[PATCH v4] x86/boot: Warn on future overlapping memcpy() use

[PATCH v4] x86/boot: Warn on future overlapping memcpy() use

[Kees Cook]

If an overlapping memcpy() is ever attempted, we should at least report
it, in case it might lead to problems, so it could be changed to a
memmove() call instead.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
v4:
- use __memcpy not memcpy since we've already done the check.
v3:
- call memmove in addition to doing the warning
v2:
- warn about overlapping region
---
 arch/x86/boot/compressed/string.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/x86/boot/compressed/string.c b/arch/x86/boot/compressed/string.c
index 2befeca1aada..5ac3acb5699f 100644
--- a/arch/x86/boot/compressed/string.c
+++ b/arch/x86/boot/compressed/string.c
@@ -8,7 +8,7 @@
 #include "../string.c"
 
 #ifdef CONFIG_X86_32
-void *memcpy(void *dest, const void *src, size_t n)
+static void *__memcpy(void *dest, const void *src, size_t n)
 {
 	int d0, d1, d2;
 	asm volatile(
@@ -22,7 +22,7 @@ void *memcpy(void *dest, const void *src, size_t n)
 	return dest;
 }
 #else
-void *memcpy(void *dest, const void *src, size_t n)
+static void *__memcpy(void *dest, const void *src, size_t n)
 {
 	long d0, d1, d2;
 	asm volatile(
@@ -53,10 +53,20 @@ void *memmove(void *dest, const void *src, size_t n)
 	const unsigned char *s = src;
 
 	if (d <= s || d - s >= n)
-		return memcpy(dest, src, n);
+		return __memcpy(dest, src, n);
 
 	while (n-- > 0)
 		d[n] = s[n];
 
 	return dest;
 }
+
+/* Detect and warn about potential overlaps, but handle them with memmove. */
+void *memcpy(void *dest, const void *src, size_t n)
+{
+	if (dest > src && dest - src < n) {
+		warn("Avoiding potentially unsafe overlapping memcpy()!");
+		return memmove(dest, src, n);
+	}
+	return __memcpy(dest, src, n);
+}
-- 
2.6.3


-- 
Kees Cook
Chrome OS & Brillo Security

Re: [PATCH v4] x86/boot: Warn on future overlapping memcpy() use

[Ingo Molnar]

* Kees Cook <keescook@chromium.org> wrote:

> If an overlapping memcpy() is ever attempted, we should at least report
> it, in case it might lead to problems, so it could be changed to a
> memmove() call instead.
> 
> Suggested-by: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> v4:
> - use __memcpy not memcpy since we've already done the check.
> v3:
> - call memmove in addition to doing the warning
> v2:
> - warn about overlapping region
> ---
>  arch/x86/boot/compressed/string.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)

Applied, thanks Kees!

Btw., can we now also remove the memmove() hack from lib/decompress_unxz.c?

Thanks,

	Ingo

Re: [PATCH v4] x86/boot: Warn on future overlapping memcpy() use

[Kees Cook]

On Thu, Apr 28, 2016 at 11:43 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Kees Cook <keescook@chromium.org> wrote:
>
>> If an overlapping memcpy() is ever attempted, we should at least report
>> it, in case it might lead to problems, so it could be changed to a
>> memmove() call instead.
>>
>> Suggested-by: Ingo Molnar <mingo@kernel.org>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>> v4:
>> - use __memcpy not memcpy since we've already done the check.
>> v3:
>> - call memmove in addition to doing the warning
>> v2:
>> - warn about overlapping region
>> ---
>>  arch/x86/boot/compressed/string.c | 16 +++++++++++++---
>>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> Applied, thanks Kees!
>
> Btw., can we now also remove the memmove() hack from lib/decompress_unxz.c?

I'll let Lasse answer for sure, but I don't think so. The original commit says:

    The XZ decompressor needs memmove(), memeq() (memcmp() == 0), and
    memzero() (memset(ptr, 0, size)), which aren't available in all
    arch-specific pre-boot environments.  I'm including simple versions in
    decompress_unxz.c, but a cleaner solution would naturally be nicer.

