x86/boot: Warn on future overlapping memcpy() use

If an overlapping memcpy() is ever attempted, we should report it and gracefully call memmove(). These cases can be found and fixed to use memmove() correctly, but in the meantime, we will not break booting.

Suggested-by Ingo Molnar <[email protected]>

Signed-off-by Kees Cook <[email protected]>

---
 arch/x86/boot/compressed/string.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

arch/x86/boot/compressed/string.c +13 -3

index 2befeca1aada..7402227fdfdb 100644
@@ -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(

@@ -60,3 +60,14 @@ void *memmove(void *dest, const void *src, size_t 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

Hi,

[auto build test ERROR on tip/auto-latest]
[cannot apply to tip/x86/core v4.6-rc5 next-20160428]
[if your patch is applied to the wrong git tree, please drop us a note to help improving the system]

url:    https://github.com/0day-ci/linux/commits/Kees-Cook/x86-boot-Warn-on-future-overlapping-memcpy-use/20160429-075026
config: i386-tinyconfig (attached as .config)
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386

All error/warnings (new ones prefixed by >>):

arch/x86/boot/compressed/string.c: In function 'memmove':

Quoted 1 line

Quoted 1 line

arch/x86/boot/compressed/string.c:56:10: warning: implicit declaration of function 'memcpy' [-Wimplicit-function-declaration]

      return memcpy(dest, src, n);
             ^

Quoted 1 line

Quoted 1 line

arch/x86/boot/compressed/string.c:56:10: warning: return makes pointer from integer without a cast [-Wint-conversion]

arch/x86/boot/compressed/string.c: At top level:

Quoted 1 line

Quoted 1 line

arch/x86/boot/compressed/string.c:65:7: error: conflicting types for 'memcpy'

void *memcpy(void *dest, const void *src, size_t n)
^
arch/x86/boot/compressed/string.c:56:10: note: previous implicit declaration of 'memcpy' was here

      return memcpy(dest, src, n);
             ^

arch/x86/boot/compressed/string.c: In function 'memcpy':

Quoted 1 line

Quoted 1 line

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

      warn("Avoiding potentially unsafe overlapping memcpy()!");
      ^

vim +/memcpy +65 arch/x86/boot/compressed/string.c

    50  void *memmove(void *dest, const void *src, size_t n)
    51  {
    52          unsigned char *d = dest;
    53          const unsigned char *s = src;
    54
    55          if (d <= s || d - s >= n)

Quoted 1 line

56 return memcpy(dest, src, n);

    57
    58          while (n-- > 0)
    59                  d[n] = s[n];
    60
    61          return dest;
    62  }
    63
    64  /* Detect and warn about potential overlaps, but handle them with memmove. */

Quoted 1 line

65 void *memcpy(void *dest, const void *src, size_t n)

    66  {
    67          if (dest > src && dest - src < n) {

Quoted 1 line

68 warn("Avoiding potentially unsafe overlapping memcpy()!");

    69                  return memmove(dest, src, n);
    70          }
    71          return __memcpy(dest, src, n);

---
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