Re: [PATCH v9 2/7] kexec: define functions to map and unmap segments

steven chen
2025-03-17T18:26:28Z

On 3/5/2025 4:24 AM, Baoquan He wrote:
> On 03/04/25 at 04:55pm, steven chen wrote:
>> On 3/4/2025 2:23 PM, Jarkko Sakkinen wrote:
>>> On Tue, Mar 04, 2025 at 11:03:46AM -0800, steven chen wrote:
>>>> The content of memory segments carried over to the new kernel during the
>>>> kexec systemcall can be changed at kexec 'execute' stage, but the size of
>>>> the memory segments cannot be changed at kexec 'execute' stage.
>>>>
>>>> To copy IMA measurement logs during the kexec operation, IMA needs to
>>>> allocate memory at the kexec 'load' stage and map the segments to the
>>>> kimage structure. The mapped address will then be used to copy IMA
>>>> measurements during the kexec 'execute' stage.
>>>>
>>>> Currently, the mechanism to map and unmap segments to the kimage
>>>> structure is not available to subsystems outside of kexec.
>>> How does IMA work with kexec without having this? Just interested
>>> (and confused).
>> Currently, all IMA-related operations during a soft reboot, such as memory
>> allocation and IMA log list copy, are handled in the kexec 'load' stage, so
>> the map/unmap mechanism is not required.
>>
>> The new design separates these two operations into different stages: memory
>> allocation remains in the kexec 'load' stage, while the IMA log list copy is
>> moved to the kexec 'execute' stage. Therefore, the map/unmap mechanism is
>> introduced.
> I think the log can be improved. About the found problem and solution
> part, we possible can describe them like below:
>
> ===
> Currently, the kernel behaviour of kexec load is the IMA measurements
> log is fetched from TPM PCRs and stored into buffer and hold. When
> kexec reboot is triggered, the stored log buffer is carried over to the
> 2nd kernel. However, the time gap between kexec load and kexec reboot
> could be very long. Then those new events extended into TPM PCRs during
> the time window misses the chance to be carried over to 2nd kernel. This
> results in mismatch between TPM PCR quotes and the actual IMA measurements
> list after kexec reboot, which in turn results in remote attestation
> failure.
>
> To solve this problem, the new design is to defer the reading TPM PCRs
> content out into kexec buffer to kexec reboot phase. While still
> allocating the necessary buffer at kexec load time because it's not
> appropriate to allocate memory at kexec reboot moment.
> ===
>
> It may still need be improved, just for your reference. You can change
> and add more detail needed and add them into your log.
>
>> Please refer to "[PATCH v9 0/7] ima: kexec: measure events between kexec
>> load and execute" for the reason why to add this.
>>
>> Steven
>>
>>>> Implement kimage_map_segment() to enable IMA to map measurement log list to
>>>> the kimage structure during kexec 'load' stage. This function takes a kimage
>>>> pointer, a memory address, and a size, then gathers the
>>>> source pages within the specified address range, creates an array of page
>>>> pointers, and maps these to a contiguous virtual address range. The
>>>> function returns the start virtual address of this range if successful, or NULL on
>>>> failure.
>>>>
>>>> Implement kimage_unmap_segment() for unmapping segments
>>>> using vunmap().
>>>>
>>>> From: Tushar Sugandhi <tusharsu@linux.microsoft.com>
>>>> Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
>>>> Cc: Eric Biederman <ebiederm@xmission.com>
>>>> Cc: Baoquan He <bhe@redhat.com>
