linux-kernel - [PATCH] usercopy: Skip HIGHMEM page checking

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Date:   Mon, 16 Sep 2019 14:32:56 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: [PATCH] usercopy: Skip HIGHMEM page checking

When running on a system with >512MB RAM with a 32-bit kernel built with:

	CONFIG_DEBUG_VIRTUAL=y
	CONFIG_HIGHMEM=y
	CONFIG_HARDENED_USERCOPY=y

all execve()s will fail due to argv copying into kmap()ed pages, and on
usercopy checking the calls ultimately of virt_to_page() will be looking
for "bad" kmap (highmem) pointers due to CONFIG_DEBUG_VIRTUAL=y:

 ------------[ cut here ]------------
 kernel BUG at ../arch/x86/mm/physaddr.c:83!
 invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
 CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.3.0-rc8 #6
 Hardware name: Dell Inc. Inspiron 1318/0C236D, BIOS A04 01/15/2009
 EIP: __phys_addr+0xaf/0x100
 ...
 Call Trace:
  __check_object_size+0xaf/0x3c0
  ? __might_sleep+0x80/0xa0
  copy_strings+0x1c2/0x370
  copy_strings_kernel+0x2b/0x40
  __do_execve_file+0x4ca/0x810
  ? kmem_cache_alloc+0x1c7/0x370
  do_execve+0x1b/0x20
  ...

fs/exec.c:
		kaddr = kmap(kmapped_page);
	...
	if (copy_from_user(kaddr+offset, str, bytes_to_copy)) ...

Without CONFIG_DEBUG_VIRTUAL=y, these pages are effectively ignored,
so now we do the same explicitly: detect and ignore kmap pages, instead
of tripping over the check later.

Reported-by: Randy Dunlap <rdunlap@...radead.org>
Fixes: f5509cc18daa ("mm: Hardened usercopy")
Cc: Matthew Wilcox <willy@...radead.org>
Cc: stable@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
Randy, I dropped your other Tested-by, since this is a different
approach. I would expect the results to be identical (i.e. my testing
shows it works), but I didn't want to assume. :)
---
 include/linux/highmem.h | 7 +++++++
 mm/highmem.c            | 2 +-
 mm/usercopy.c           | 3 ++-
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index ea5cdbd8c2c3..c881698b8023 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -64,12 +64,19 @@ static inline void totalhigh_pages_set(long val)
 
 void kmap_flush_unused(void);
 
+static inline bool is_kmap(unsigned long addr)
+{
+	return (addr >= PKMAP_ADDR(0) && addr < PKMAP_ADDR(LAST_PKMAP));
+}
+
 struct page *kmap_to_page(void *addr);
 
 #else /* CONFIG_HIGHMEM */
 
 static inline unsigned int nr_free_highpages(void) { return 0; }
 
+static inline bool is_kmap(unsigned long addr) { return false; }
+
 static inline struct page *kmap_to_page(void *addr)
 {
 	return virt_to_page(addr);
diff --git a/mm/highmem.c b/mm/highmem.c
index 107b10f9878e..e99eca4f63fa 100644
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -151,7 +151,7 @@ struct page *kmap_to_page(void *vaddr)
 {
 	unsigned long addr = (unsigned long)vaddr;
 
-	if (addr >= PKMAP_ADDR(0) && addr < PKMAP_ADDR(LAST_PKMAP)) {
+	if (is_kmap(addr)) {
 		int i = PKMAP_NR(addr);
 		return pte_page(pkmap_page_table[i]);
 	}
diff --git a/mm/usercopy.c b/mm/usercopy.c
index 98e924864554..924e634cc95d 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -11,6 +11,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/mm.h>
+#include <linux/highmem.h>
 #include <linux/slab.h>
 #include <linux/sched.h>
 #include <linux/sched/task.h>
@@ -224,7 +225,7 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
 {
 	struct page *page;
 
-	if (!virt_addr_valid(ptr))
+	if (!virt_addr_valid(ptr) || is_kmap((unsigned long)ptr))
 		return;
 
 	page = virt_to_head_page(ptr);
-- 
2.17.1


-- 
Kees Cook