[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 <mi...@kernel.org>
Signed-off-by: Kees Cook <keesc...@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