>>>> Cc: Vivek Goyal <vgoyal@redhat.com>
>>>> Cc: Dave Young <dyoung@redhat.com>
>>>> Signed-off-by: steven chen <chenste@linux.microsoft.com>
>>>> ---
>>>> include/linux/kexec.h | 6 +++++
>>>> kernel/kexec_core.c | 54 +++++++++++++++++++++++++++++++++++++++++++
>>>> 2 files changed, 60 insertions(+)
>>>>
>>>> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
>>>> index f0e9f8eda7a3..7d6b12f8b8d0 100644
>>>> --- a/include/linux/kexec.h
>>>> +++ b/include/linux/kexec.h
>>>> @@ -467,13 +467,19 @@ extern bool kexec_file_dbg_print;
>>>> #define kexec_dprintk(fmt, arg...) \
>>>> do { if (kexec_file_dbg_print) pr_info(fmt, ##arg); } while (0)
>>>> +extern void *kimage_map_segment(struct kimage *image, unsigned long addr, unsigned long size);
>>>> +extern void kimage_unmap_segment(void *buffer);
>>>> #else /* !CONFIG_KEXEC_CORE */
>>>> struct pt_regs;
>>>> struct task_struct;
>>>> +struct kimage;
>>>> static inline void __crash_kexec(struct pt_regs *regs) { }
>>>> static inline void crash_kexec(struct pt_regs *regs) { }
>>>> static inline int kexec_should_crash(struct task_struct *p) { return 0; }
>>>> static inline int kexec_crash_loaded(void) { return 0; }
>>>> +static inline void *kimage_map_segment(struct kimage *image, unsigned long addr, unsigned long size)
>>>> +{ return NULL; }
>>>> +static inline void kimage_unmap_segment(void *buffer) { }
>>>> #define kexec_in_progress false
>>>> #endif /* CONFIG_KEXEC_CORE */
>>>> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
>>>> index c0bdc1686154..63e4d16b6023 100644
>>>> --- a/kernel/kexec_core.c
>>>> +++ b/kernel/kexec_core.c
>>>> @@ -867,6 +867,60 @@ int kimage_load_segment(struct kimage *image,
>>>> return result;
>>>> }
>>>> +void *kimage_map_segment(struct kimage *image,
>>>> + unsigned long addr, unsigned long size)
>>>> +{
>>>> + unsigned long eaddr = addr + size;
>>>> + unsigned long src_page_addr, dest_page_addr;
>>>> + unsigned int npages;
>>>> + struct page **src_pages;
>>>> + int i;
>>>> + kimage_entry_t *ptr, entry;
>>>> + void *vaddr = NULL;
> When adding a new function, it's suggested to take the reverse xmas tree
> style for local variable ordering usually.
>
>>>> +
>>>> + /*
>>>> + * Collect the source pages and map them in a contiguous VA range.
>>>> + */
>>>> + npages = PFN_UP(eaddr) - PFN_DOWN(addr);
>>>> + src_pages = kmalloc_array(npages, sizeof(*src_pages), GFP_KERNEL);
>>>> + if (!src_pages) {
>>>> + pr_err("Could not allocate ima pages array.\n");
>>>> + return NULL;
>>>> + }
>>>> +
>>>> + i = 0;
>>>> + for_each_kimage_entry(image, ptr, entry) {
>>>> + if (entry & IND_DESTINATION) {
>>>> + dest_page_addr = entry & PAGE_MASK;
>>>> + } else if (entry & IND_SOURCE) {
>>>> + if (dest_page_addr >= addr && dest_page_addr < eaddr) {
>>>> + src_page_addr = entry & PAGE_MASK;
>>>> + src_pages[i++] =
>>>> + virt_to_page(__va(src_page_addr));
>>>> + if (i == npages)
>>>> + break;
>>>> + dest_page_addr += PAGE_SIZE;
>>>> + }
>>>> + }
>>>> + }
>>>> +
>>>> + /* Sanity check. */
>>>> + WARN_ON(i < npages);
>>>> +
>>>> + vaddr = vmap(src_pages, npages, VM_MAP, PAGE_KERNEL);
>>>> + kfree(src_pages);
>>>> +
>>>> + if (!vaddr)
>>>> + pr_err("Could not map ima buffer.\n");
>>>> +
>>>> + return vaddr;
>>>> +}
>>>> +
>>>> +void kimage_unmap_segment(void *segment_buffer)
>>>> +{
>>>> + vunmap(segment_buffer);
>>>> +}
>>>> +
>>>> struct kexec_load_limit {
>>>> /* Mutex protects the limit count. */
>>>> struct mutex mutex;
>>>> --
>>>> 2.25.1
>>>>
>>>>
>>> BR, Jarkko
>>
Hi Baoquan,

Thanks for your comments. I will update it in next version.

Steven