linux-kernel - [PATCH] crash: fix crashkernel resource shrink

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

Message-ID: <20251101193741.289252-1-sourabhjain@linux.ibm.com>
Date: Sun,  2 Nov 2025 01:07:41 +0530
From: Sourabh Jain <sourabhjain@...ux.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>, Baoquan He <bhe@...hat.com>,
        Zhen Lei <thunder.leizhen@...wei.com>, kexec@...ts.infradead.org
Subject: [PATCH] crash: fix crashkernel resource shrink

When crashkernel is configured with a high reservation, shrinking its
value below the low crashkernel reservation causes two issues:

1. Invalid crashkernel resource objects
2. Kernel crash if crashkernel shrinking is done twice

For example, with crashkernel=200M,high, the kernel reserves 200MB of
high memory and some default low memory (say 256MB). The reservation
appears as:

cat /proc/iomem | grep -i crash
af000000-beffffff : Crash kernel
433000000-43f7fffff : Crash kernel

If crashkernel is then shrunk to 50MB (echo 52428800 >
/sys/kernel/kexec_crash_size), /proc/iomem still shows 256MB reserved:
af000000-beffffff : Crash kernel

Instead, it should show 50MB:
af000000-b21fffff : Crash kernel

Further shrinking crashkernel to 40MB causes a kernel crash with the
following trace (x86):

BUG: kernel NULL pointer dereference, address: 0000000000000038
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP NOPTI
<snip...>
Call Trace: <TASK>
? __die_body.cold+0x19/0x27
? page_fault_oops+0x15a/0x2f0
? search_module_extables+0x19/0x60
? search_bpf_extables+0x5f/0x80
? exc_page_fault+0x7e/0x180
? asm_exc_page_fault+0x26/0x30
? __release_resource+0xd/0xb0
release_resource+0x26/0x40
__crash_shrink_memory+0xe5/0x110
crash_shrink_memory+0x12a/0x190
kexec_crash_size_store+0x41/0x80
kernfs_fop_write_iter+0x141/0x1f0
vfs_write+0x294/0x460
ksys_write+0x6d/0xf0
<snip...>

This happens because __crash_shrink_memory()/kernel/crash_core.c
incorrectly updates the crashk_res resource object even when
crashk_low_res should be updated.

Fix this by ensuring the correct crashkernel resource object is updated
when shrinking crashkernel memory.

Fixes: 16c6006af4d4 ("kexec: enable kexec_crash_size to support two crash kernel regions")
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Baoquan He <bhe@...hat.com>
Cc: Zhen Lei <thunder.leizhen@...wei.com>
Cc: kexec@...ts.infradead.org
Signed-off-by: Sourabh Jain <sourabhjain@...ux.ibm.com>
---
 kernel/crash_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 3b1c43382eec..99dac1aa972a 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -373,7 +373,7 @@ static int __crash_shrink_memory(struct resource *old_res,
 		old_res->start = 0;
 		old_res->end   = 0;
 	} else {
-		crashk_res.end = ram_res->start - 1;
+		old_res->end = ram_res->start - 1;
 	}
 
 	crash_free_reserved_phys_range(ram_res->start, ram_res->end);
-- 
2.51.0