Introduction to Kernel Page Faults in User Space
Kernel page faults in user space are critical errors that occur when a process accesses memory that the kernel cannot map. These faults often lead to system instability, crashes, or panics, and are common in kernel development or system administration scenarios involving custom kernel modules, hardware drivers, or low-level memory operations. Understanding their root causes requires familiarity with virtual memory systems, page tables, and the Linux kernel’s memory management subsystem.
Symptoms of Kernel Page Faults
The symptoms of a kernel page fault are typically severe and include:
-
BUG: unable to handle kernel paging request at virtual address [hex address]
in kernel logs (dmesg) -
Process crashes with
Segmentation Fault (SIGSEGV)
orGeneral Protection Fault (GPF)
-
System-wide freezes or kernel panics (if the fault occurs in kernel space)
-
Userspace applications failing to allocate memory, even with sufficient physical RAM
Root Cause Analysis
Page faults in user space often stem from invalid memory access patterns. Common root causes include:
Use-After-Free Vulnerabilities
Accessing memory after it has been freed by the kernel, leading to undefined behavior.
Null Pointer Dereference
Attempting to access a null pointer in a kernel module or driver, which results in a fault when the MMU tries to map the address.
Incorrect Page Table Entries
Malformed or improperly configured page tables, often due to custom memory management code or hardware-specific issues.
Invalid User-Space Address Handling
Improper use of copy_from_user()
or copy_to_user()
functions, leading to malformed virtual addresses.
Diagnosis Tools and Techniques
Effective diagnosis requires specialized tools and methods:
Kernel Logs (dmesg)
Inspect dmesg
output for error messages indicating the faulty address and context. Example:
BUG: unable to handle kernel paging request at virtual address ffffc900018c5000
GNU Debugger (gdb)
Use gdb
to analyze kernel crash dumps or core files with debug symbols enabled. Example command:
gdb -k /usr/lib/debug/lib/modules/$(uname -r)/vmlinux /var/crash/vmcore
SystemTap or eBPF Tracing
Instrument kernel functions like handle_mm_fault()
or do_page_fault()
to track memory access patterns.
probe kernel.function("do_page_fault") { printf("Page fault at %p\n", $address) }
Kernel Oops Analysis
Parse Oops
messages to identify the instruction pointer (RIP) and stack trace. Tools like crash
or gdb
are essential here.
Example Code: Reproducing a Use-After-Free Page Fault
The following kernel module demonstrates a use-after-free scenario:
#include <linux/module.h>
#include <linux/slab.h>
static struct my_struct *ptr;
static int __init faulty_init(void) {
ptr = kmalloc(sizeof(struct my_struct), GFP_KERNEL);
if (!ptr)
return -ENOMEM;
kfree(ptr);
ptr->data = 0; // This line causes a page fault after free
return 0;
}
static void __exit faulty_exit(void) {
if (ptr)
kfree(ptr);
}
MODULE_LICENSE("GPL");
module_init(faulty_init);
module_exit(faulty_exit);
This code allocates memory with kmalloc()
, frees it, and then attempts to access the freed pointer, triggering a page fault.
Step-by-Step Solution to Resolve the Issue
To resolve kernel page faults in userspace, follow these steps:
1. Reproduce the Fault in a Controlled Environment
Test the issue in a virtual machine or isolated system to avoid data loss. Use modprobe
to load the faulty module and observe dmesg
output.
2. Analyze the Kernel Log
Identify the virtual address and the context of the fault. Example:
[23456.789012] BUG: unable to handle kernel paging request at virtual address ffffc900018c5000
Use addr2line
with debug symbols to map the address to a source file and line:
addr2line -f -e /usr/lib/debug/lib/modules/$(uname -r)/vmlinux -s ffffc900018c5000
3. Locate the Faulty Code Path
Use gdb
or crash
to inspect the stack trace. For example:
(gdb) bt
#0 do_page_fault () at arch/x86/mm/fault.c:123
#1 page_fault_handler () at arch/x86/mm/fault.c:245
Identify the module and function responsible for the invalid access.
4. Validate Memory Allocation Logic
Review code for improper use of memory management functions. Ensure kmalloc()
is paired with kfree()
and that pointers are set to NULL
after freeing.
5. Apply Fixes and Recompile
Modify the code to avoid use-after-free, such as:
ptr = kmalloc(...);
if (ptr) {
ptr->data = 0;
kfree(ptr);
ptr = NULL; // Explicitly nullify after free
}
Recompile the module and test the fix using modprobe
.
6. Monitor with Perf or ftrace
Use perf
or ftrace
to trace memory allocation and deallocation events post-fix:
perf record -e mm:kmalloc -e mm:kfree
Validate that the fault no longer occurs under load.