Re: [PATCH v13 04/15] arm64: kexec_file: Fix potential buffer overflow in prepare_elf_headers()

[Breno Leitao <leitao@debian.org>]

On Mon, May 11, 2026 at 07:30:44PM +0800, Jinjie Ruan wrote:
> 
> 
> On 5/11/2026 5:46 PM, Breno Leitao wrote:
> > On Mon, May 11, 2026 at 11:04:43AM +0800, Jinjie Ruan wrote:
> >> There is a race condition between the kexec_load() system call
> >> (crash kernel loading path) and memory hotplug operations that can
> >> lead to buffer overflow and potential kernel crash.
> >>
> >> During prepare_elf_headers(), the following steps occur:
> >> 1. The first for_each_mem_range() queries current System RAM memory ranges
> >> 2. Allocates buffer based on queried count
> >> 3. The 2st for_each_mem_range() populates ranges from memblock
> >>
> >> If memory hotplug occurs between step 1 and step 3, the number of ranges
> >> can increase, causing out-of-bounds write when populating cmem->ranges[].
> >>
> >> This happens because kexec_load() uses kexec_trylock (atomic_t) while
> >> memory hotplug uses device_hotplug_lock (mutex), so they don't serialize
> >> with each other.
> >>
> >> Add the explicit bounds checking to prevent out-of-bounds access.
> > 
> > It seems you have a TOCTOU type of issue, and this seems to be shrinking
> > the window, but not fully solving it?
> 
> Hi Breno,
> 
> Thanks for your comments regarding the TOCTOU issue.
> 
> You are correct that the current bounds checking only "shrinks the
> window" and prevents a kernel crash, but doesn't fully guarantee header
> consistency if a race occurs.
> 
> In my local environment, this race is extremely difficult to reproduce,
> but it is theoretically possible.
> 
> To address this properly for arm64, I am considering two steps:
> 
> - For this patch: I will change the return value to -EAGAIN and keep the
> bounds check. This ensures that even if a race happens, the kernel
> remains safe (no OOB access), and user-space is notified to retry.
> 
> - Long-term solution: A better way to solve this is to implement ARM64
> CRASH_HOTPLUG support (similar to x86). With crash hotplug, the kernel
> will automatically re-generate the crash headers whenever a memory
> hotplug event occurs. This makes the TOCTOU during the initial
> kexec_load less critical, as any transient inconsistency will be
> immediately corrected by the subsequent hotplug handler.
> 
> Does it make sense to you to use this patch as a safety guard first, and
> then I (or someone else) follow up with the full CRASH_HOTPLUG support
> for arm64 as [1]?

It would be OK for me, but, make it explict that there is a TOCTOU
issue, that depends on CRASH_HOTPLUG.

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