proc/vmcore: fix signedness bug in read_from_oldmem()

JIRA: https://issues.redhat.com/browse/RHEL-32199

Upstream Status: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

commit 641db40
Author: Dan Carpenter <dan.carpenter@linaro.org>
Date:   Tue Jul 25 20:03:16 2023 +0300

    proc/vmcore: fix signedness bug in read_from_oldmem()

    The bug is the error handling:

            if (tmp < nr_bytes) {

    "tmp" can hold negative error codes but because "nr_bytes" is type size_t
    the negative error codes are treated as very high positive values
    (success).  Fix this by changing "nr_bytes" to type ssize_t.  The
    "nr_bytes" variable is used to store values between 1 and PAGE_SIZE and
    they can fit in ssize_t without any issue.

    Link: https://lkml.kernel.org/r/b55f7eed-1c65-4adc-95d1-6c7c65a54a6e@moroto.mountain
    Fixes: 5d8de29 ("vmcore: convert copy_oldmem_page() to take an iov_iter")
    Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
    Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
    Acked-by: Baoquan He <bhe@redhat.com>
    Cc: Dave Young <dyoung@redhat.com>
    Cc: Vivek Goyal <vgoyal@redhat.com>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Baoquan He <bhe@redhat.com>

Author: Baoquan He

fs/proc/vmcore.c

@@ -133,7 +133,7 @@ ssize_t read_from_oldmem(struct iov_iter *iter, size_t count,
 			 u64 *ppos, bool encrypted)
 {
 	unsigned long pfn, offset;
-	size_t nr_bytes;
+	ssize_t nr_bytes;
 	ssize_t read = 0, tmp;
 	int idx;