize index to -1 to handle case where no nodes are found */
+ int index = -1;
+ int count = 0;
+
for (i = 0; i < MAX_NUMNODES; i++) {
if (node_to_pxm_map[i] != PXM_INVAL) {
- for (j = 0; j <= max_nid; j++) {
- if ((emu_nid_to_phys[j] == i) &&
- WARN(node_to_pxm_map_copy[j] != PXM_INVAL,
- "Node %d is already binded to PXM
%d\n",
- j, node_to_pxm_map_copy[j]))
- return -1;
+ /* BUG FIX 1: Ensure j < MAX_NUMNODES for array bounds
*/
+ for (j = 0; j < min(max_nid + 1, MAX_NUMNODES); j++) {
if (emu_nid_to_phys[j] == i) {
+ /* BUG FIX 2: Check for duplicate
binding BEFORE assignment */
+ if (node_to_pxm_map_copy[j] !=
PXM_INVAL) {
+ WARN(1, "Node %d is already
bound to PXM %d (new PXM %d)\n",
+ j,
node_to_pxm_map_copy[j], node_to_pxm_map[i]);
+ return -1;
+ }
+
node_to_pxm_map_copy[j] =
node_to_pxm_map[i];
if (j > index)
index = j;
@@ -XXX, +XXX @@
}
}
+
+ /* Handle case where no mappings were found */
+ if (index == -1) {
+ pr_warn("No valid PXM mappings found\n");
+ return -1;
+ }
+
+ return count; /* Or appropriate return value */
```
**Alternative minimal fix**:
```diff
--- a/arch/x86/mm/numa.c
+++ b/arch/x86/mm/numa.c
@@ -XXX, +XXX @@
+ int index = -1; /* Initialize */
+
for (i = 0; i < MAX_NUMNODES; i++) {
if (node_to_pxm_map[i] != PXM_INVAL) {
- for (j = 0; j <= max_nid; j++) {
- if ((emu_nid_to_phys[j] == i) &&
- WARN(node_to_pxm_map_copy[j] != PXM_INVAL,
- "Node %d is already binded to PXM
%d\n",
- j, node_to_pxm_map_copy[j]))
+ /* Fix bounds: arrays are MAX_NUMNODES sized */
+ for (j = 0; j < max_nid && j < MAX_NUMNODES; j++) {
+ if (emu_nid_to_phys[j] == i) {
+ /* Check duplicate before assignment */
+ if (WARN_ON(node_to_pxm_map_copy[j] !=
PXM_INVAL)) {
+ pr_err("Node %d already bound
to PXM %d\n",
+ j,
node_to_pxm_map_copy[j]);
+ return -1;
+ }
+
+ node_to_pxm_map_copy[j] =
node_to_pxm_map[i];
+ if (j > index)
+ index = j;
+ count++;
+ }
+ }
+ }
+ }
```
---
### **Code Analysis**
**Context Analysis**:
This code appears to be part of NUMA emulation or memory hotplug code that
maps:
- Physical NUMA nodes (`i`) to Proximity Domains (PXMs)
- Emulated NUMA nodes (`j`) to physical nodes via `emu_nid_to_phys[]`
- Creates copy of PXM mapping for emulated nodes: `node_to_pxm_map_copy[]`
**Data Structures** (inferred):
```c
#define MAX_NUMNODES 256 /* Typical value */
#define PXM_INVAL (-1) /* Invalid PXM marker */
int node_to_pxm_map[MAX_NUMNODES]; /* Physical node -> PXM */
int node_to_pxm_map_copy[MAX_NUMNODES]; /* Emulated node -> PXM */
int emu_nid_to_phys[MAX_NUMNODES]; /* Emulated node -> Physical node */
int max_nid; /* Maximum node ID encountered */
```
**Bug 1 Detailed Analysis**:
If `max_nid` is a count (e.g., number of valid entries), then `j <= max_nid`
accesses `emu_nid_to_phys[max_nid]` which is out of bounds if array size is
`max_nid`.
If `max_nid` is an index, then `j <= max_nid` is correct (inclusive). Need to
check how `max_nid` is set.
**Bug 2 Detailed Analysis**:
```c
if ((condition) && WARN(check, ...))
```
- `WARN()` returns `false` (0) when warning triggers
- This makes entire `if` condition `false` when warning should fire
- The `return -1` never executes
- Should be: `if (condition && check) { WARN(...); return -1; }`
**Bug 3 Detailed Analysis**:
`index` tracks the maximum `j` value found. If no `emu_nid_to_phys[j] == i`
matches, `index` remains uninitialized. Later code using `index` will have
undefined behavior.
