Re: [PATCH] kexec: should use uchunk for user buffer increasing

[Eric W. Biederman]

Baoquan He <bhe@xxxxxxxxxx> writes:

> On 01/30/24 at 06:18pm, yang.zhang wrote:
>> From: "yang.zhang" <yang.zhang@xxxxxxxxxxxx>
>> 
>> Because of alignment requirement in kexec-tools, there is
>> no problem for user buffer increasing when loading segments.
>> But when coping, the step is uchunk, so we should use uchunk
>> not mchunk.
>
> In theory, ubytes is <= mbytes. So uchunk is always <= mchunk. If ubytes
> is exhausted, while there's still remaining mbytes, then uchunk is 0,
> there's still mchunk stepping forward. If I understand it correctly,
> this is a good catch. Not sure if Eric has comment on this to confirm.

As far as I can read the code the proposed change is a noop.

I agree it is more correct to not advance the pointers we read from,
but since we never read from them after that point it does not
matter.

>
> static int kimage_load_normal_segment(struct kimage *image,
>                                          struct kexec_segment *segment)
> {
> ......
>
>                 ptr += maddr & ~PAGE_MASK;
>                 mchunk = min_t(size_t, mbytes,
>                                 PAGE_SIZE - (maddr & ~PAGE_MASK));
>                 uchunk = min(ubytes, mchunk);
> ......}

If we are going to improve the code for clarity.  We probably
want to do something like:

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index d08fc7b5db97..1a8b8ce6bf15 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -800,22 +800,24 @@ static int kimage_load_normal_segment(struct kimage *image,
                                PAGE_SIZE - (maddr & ~PAGE_MASK));
                uchunk = min(ubytes, mchunk);
 
-               /* For file based kexec, source pages are in kernel memory */
-               if (image->file_mode)
-                       memcpy(ptr, kbuf, uchunk);
-               else
-                       result = copy_from_user(ptr, buf, uchunk);
+               if (uchunk) {
+                       /* For file based kexec, source pages are in kernel memory */
+                       if (image->file_mode)
+                               memcpy(ptr, kbuf, uchunk);
+                       else
+                               result = copy_from_user(ptr, buf, uchunk);
+                       ubytes -= uchunk;
+                       if (image->file_mode)
+                               kbuf += uchunk;
+                       else
+                               buf += uchunk;
+               }
                kunmap_local(ptr);
                if (result) {
                        result = -EFAULT;
                        goto out;
                }
-               ubytes -= uchunk;
                maddr  += mchunk;
-               if (image->file_mode)
-                       kbuf += mchunk;
-               else
-                       buf += mchunk;
                mbytes -= mchunk;
 
                cond_resched();

And make it exceedingly clear that all of the copying and the rest
only happens before uchunk goes to zero.  Otherwise we are relying
on a lot of operations becoming noops when uchunk goes to zero.

Eric