Re: [PATCH v4] x86/kexec: fix potential cmem->ranges out of bounds

Coiby Xu
2025-05-09T04:05:32Z

On Wed, May 07, 2025 at 10:59:59PM -0700, Andrew Morton wrote:
>On Thu, 8 May 2025 12:25:15 +0800 Coiby Xu <coxu@redhat.com> wrote:
>
>> >
>> >Acked-by: Baoquan He <bhe@redhat.com>
>>
>> Hi Andrew,
>>
>> It seems this patch was missed.
>
>January 2024. Yes, it's fair to assume that it was missed ;)
>
>> Will you pick it up?
>
>Sure.

Thanks for quickly processing this patch! Sorry I didn't reply yesterday
as I was trying to reproduce the UBSAN warning and truly understand the
it.

>
>> Without this patch,
>> kdump kernel will fail to be loaded by the kexec_file_load,

As already pointed out by Baoquan, a manual test shows kexec_file_load
actually works despite the UBSAN warning. Sorry I misinterpreted the
UBSAN warning and the automated test result failure (somehow sysrq
wasn't be triggered and vmcore wasn't saved either).

>>
>> [ 139.736948] UBSAN: array-index-out-of-bounds in arch/x86/kernel/crash.c:350:25
>> [ 139.742360] index 0 is out of range for type 'range [*]'
[...]
>>
>
>Do we know why this has appeared at such a late date? The reporter
>must be doing something rare.

The UBSAN warning happens because flexible array members annotated with
__counted_by are accessed without assigning an array element count i.e.
crash_mem->ranges[0] is accessed without setting max_nr_ranges after
vzalloc,

// include/linux/crash_core.h
struct crash_mem {
unsigned int max_nr_ranges;
unsigned int nr_ranges;
struct range ranges[] __counted_by(max_nr_ranges);
};

The bad commit was introduced in 2021 but only recent gcc-15 supports
__counted_by. That's why we don't see this UBSAN warning until this
year. And although this UBSAN warning is scary enough, fortunately it
doesn't cause a real problem.

>
>Baoquan, please re-review this?
>
>A -stable backport is clearly required. A Fixes: would be nice, but I
>assume this goes back a long time so it isn't worth spending a lot of
>time working out when this was introduced.

So I believe the correct fix should be as follows,

--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -301,6 +301,7 @@ int crash_setup_memmap_entries(struct kimage *image, struct boot_params *params)
cmem = vzalloc(struct_size(cmem, ranges, 1));
if (!cmem)
return -ENOMEM;
+ cmem->max_nr_ranges = 1;

memset(&cmd, 0, sizeof(struct crash_memmap_data));
cmd.params = params;

And a Fixes tag should be dedicated to commit
5849cdf8c120 ("x86/crash: Fix crash_setup_memmap_entries() out-of-bounds access")
which forgot to set cmem->max_nr_ranges=1.

>
>The patch needed a bit of work to apply to current code. I did the
>below. It compiles.
>
>--- a/arch/x86/kernel/crash.c~x86-kexec-fix-potential-cmem-ranges-out-of-bounds
>+++ a/arch/x86/kernel/crash.c
>@@ -165,8 +165,18 @@ static struct crash_mem *fill_up_crash_e
> /*
> * Exclusion of crash region and/or crashk_low_res may cause
> * another range split. So add extra two slots here.
>+ *
>+ * Exclusion of low 1M may not cause another range split, because the
>+ * range of exclude is [0, 1M] and the condition for splitting a new
>+ * region is that the start, end parameters are both in a certain
>+ * existing region in cmem and cannot be equal to existing region's
>+ * start or end. Obviously, the start of [0, 1M] cannot meet this
>+ * condition.
>+ *
>+ * But in order to lest the low 1M could be changed in the future,
>+ * (e.g. [stare, 1M]), add a extra slot.
> */
>- nr_ranges += 2;
>+ nr_ranges += 3;
> cmem = vzalloc(struct_size(cmem, ranges, nr_ranges));
> if (!cmem)
> return NULL;
>@@ -317,9 +327,16 @@ int crash_setup_memmap_entries(struct ki
> * split. So use two slots here.
> */
> nr_ranges = 2;
>- cmem = vzalloc(struct_size(cmem, ranges, nr_ranges));
>+ /*
>+ * In the current x86 architecture code, the elfheader is always
>+ * allocated at crashk_res.start. But it depends on the allocation
>+ * position of elfheader in crashk_res. To avoid potential out of
>+ * bounds in future, add a extra slot.
>+ */
>+ cmem = vzalloc(struct_size(cmem, ranges, 2));
> if (!cmem)
> return -ENOMEM;
>+ cmem->max_nr_ranges = 2;

Thanks for coming up with the above patch! I think the goal of this
patch is addressing a different issue but it also fixes the UBSAN
warning because cmem->max_nr_ranges is now set.

>
> cmem->max_nr_ranges = nr_ranges;
> cmem->nr_ranges = 0;
>_

--
Best regards,
Coiby