fs/proc/kcore.c: fix invalid memory access in multi-page read optimization

The 'm' kcore_list item could point to kclist_head, and it is incorrect to
look at m->addr / m->size in this case.

There is no choice but to run through the list of entries for every
address if we did not find any entry in the previous iteration

Reset 'm' to NULL in that case at Omar Sandoval's suggestion.

[akpm@linux-foundation.org: add comment]
Link: http://lkml.kernel.org/r/1536100702-28706-1-git-send-email-asmadeus@codewreck.org
Fixes: bf991c22 ("proc/kcore: optimize multiple page reads")
Signed-off-by: NDominique Martinet <asmadeus@codewreck.org>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: Omar Sandoval <osandov@osandov.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: James Morse <james.morse@arm.com>
Cc: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/proc/kcore.c

@@ -464,6 +464,7 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 				ret = -EFAULT;
 				goto out;
 			}
+			m = NULL;	/* skip the list anchor */
 		} else if (m->type == KCORE_VMALLOC) {
 			vread(buf, (char *)start, tsz);
 			/* we have to zero-fill user buffer even if no read */