[PATCH v4] riscv: Prevent NULL pointer dereference in machine_kexec_prepare

A NULL pointer dereference issue is noticed in riscv's machine_kexec_prepare(),
where image->segment[i].buf might be NULL and copied unchecked.

The NULL buf comes from ima_add_kexec_buffer(), where kbuf is added by
kexec_add_buffer(), but kbuf.buffer is NULL, then it is copied without
a check in machine_kexec_prepare().

Relevant path:

  kexec_file_load
    -> kimage_file_alloc_init()
       -> kimage_file_prepare_segments()
          -> ima_add_kexec_buffer()
             -> kexec_add_buffer()
    -> machine_kexec_prepare()
       -> memcpy()

Address this by adding a check before the data copy attempt.

Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")
Cc: [email protected]
Closes: https://lore.kernel.org/kexec/CAO7dBbVftLUhd2qrh7hmijTB3PEPfZAhykCGqEfrPoOcSrrj-w@mail.gmail.com/
Acked-by: Baoquan He <[email protected]>
Acked-by: Pratyush Yadav <[email protected]>
Reviewed-by: Nutty Liu <[email protected]>
Signed-off-by: Tao Liu <[email protected]>
---
v4 -> v3: 1) Remove code comment.
          2) Replace (buf == NULL) to (!buf).
          3) Reword commit message.

link to v1: https://lore.kernel.org/linux-riscv/[email protected]/
link to v2: https://lore.kernel.org/linux-riscv/[email protected]/
link to v3: https://lore.kernel.org/linux-riscv/[email protected]/
---
 arch/riscv/kernel/machine_kexec.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/riscv/kernel/machine_kexec.c b/arch/riscv/kernel/machine_kexec.c
index 2306ce3e5f22..738df176ff6f 100644
--- a/arch/riscv/kernel/machine_kexec.c
+++ b/arch/riscv/kernel/machine_kexec.c
@@ -41,6 +41,9 @@ machine_kexec_prepare(struct kimage *image)
 		if (image->segment[i].memsz <= sizeof(fdt))
 			continue;
 
+		if (!image->segment[i].buf)
+			continue;
+
 		if (image->file_mode)
 			memcpy(&fdt, image->segment[i].buf, sizeof(fdt));
 		else if (copy_from_user(&fdt, image->segment[i].buf, sizeof(fdt)))
-- 
2.54.0