On Wed, May 07, 2025 at 10:59:59PM -0700, Andrew Morton wrote:
On Thu, 8 May 2025 12:25:15 +0800 Coiby Xu <c...@redhat.com> wrote:
>
>Acked-by: Baoquan He <b...@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
