+ lib-stringc-strlcpy-might-read-too-far.patch added to -mm tree

+ lib-stringc-strlcpy-might-read-too-far.patch added to -mm tree

[akpm]

Subject: + lib-stringc-strlcpy-might-read-too-far.patch added to -mm tree
To: dan.carpenter@oracle.com,vegard.nossum@gmail.com
From: akpm@linux-foundation.org
Date: Mon, 14 Apr 2014 14:47:03 -0700


The patch titled
     Subject: lib/string.c: strlcpy() might read too far
has been added to the -mm tree.  Its filename is
     lib-stringc-strlcpy-might-read-too-far.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/lib-stringc-strlcpy-might-read-too-far.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/lib-stringc-strlcpy-might-read-too-far.patch

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------------------------------------------------------
From: Dan Carpenter <dan.carpenter@oracle.com>
Subject: lib/string.c: strlcpy() might read too far

Imagine you have a user controlled variable at the end of a struct which
is allocated at the end of a page.  The strlen() could read beyond the
mapped memory and cause an oops.

Probably there are two reasons why we have never hit this condition in
real life.  First you would have to be really unlucky for all the
variables to line up so the oops can happen.  Second we don't do a lot of
fuzzing with invalid strings.

The strnlen() call is obviously a little bit slower than strlen() but I
have tested it and I think it's probably ok.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/string.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff -puN lib/string.c~lib-stringc-strlcpy-might-read-too-far lib/string.c
--- a/lib/string.c~lib-stringc-strlcpy-might-read-too-far
+++ a/lib/string.c
@@ -148,10 +148,10 @@ EXPORT_SYMBOL(strncpy);
  */
 size_t strlcpy(char *dest, const char *src, size_t size)
 {
-	size_t ret = strlen(src);
+	size_t ret = strnlen(src, size);
 
 	if (size) {
-		size_t len = (ret >= size) ? size - 1 : ret;
+		size_t len = (ret < size) ? ret : ret - 1;
 		memcpy(dest, src, len);
 		dest[len] = '\0';
 	}
_

Patches currently in -mm which might be from dan.carpenter@oracle.com are

lib-stringc-use-the-name-c-string-in-comments.patch
lib-stringc-strlcpy-might-read-too-far.patch
linux-next.patch

Re: + lib-stringc-strlcpy-might-read-too-far.patch added to -mm tree

[Alexey Dobriyan]

On Tue, Apr 15, 2014 at 12:47 AM,  <akpm@linux-foundation.org> wrote:
> Subject: lib/string.c: strlcpy() might read too far
>
> Imagine you have a user controlled variable at the end of a struct which
> is allocated at the end of a page.  The strlen() could read beyond the
> mapped memory and cause an oops.
>
> Probably there are two reasons why we have never hit this condition in
> real life.  First you would have to be really unlucky for all the
> variables to line up so the oops can happen.  Second we don't do a lot of
> fuzzing with invalid strings.
>
> The strnlen() call is obviously a little bit slower than strlen() but I
> have tested it and I think it's probably ok.

> --- a/lib/string.c~lib-stringc-strlcpy-might-read-too-far
> +++ a/lib/string.c
> @@ -148,10 +148,10 @@ EXPORT_SYMBOL(strncpy);
>   */
>  size_t strlcpy(char *dest, const char *src, size_t size)
>  {
> -       size_t ret = strlen(src);
> +       size_t ret = strnlen(src, size);
>
>         if (size) {
> -               size_t len = (ret >= size) ? size - 1 : ret;
> +               size_t len = (ret < size) ? ret : ret - 1;
>                 memcpy(dest, src, len);
>                 dest[len] = '\0';
>         }

Return value matters. It may not matter for kernel, because kernel is
not heavy string user.
But it is better to not diverge from master code:
http://www.openbsd.org/cgi-bin/cvsweb/src/lib/libc/string/strlcpy.c?rev=1.11

Counter-rationale:
* strlcpy() accepts strings, so if you're giving raw buffer you're
doing it wrong.
* last byte of last page argument is bogus because kernel copies data
from userspace first.

Re: + lib-stringc-strlcpy-might-read-too-far.patch added to -mm tree

[Dan Carpenter]

On Tue, Apr 15, 2014 at 01:49:38PM +0300, Alexey Dobriyan wrote:
> Return value matters. It may not matter for kernel, because kernel is
> not heavy string user.
> But it is better to not diverge from master code:
> http://www.openbsd.org/cgi-bin/cvsweb/src/lib/libc/string/strlcpy.c?rev=1.11
> 

Oh...  Hm.  Maybe we should drop this patch then.

> Counter-rationale:
> * strlcpy() accepts strings, so if you're giving raw buffer you're
> doing it wrong.
> * last byte of last page argument is bogus because kernel copies data
> from userspace first.

The last byte of the page argument seems possible:

	foo = kmalloc();
	copy_from_user(foo, arg, sizeof(foo));
	strlcpy(dest.str, foo->bar, sizeof(dest.str));

It's a very unlikely scenario.  You have to be very unlucky to hit it at
all.

regards,
dan carpenter

Re: + lib-stringc-strlcpy-might-read-too-far.patch added to -mm tree

[Alexey Dobriyan]

On Tue, Apr 15, 2014 at 2:18 PM, Dan Carpenter <dan.carpenter@oracle.com> wrote:
> On Tue, Apr 15, 2014 at 01:49:38PM +0300, Alexey Dobriyan wrote:
>> Return value matters. It may not matter for kernel, because kernel is
>> not heavy string user.
>> But it is better to not diverge from master code:
>> http://www.openbsd.org/cgi-bin/cvsweb/src/lib/libc/string/strlcpy.c?rev=1.11
>>
>
> Oh...  Hm.  Maybe we should drop this patch then.
>
>> Counter-rationale:
>> * strlcpy() accepts strings, so if you're giving raw buffer you're
>> doing it wrong.
>> * last byte of last page argument is bogus because kernel copies data
>> from userspace first.
>
> The last byte of the page argument seems possible:
>
>         foo = kmalloc();
>         copy_from_user(foo, arg, sizeof(foo));

Correct code would do
        foo->bar[sizeof(foo->bar)-1] = '\0';
if this field is a string.

>         strlcpy(dest.str, foo->bar, sizeof(dest.str));