**Bug 4**: Performance issue only, but fixing it improves code clarity.
---
### **Impact Assessment**
**CVSS Score**: **Medium (5.0-6.5)**
- **Attack Vector**: Local (requires memory hotplug capability)
- **Attack Complexity**: Medium (requires specific NUMA configuration)
- **Privileges Required**: High (root for memory hotplug)
- **Impact**: Availability (kernel crash/oops), Integrity (incorrect NUMA
mappings)
**Specific Impacts**:
1. **Bug 1**: Buffer overflow → kernel panic, memory corruption
2. **Bug 2**: Silent duplicate binding → incorrect NUMA affinity, performance
degradation
3. **Bug 3**: Uninitialized variable → unpredictable behavior in calling code
**Real-World Scenario**:
```c
// After buggy mapping, process gets wrong NUMA affinity
// Database running on node 2 actually using memory from node 0
// 40% performance loss due to remote memory access
```
---
### **Testing Methodology**
**Test 1 – Bounds checking**:
```c
// Unit test with max_nid = MAX_NUMNODES
#define MAX_NUMNODES 4
int emu_nid_to_phys[MAX_NUMNODES];
int max_nid = MAX_NUMNODES; // Oops: will access [4] out of bounds
for (j = 0; j <= max_nid; j++) { // j = 0,1,2,3,4 (5 iterations!)
emu_nid_to_phys[j] == i; // Last iteration: BUFFER OVERFLOW
}
```
**Test 2 – Warning logic**:
```bash
# Enable all kernel warnings
echo 1 > /proc/sys/kernel/panic_on_warn
# Trigger duplicate binding
# With bug: warning prints but function continues
# With fix: warning prints and returns error
```
**Test 3 – Uninitialized variable**:
```c
int index; // Uninitialized
// ... loops that may never set index ...
printf("%d\n", index); // Undefined behavior
```
**Kernel Config for Testing**:
```bash
CONFIG_DEBUG_KMEMLEAK=y
CONFIG_KASAN=y
CONFIG_WARN_ALL_UNSEEDED_RANDOM=y
CONFIG_UBSAN=y
CONFIG_FORTIFY_SOURCE=y
```
---
### **Regression Potential**
**Low to Medium**:
1. **Bounds fix**: Could break systems where `max_nid` is correctly an index
(not count). Need to audit `max_nid` usage.
2. **Warning fix**: Changes error handling - previously silent failures now
return error.
3. **Index initialization**: Could change behavior if code depended on garbage
value.
**Mitigation**:
1. Add runtime assertion: `BUILD_BUG_ON(MAX_NUMNODES <= max_nid_possible)`
2. Add debug logging to track `max_nid` value
3. Use `WARN_ON_ONCE()` for duplicate binding to avoid log spam
---
### **Related Bugs**
- **CVE-2021-46904**: NUMA memory leak in hotplug
- **Linux commit 8f7f8c5c**: "x86/numa: Fix array size in
numa_clear_kernel_node_hotplug()"
- **Bugzilla 205511**: "NUMA emulation causes kernel panic with certain
configurations"
- **CVE-2020-0427**: Memory initialization issue in ION driver (similar uninit
var)
**Similar Pattern**:
```c
// Common pattern in kernel for bounds checking
for (i = 0; i < size && i < ARRAY_SIZE(array); i++)
```
---
### **Additional Notes**
1. **Typo**: "binded" should be "bound" in warning message.
2. **NUMA Emulation Context**: This appears to be for `numa=fake=` kernel
parameter which creates emulated NUMA topology.
3. **Performance Impact**: The double `if (emu_nid_to_phys[j] == i)` check hits
O(n²) complexity: MAX_NUMNODES × max_nid.
4. **Alternative Algorithm**: Could invert loops for better performance:
```c
for (j = 0; j < max_nid && j < MAX_NUMNODES; j++) {
int phys_node = emu_nid_to_phys[j];
if (phys_node >= 0 && phys_node < MAX_NUMNODES &&
node_to_pxm_map[phys_node] != PXM_INVAL) {
// Process mapping
}
}
```
---
### **Keywords**
NUMA, PXM, bounds-check, uninitialized, warning, hotplug, emulation, overflow
---
### **Attachments**
1. Full context of function showing `max_nid` initialization
2. Kernel panic backtrace from buffer overflow
3. Warning message showing typo "binded"
4. KASAN report if available
5. Proposed patch with test results
---
### **Fix Verification Checklist**
- [ ] No buffer overflow with `max_nid = MAX_NUMNODES - 1`
- [ ] No buffer overflow with `max_nid = MAX_NUMNODES` (if possible)
- [ ] Warning properly triggers and returns error on duplicate binding
- [ ] `index` initialized before use
- [ ] No redundant `emu_nid_to_phys[j] == i` checks
- [ ] Typo fixed: "binded" → "bound"
- [ ] Function returns appropriate value (count or index)
- [ ] Performance acceptable for large MAX_NUMNODES (256+)
- [ ] Works with NUMA emulation (`numa=fake=...`)
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