kexec_file: fix elfcorehdr digest exclusion when CONFIG_CRASH_HOTPLUG=y

Fix the condition to exclude the elfcorehdr segment from the SHA digest
calculation.

The j iterator is an index into the output sha_regions[] array, not into
the input image->segment[] array.  Once it reaches
image->elfcorehdr_index, all subsequent segments are excluded.  Besides,
if the purgatory segment precedes the elfcorehdr segment, the elfcorehdr
may be wrongly included in the calculation.

Link: https://lkml.kernel.org/r/20240805150750.170739-1-petr.tesarik@suse.com
Fixes: f7cc804 ("kexec: exclude elfcorehdr from the segment digest")
Signed-off-by: Petr Tesarik <ptesarik@suse.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Sourabh Jain <sourabhjain@linux.ibm.com>
Cc: Eric DeVolder <eric_devolder@yahoo.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ptesarik

kernel/kexec_file.c

@@ -752,7 +752,7 @@ static int kexec_calculate_store_digests(struct kimage *image)
 
 #ifdef CONFIG_CRASH_HOTPLUG
 		/* Exclude elfcorehdr segment to allow future changes via hotplug */
-		if (j == image->elfcorehdr_index)
+		if (i == image->elfcorehdr_index)
 			continue;
 #endif