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.

---
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(+)

index 2306ce3e5f22..738df176ff6f 100644
@@ -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

…

Would it be helpful to append parentheses to a function name also in the summary phrase?

Why was such a hint not be taken into account from a previous suggestion?

Regards,
Markus

Oh come on, this is a really really minor nitpick. The fact that machine_kexec_prepare is a function is fairly obvious when reading the patch. In fact, the first sentence of the commit message uses the parentheses.

So yes, it would have been nice to have them in the subject. But no, this is not at all worth anyone's time to fix. Certainly not at all worth a v5.

Please, let's not nitpick our contributors into giving up.

Regards,
Pratyush Yadav

Hi Pratyush & Markus,

Thanks for the review and for pointing out the issues in my patch.

I'm still new to kernel patch submission and clearly have a lot to learn. I appreciate all the feedback. Thanks again for your patience and guidance.

Thanks,
Tao Liu