-Kees


-- 
Kees Cook
Chrome OS & Brillo Security

Re: [PATCH v4] x86/boot: Warn on future overlapping memcpy() use

[Ingo Molnar]

* Kees Cook <keescook@chromium.org> wrote:

> On Thu, Apr 28, 2016 at 11:43 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Kees Cook <keescook@chromium.org> wrote:
> >
> >> If an overlapping memcpy() is ever attempted, we should at least report
> >> it, in case it might lead to problems, so it could be changed to a
> >> memmove() call instead.
> >>
> >> Suggested-by: Ingo Molnar <mingo@kernel.org>
> >> Signed-off-by: Kees Cook <keescook@chromium.org>
> >> ---
> >> v4:
> >> - use __memcpy not memcpy since we've already done the check.
> >> v3:
> >> - call memmove in addition to doing the warning
> >> v2:
> >> - warn about overlapping region
> >> ---
> >>  arch/x86/boot/compressed/string.c | 16 +++++++++++++---
> >>  1 file changed, 13 insertions(+), 3 deletions(-)
> >
> > Applied, thanks Kees!
> >
> > Btw., can we now also remove the memmove() hack from lib/decompress_unxz.c?
> 
> I'll let Lasse answer for sure, but I don't think so. The original commit says:
> 
>     The XZ decompressor needs memmove(), memeq() (memcmp() == 0), and
>     memzero() (memset(ptr, 0, size)), which aren't available in all
>     arch-specific pre-boot environments.  I'm including simple versions in
>     decompress_unxz.c, but a cleaner solution would naturally be nicer.

I see, so non-x86 architectures might not have the proper runtime environment 
implemented, right?

Fair enough!

Thanks,

	Ingo

Re: [PATCH v4] x86/boot: Warn on future overlapping memcpy() use

[Ingo Molnar]

* Kees Cook <keescook@chromium.org> wrote:

> If an overlapping memcpy() is ever attempted, we should at least report
> it, in case it might lead to problems, so it could be changed to a
> memmove() call instead.
> 
> Suggested-by: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> v4:
> - use __memcpy not memcpy since we've already done the check.
> v3:
> - call memmove in addition to doing the warning
> v2:
> - warn about overlapping region
> ---
>  arch/x86/boot/compressed/string.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)

I think you'll hate this patch some more:

 arch/x86/boot/compressed/string.c:68:3: warning: implicit declaration of function ‘warn’ [-Wimplicit-function-declaration]

:-)

Can we do the trick below? Because misc.h also includes the regular kernel memcpy 
functions, we can remove the decompressor specific __memcpy() - but the question 
is, is it safe to do?

If it's not safe to do, we are playing with fire already I suspect:

 arch/x86/boot/compressed/cmdline.c:#include "misc.h"
 arch/x86/boot/compressed/early_serial_console.c:#include "misc.h"
 arch/x86/boot/compressed/kaslr.c:#include "misc.h"
 arch/x86/boot/compressed/misc.c:#include "misc.h"

?

Thanks,

	Ingo

 arch/x86/boot/compressed/string.c | 31 +------------------------------
 1 file changed, 1 insertion(+), 30 deletions(-)

diff --git a/arch/x86/boot/compressed/string.c b/arch/x86/boot/compressed/string.c
index 952510976732..f4b95ed4e7a2 100644
--- a/arch/x86/boot/compressed/string.c
+++ b/arch/x86/boot/compressed/string.c
@@ -6,37 +6,8 @@
  * (e.g. FPU ops) in the minimal decompression stub execution environment.
  */
 #include "../string.c"
-#include "misc.h"
-
-#ifdef CONFIG_X86_32
-static void *__memcpy(void *dest, const void *src, size_t n)
-{
-	int d0, d1, d2;
-	asm volatile(
-		"rep ; movsl\n\t"
-		"movl %4,%%ecx\n\t"
-		"rep ; movsb\n\t"
-		: "=&c" (d0), "=&D" (d1), "=&S" (d2)
-		: "0" (n >> 2), "g" (n & 3), "1" (dest), "2" (src)
-		: "memory");
-
-	return dest;
-}
-#else
-static void *__memcpy(void *dest, const void *src, size_t n)
-{
-	long d0, d1, d2;
-	asm volatile(
-		"rep ; movsq\n\t"
-		"movq %4,%%rcx\n\t"
-		"rep ; movsb\n\t"
-		: "=&c" (d0), "=&D" (d1), "=&S" (d2)
-		: "0" (n >> 3), "g" (n & 7), "1" (dest), "2" (src)
-		: "memory");
 
