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

Baoquan He
2025-05-08T07:12:41Z

On 05/07/25 at 10:59pm, 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.
>
> > Without this patch,
> > kdump kernel will fail to be loaded by the kexec_file_load,
> >
> > [ 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 [*]'
> > [ 139.745695] CPU: 0 UID: 0 PID: 5778 Comm: kexec Not tainted 6.15.0-0.rc3.20250425git02ddfb981de8.32.fc43.x86_64 #1 PREEMPT(lazy)
> > [ 139.745698] Hardware name: Amazon EC2 c5.large/, BIOS 1.0 10/16/2017
> > [ 139.745699] Call Trace:
> > [ 139.745700] <TASK>
> > [ 139.745701] dump_stack_lvl+0x5d/0x80
> > [ 139.745706] ubsan_epilogue+0x5/0x2b
> > [ 139.745709] __ubsan_handle_out_of_bounds.cold+0x54/0x59
> > [ 139.745711] crash_setup_memmap_entries+0x2d9/0x330
> > [ 139.745716] setup_boot_parameters+0xf8/0x6a0
> > [ 139.745720] bzImage64_load+0x41b/0x4e0
> > [ 139.745722] ? find_next_iomem_res+0x109/0x140
> > [ 139.745727] ? locate_mem_hole_callback+0x109/0x170
> > [ 139.745737] kimage_file_alloc_init+0x1ef/0x3e0
> > [ 139.745740] __do_sys_kexec_file_load+0x180/0x2f0
> > [ 139.745742] do_syscall_64+0x7b/0x160
> > [ 139.745745] ? do_user_addr_fault+0x21a/0x690
> > [ 139.745747] ? exc_page_fault+0x7e/0x1a0
> > [ 139.745749] entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > [ 139.745751] RIP: 0033:0x7f7712c84e4d
> >
>
> Do we know why this has appeared at such a late date? The reporter
> must be doing something rare.
>
> 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.

No need for stable kernel. The UBSAN only warns a potential risk, it
won't happen in reality. I am talking to Coiby, he got it wrong about
the testing result. He saw the UBSAN warning when UBSAN is enabled,
while vmcore is till saved successfully.

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