Re: ether_addr_equal_64bits breakage with gcc-12

[Kees Cook <keescook@chromium.org>]

On Fri, Feb 11, 2022 at 04:35:41PM -0800, Jakub Kicinski wrote:
> On Fri, 11 Feb 2022 15:12:13 +0100 Marc Kleine-Budde wrote:
> > Hello,
> > 
> > the current arm-linux-gnueabihf-gcc 12 snapshot in Debian breaks (at
> > least with CONFIG_WERROR=y):
> > 
> > |   CC      net/core/dev.o
> > | net/core/dev.c: In function ‘bpf_prog_run_generic_xdp’:
> > | net/core/dev.c:4618:21: warning: ‘ether_addr_equal_64bits’ reading 8 bytes from a region of size 6 [-Wstringop-overread]
> > |  4618 |         orig_host = ether_addr_equal_64bits(eth->h_dest, skb->dev->dev_addr);
> > |       |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > | net/core/dev.c:4618:21: note: referencing argument 1 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> > | net/core/dev.c:4618:21: note: referencing argument 2 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> > | In file included from net/core/dev.c:91:
> > | include/linux/etherdevice.h:375:20: note: in a call to function ‘ether_addr_equal_64bits’
> > |   375 | static inline bool ether_addr_equal_64bits(const u8 addr1[6+2],
> > |       |                    ^~~~~~~~~~~~~~~~~~~~~~~
> > | net/core/dev.c:4619:22: warning: ‘is_multicast_ether_addr_64bits’ reading 8 bytes from a region of size 6 [-Wstringop-overread]
> > |  4619 |         orig_bcast = is_multicast_ether_addr_64bits(eth->h_dest);
> > |       |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > | net/core/dev.c:4619:22: note: referencing argument 1 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> > | include/linux/etherdevice.h:137:20: note: in a call to function ‘is_multicast_ether_addr_64bits’
> > |   137 | static inline bool is_multicast_ether_addr_64bits(const u8 addr[6+2])
> > |       |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > | net/core/dev.c:4646:27: warning: ‘ether_addr_equal_64bits’ reading 8 bytes from a region of size 6 [-Wstringop-overread]
> > |  4646 |             (orig_host != ether_addr_equal_64bits(eth->h_dest,
> > |       |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > |  4647 |                                                   skb->dev->dev_addr)) ||
> > |       |                                                   ~~~~~~~~~~~~~~~~~~~
> > | net/core/dev.c:4646:27: note: referencing argument 1 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> > | net/core/dev.c:4646:27: note: referencing argument 2 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> > | include/linux/etherdevice.h:375:20: note: in a call to function ‘ether_addr_equal_64bits’
> > |   375 | static inline bool ether_addr_equal_64bits(const u8 addr1[6+2],
> > |       |                    ^~~~~~~~~~~~~~~~~~~~~~~
> > | net/core/dev.c:4648:28: warning: ‘is_multicast_ether_addr_64bits’ reading 8 bytes from a region of size 6 [-Wstringop-overread]
> > |  4648 |             (orig_bcast != is_multicast_ether_addr_64bits(eth->h_dest))) {
> > |       |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > | net/core/dev.c:4648:28: note: referencing argument 1 of type ‘const u8[8]’ {aka ‘const unsigned char[8]’}
> > | include/linux/etherdevice.h:137:20: note: in a call to function ‘is_multicast_ether_addr_64bits’
> > |   137 | static inline bool is_multicast_ether_addr_64bits(const u8 addr[6+2])
> > |       |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > 
> > | arm-linux-gnueabihf-gcc -v
> > | Using built-in specs.
> > | COLLECT_GCC=/usr/bin/arm-linux-gnueabihf-gcc
> > | COLLECT_LTO_WRAPPER=/usr/lib/gcc-cross/arm-linux-gnueabihf/12/lto-wrapper
> > | Target: arm-linux-gnueabihf
> > | Configured with: ../src/configure -v --with-pkgversion='Debian 12-20220126-1' --with-bugurl=file:///usr/share/doc/gcc-12/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-12 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-libitm --disable-libquadmath --disable-libquadmath-support --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --without-target-system-zlib --enable-multiarch --disable-sjlj-exceptions --with-arch=armv7-a+fp --with-float=hard --with-mode=thumb --disable-werror --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=arm-linux-gnueabihf --program-prefix=arm-linux-gnueabihf- --includedir=/usr/arm-linux-gnueabihf/include
> > | Thread model: posix
> > | Supported LTO compression algorithms: zlib zstd
> > | gcc version 12.0.1 20220126 (experimental) [master r12-6872-gf3e6ef7d873] (Debian 12-20220126-1)
> 
> Maybe Kees will have as suggestion - Kees, are there any best practices
> for dealing with such issues? For the reference we do a oversized load
> from a structure (read 8B of a 6B array):

Wheee.

So, the short theoretical "don't do that" scenario would be "what
happens if":

        struct page *page;
        void *ptr;
	unsigned char *eth_addr;

        page = alloc_pages(GFP_KERNEL, 0);
	...
        ptr = page_address(page);
	...
	/* "eth_addr" at end of allocated memory */
	eth_addr = ptr + PAGE_SIZE - 6;
	/* access fault... */
	ether_addr_equal_64bits(eth_addr, ...);

But, yes, pragmatically, this is likely extremely rare.

Regardless, with the other cases like this that got fixed like this, it
was a matter of finding a way to represent the "actual" available memory
(best), or telling the compiler what real contract is (less good).

It looks like alignment isn't a concern, so I'd say adjust the prototype
to reflect the reality, and go with:


diff --git a/include/linux/etherdevice.h b/include/linux/etherdevice.h
index 2ad71cc90b37..92b10e67d5f8 100644
--- a/include/linux/etherdevice.h
+++ b/include/linux/etherdevice.h
@@ -134,7 +134,7 @@ static inline bool is_multicast_ether_addr(const u8 *addr)
 #endif
 }
 
-static inline bool is_multicast_ether_addr_64bits(const u8 addr[6+2])
+static inline bool is_multicast_ether_addr_64bits(const u8 *addr)
 {
 #if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && BITS_PER_LONG == 64
 #ifdef __BIG_ENDIAN
@@ -372,8 +372,7 @@ static inline bool ether_addr_equal(const u8 *addr1, const u8 *addr2)
  * Please note that alignment of addr1 & addr2 are only guaranteed to be 16 bits.
  */
 
-static inline bool ether_addr_equal_64bits(const u8 addr1[6+2],
-					   const u8 addr2[6+2])
+static inline bool ether_addr_equal_64bits(const u8 *addr1, const u8 *addr2)
 {
 #if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && BITS_PER_LONG == 64
 	u64 fold = (*(const u64 *)addr1) ^ (*(const u64 *)addr2);


-- 
Kees Cook