pstore fixes for v4.19-rc4:

- Handle page-vs-byte offset handling between iomap and vmap (Bin Yang)
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 Comment: Kees Cook <kees@outflux.net>
 
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Merge tag 'pstore-v4.19-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull pstore fix from Kees Cook:
 "This fixes a 6 year old pstore bug that everyone just got lucky in
  avoiding, likely due only using page-aligned persistent ram regions:

   - Handle page-vs-byte offset handling between iomap and vmap (Bin Yang)"

* tag 'pstore-v4.19-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  pstore: Fix incorrect persistent ram buffer mapping

fs/pstore/ram_core.c

@@ -429,7 +429,12 @@ static void *persistent_ram_vmap(phys_addr_t start, size_t size,
 	vaddr = vmap(pages, page_count, VM_MAP, prot);
 	kfree(pages);
 
-	return vaddr;
+	/*
+	 * Since vmap() uses page granularity, we must add the offset
+	 * into the page here, to get the byte granularity address
+	 * into the mapping to represent the actual "start" location.
+	 */
+	return vaddr + offset_in_page(start);
 }
 
 static void *persistent_ram_iomap(phys_addr_t start, size_t size,
@@ -448,6 +453,11 @@ static void *persistent_ram_iomap(phys_addr_t start, size_t size,
 	else
 		va = ioremap_wc(start, size);
 
+	/*
+	 * Since request_mem_region() and ioremap() are byte-granularity
+	 * there is no need handle anything special like we do when the
+	 * vmap() case in persistent_ram_vmap() above.
+	 */
 	return va;
 }
 
@@ -468,7 +478,7 @@ static int persistent_ram_buffer_map(phys_addr_t start, phys_addr_t size,
 		return -ENOMEM;
 	}
 
-	prz->buffer = prz->vaddr + offset_in_page(start);
+	prz->buffer = prz->vaddr;
 	prz->buffer_size = size - sizeof(struct persistent_ram_buffer);
 
 	return 0;
@@ -515,7 +525,8 @@ void persistent_ram_free(struct persistent_ram_zone *prz)
 
 	if (prz->vaddr) {
 		if (pfn_valid(prz->paddr >> PAGE_SHIFT)) {
-			vunmap(prz->vaddr);
+			/* We must vunmap() at page-granularity. */
+			vunmap(prz->vaddr - offset_in_page(prz->paddr));
 		} else {
 			iounmap(prz->vaddr);
 			release_mem_region(prz->paddr, prz->size);