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Subject[PATCH 4.19 031/114] usercopy: Avoid HIGHMEM pfn warning
Date
From: Kees Cook <keescook@chromium.org>

commit 314eed30ede02fa925990f535652254b5bad6b65 upstream.

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
...

The check is from arch/x86/mm/physaddr.c:

VIRTUAL_BUG_ON((phys_addr >> PAGE_SHIFT) > max_low_pfn);

Due to the kmap() in fs/exec.c:

kaddr = kmap(kmapped_page);
...
if (copy_from_user(kaddr+offset, str, bytes_to_copy)) ...

Now we can fetch the correct page to avoid the pfn check. In both cases,
hardened usercopy will need to walk the page-span checker (if enabled)
to do sanity checking.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: f5509cc18daa ("mm: Hardened usercopy")
Cc: Matthew Wilcox <willy@infradead.org>
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/201909171056.7F2FFD17@keescook
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/usercopy.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -15,6 +15,7 @@
#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>
@@ -231,7 +232,12 @@ static inline void check_heap_object(con
if (!virt_addr_valid(ptr))
return;

- page = virt_to_head_page(ptr);
+ /*
+ * When CONFIG_HIGHMEM=y, kmap_to_page() will give either the
+ * highmem page or fallback to virt_to_page(). The following
+ * is effectively a highmem-aware virt_to_head_page().
+ */
+ page = compound_head(kmap_to_page((void *)ptr));

if (PageSlab(page)) {
/* Check slab allocator for flags and size. */

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