-	return dest;
-}
-#endif
+#include "misc.h"
 
 void *memset(void *s, int c, size_t n)
 {

Re: [PATCH v4] x86/boot: Warn on future overlapping memcpy() use

[Kees Cook]

On Fri, Apr 29, 2016 at 12:57 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Kees Cook <keescook@chromium.org> wrote:
>
>> If an overlapping memcpy() is ever attempted, we should at least report
>> it, in case it might lead to problems, so it could be changed to a
>> memmove() call instead.
>>
>> Suggested-by: Ingo Molnar <mingo@kernel.org>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>> v4:
>> - use __memcpy not memcpy since we've already done the check.
>> v3:
>> - call memmove in addition to doing the warning
>> v2:
>> - warn about overlapping region
>> ---
>>  arch/x86/boot/compressed/string.c | 16 +++++++++++++---
>>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> I think you'll hate this patch some more:
>
>  arch/x86/boot/compressed/string.c:68:3: warning: implicit declaration of function ‘warn’ [-Wimplicit-function-declaration]
>
> :-)

Argh, I didn't get that warning when I built. But yes, this patch
hates me too, it seems.

>
> Can we do the trick below? Because misc.h also includes the regular kernel memcpy
> functions, we can remove the decompressor specific __memcpy() - but the question
> is, is it safe to do?
>
> If it's not safe to do, we are playing with fire already I suspect:
>
>  arch/x86/boot/compressed/cmdline.c:#include "misc.h"
>  arch/x86/boot/compressed/early_serial_console.c:#include "misc.h"
>  arch/x86/boot/compressed/kaslr.c:#include "misc.h"
>  arch/x86/boot/compressed/misc.c:#include "misc.h"

Hrm, let me poke at it. I think the better thing to do would be to
split up header files. I would rather have the warn/error/etc
functions in a separate header than to put the memcpy code in a
header.

-Kees

>
> ?
>
> Thanks,
>
>         Ingo
>
>  arch/x86/boot/compressed/string.c | 31 +------------------------------
>  1 file changed, 1 insertion(+), 30 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/string.c b/arch/x86/boot/compressed/string.c
> index 952510976732..f4b95ed4e7a2 100644
> --- a/arch/x86/boot/compressed/string.c
> +++ b/arch/x86/boot/compressed/string.c
> @@ -6,37 +6,8 @@
>   * (e.g. FPU ops) in the minimal decompression stub execution environment.
>   */
>  #include "../string.c"
> -#include "misc.h"
> -
> -#ifdef CONFIG_X86_32
> -static void *__memcpy(void *dest, const void *src, size_t n)
> -{
> -       int d0, d1, d2;
> -       asm volatile(
> -               "rep ; movsl\n\t"
> -               "movl %4,%%ecx\n\t"
> -               "rep ; movsb\n\t"
> -               : "=&c" (d0), "=&D" (d1), "=&S" (d2)
> -               : "0" (n >> 2), "g" (n & 3), "1" (dest), "2" (src)
> -               : "memory");
> -
> -       return dest;
> -}
> -#else
> -static void *__memcpy(void *dest, const void *src, size_t n)
> -{
> -       long d0, d1, d2;
> -       asm volatile(
> -               "rep ; movsq\n\t"
> -               "movq %4,%%rcx\n\t"
> -               "rep ; movsb\n\t"
> -               : "=&c" (d0), "=&D" (d1), "=&S" (d2)
> -               : "0" (n >> 3), "g" (n & 7), "1" (dest), "2" (src)
> -               : "memory");
>
> -       return dest;
> -}
> -#endif
> +#include "misc.h"
>
>  void *memset(void *s, int c, size_t n)
>  {



-- 
Kees Cook
Chrome OS & Brillo Security