LoongArch: Fix address space mismatch in kexec command line lookup

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 *

Here 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.

Cc: stable@vger.kernel.org
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>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

diff --git a/arch/loongarch/kernel/machine_kexec.c b/arch/loongarch/kernel/machine_kexec.c
index d7fafda1d5417c99b48d842b2a9cb84d464e4cca..1883cae93bc31a0085afc4104ab15ff3de68e028 100644 (file)
--- a/arch/loongarch/kernel/machine_kexec.c
+++ b/arch/loongarch/kernel/machine_kexec.c
@@ -42,6 +42,7 @@ static unsigned long first_ind_entry;
 int machine_kexec_prepare(struct kimage *kimage)
 {
        int i;
+       char head[8];
        char *bootloader = "kexec";
        void *cmdline_ptr = (void *)KEXEC_CMDLINE_ADDR;
 
@@ -59,7 +60,9 @@ int machine_kexec_prepare(struct kimage *kimage)
        } else {
                /* 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;