[PATCH v2] x86: __memcpy_flushcache: fix wrong alignment if size > 2^32

[Mikulas Patocka <mpatocka@redhat.com>]

On Tue, 19 Apr 2022, Linus Torvalds wrote:

> On Tue, Apr 19, 2022 at 6:56 AM Mikulas Patocka <mpatocka@redhat.com> wrote:
> >
> > The first "if" condition in __memcpy_flushcache is supposed to align the
> > "dest" variable to 8 bytes and copy data up to this alignment. However,
> > this condition may misbehave if "size" is greater than 4GiB.
> 
> You're not wrong, but I also don't think it would be wrong to just have a
> 
>         if (WARN_ON_ONCE(size > MAX_INT))
>                 return;
> 
> in there instead.

If you skip copying, it could in theory have security or reliability 
implications (the user may be reading stale data that he is not supposed 
to read). So, I think it's better to just add WARN_ON_ONCE and proceed 
with the copying.

> It' not like "> 2**32" should ever really be a valid thing for any
> kind of copy in the kernel. Even if that were to be what you actually

For example, the dm-stats subsystem (drivers/md/dm-stats.c) can allocate 
extremely large blocks of memory using vmalloc (it is only limited to 1/4 
of total system memory) - though, it doesn't use memcpy on it.

> wanted to do (which sounds very unlikely), you'd need to split it up
> with cond_resched() just for latency reasons.
> 
>              Linus



From: Mikulas Patocka <mpatocka@redhat.com>

The first "if" condition in __memcpy_flushcache is supposed to align the
"dest" variable to 8 bytes and copy data up to this alignment. However,
this condition may misbehave if "size" is greater than 4GiB.

The statement min_t(unsigned, size, ALIGN(dest, 8) - dest); casts both
arguments to unsigned int and selects the smaller one. However, the cast
truncates high bits in "size" and it results in misbehavior.

For example:
	suppose that size == 0x100000001, dest == 0x200000002
	min_t(unsigned, size, ALIGN(dest, 8) - dest) == min_t(0x1, 0xe) == 0x1;
	...
	dest += 0x1;
so we copy just one byte "and" dest remains unaligned.

This patch fixes the bug by replacing unsigned with size_t.

As the function is not supposed to be used with large size, the patch also
adds a WARN_ON_ONCE there.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 arch/x86/lib/usercopy_64.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6/arch/x86/lib/usercopy_64.c
===================================================================
--- linux-2.6.orig/arch/x86/lib/usercopy_64.c
+++ linux-2.6/arch/x86/lib/usercopy_64.c
@@ -117,9 +117,11 @@ void __memcpy_flushcache(void *_dst, con
 	unsigned long dest = (unsigned long) _dst;
 	unsigned long source = (unsigned long) _src;
 
+	WARN_ON_ONCE(size > INT_MAX);
+
 	/* cache copy and flush to align dest */
 	if (!IS_ALIGNED(dest, 8)) {
-		unsigned len = min_t(unsigned, size, ALIGN(dest, 8) - dest);
+		size_t len = min_t(size_t, size, ALIGN(dest, 8) - dest);
 
 		memcpy((void *) dest, (void *) source, len);
 		clean_cache_range((void *) dest, len);