[PATCH v2] LoongArch: kexec: Fix address space mismatch in command line lookup

[George Guo <dongtai.guo@linux.dev>]

From: George Guo <guodongtai@kylinos.cn>

When searching the loaded segments for the "kexec" command line marker,
the kexec_load(2) path (file_mode == 0) passes the user-space segment
buffer straight to strncmp() through a bogus (char __user *) cast. This
dereferences a user pointer in kernel context, which is wrong and is
flagged by sparse:

  arch/loongarch/kernel/machine_kexec.c:84:51: sparse: incorrect type in
  argument 2 (different address spaces) @@ expected char const * @@ got
  char [noderef] __user *

Copy the marker-sized prefix of each segment into a small on-stack
buffer with copy_from_user() before comparing, and skip segments that
fault. The subsequent copy_from_user() that stages the full command line
into the safe area is left unchanged.

Fixes: 4a03b2ac06a5 ("LoongArch: Add kexec support")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202605051639.aEPioXdD-lkp@intel.com/
Co-developed-by: Kexin Liu <liukexin@kylinos.cn>
Signed-off-by: Kexin Liu <liukexin@kylinos.cn>
Signed-off-by: George Guo <guodongtai@kylinos.cn>
---
Changes in v2:
- Keep `bootloader` unchanged and copy into a fixed `char head[8]` instead of
  turning `bootloader` into an array just to size the temporary buffer (Huacai).

v1: https://lore.kernel.org/all/20260626105150.362203-1-dongtai.guo@linux.dev/

 arch/loongarch/kernel/machine_kexec.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/loongarch/kernel/machine_kexec.c b/arch/loongarch/kernel/machine_kexec.c
index d7fafda1d541..ced49194d9de 100644
--- a/arch/loongarch/kernel/machine_kexec.c
+++ b/arch/loongarch/kernel/machine_kexec.c
@@ -57,9 +57,13 @@ int machine_kexec_prepare(struct kimage *kimage)
 					strlen((char *)kimage->arch.cmdline_ptr) + 1);
 		kimage->arch.cmdline_ptr = (unsigned long)KEXEC_CMDLINE_ADDR;
 	} else {
+		char head[8];
+
 		/* Find the command line */
 		for (i = 0; i < kimage->nr_segments; i++) {
-			if (!strncmp(bootloader, (char __user *)kimage->segment[i].buf, strlen(bootloader))) {
+			if (copy_from_user(head, kimage->segment[i].buf, strlen(bootloader)))
+				continue;
+			if (!strncmp(bootloader, head, strlen(bootloader))) {
 				if (!copy_from_user(cmdline_ptr, kimage->segment[i].buf, COMMAND_LINE_SIZE))
 					kimage->arch.cmdline_ptr = (unsigned long)cmdline_ptr;
 				break;
-- 
2.25.1