The patch titled
Subject: slub: improve bit diffusion for freelist ptr obfuscation
has been added to the -mm tree. Its filename is
slub-improve-bit-diffusion-for-freelist-ptr-obfuscation.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/slub-improve-bit-diffusion-for-freelist-ptr-obfuscation.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/slub-improve-bit-diffusion-for-freelist-ptr-obfuscation.patch
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------------------------------------------------------
From: Kees Cook <keescook@xxxxxxxxxxxx>
Subject: slub: improve bit diffusion for freelist ptr obfuscation
Under CONFIG_SLAB_FREELIST_HARDENED=y, the obfuscation was relatively weak
in that the ptr and ptr address were usually so close that the first XOR
would result in an almost entirely 0-byte value[1], leaving most of the
"secret" number ultimately being stored after the third XOR. A single
blind memory content exposure of the freelist was generally sufficient to
learn the secret.
Add a swab() call to mix bits a little more. This is a cheap way (1
cycle) to make attacks need more than a single exposure to learn the
secret (or to know _where_ the exposure is in memory).
kmalloc-32 freelist walk, before:
ptr ptr_addr stored value secret
ffff90c22e019020@ffff90c22e019000 is 86528eb656b3b5bd (86528eb656b3b59d)
ffff90c22e019040@ffff90c22e019020 is 86528eb656b3b5fd (86528eb656b3b59d)
ffff90c22e019060@ffff90c22e019040 is 86528eb656b3b5bd (86528eb656b3b59d)
ffff90c22e019080@ffff90c22e019060 is 86528eb656b3b57d (86528eb656b3b59d)
ffff90c22e0190a0@ffff90c22e019080 is 86528eb656b3b5bd (86528eb656b3b59d)
...
after:
ptr ptr_addr stored value secret
ffff9eed6e019020@ffff9eed6e019000 is 793d1135d52cda42 (86528eb656b3b59d)
ffff9eed6e019040@ffff9eed6e019020 is 593d1135d52cda22 (86528eb656b3b59d)
ffff9eed6e019060@ffff9eed6e019040 is 393d1135d52cda02 (86528eb656b3b59d)
ffff9eed6e019080@ffff9eed6e019060 is 193d1135d52cdae2 (86528eb656b3b59d)
ffff9eed6e0190a0@ffff9eed6e019080 is f93d1135d52cdac2 (86528eb656b3b59d)
[1] https://blog.infosectcbr.com.au/2020/03/weaknesses-in-linux-kernel-heap.html
Link: http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook
Fixes: 2482ddec670f ("mm: add SLUB free list pointer obfuscation")
Reported-by: Silvio Cesare <silvio.cesare@xxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/slub.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/mm/slub.c~slub-improve-bit-diffusion-for-freelist-ptr-obfuscation
+++ a/mm/slub.c
@@ -259,7 +259,7 @@ static inline void *freelist_ptr(const s
* freepointer to be restored incorrectly.
*/
return (void *)((unsigned long)ptr ^ s->random ^
- (unsigned long)kasan_reset_tag((void *)ptr_addr));
+ swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
#else
return ptr;
#endif
_
Patches currently in -mm which might be from keescook@xxxxxxxxxxxx are
slub-improve-bit-diffusion-for-freelist-ptr-obfuscation.patch
slub-relocate-freelist-pointer-to-middle-of-object.patch
shmem-distribute-switch-variables-for-initialization.patch
lib-test_stackinitc-xfail-switch-variable-init-tests.patch
ubsan-add-trap-instrumentation-option.patch
ubsan-split-bounds-checker-from-other-options.patch
lkdtm-bugs-add-arithmetic-overflow-and-array-bounds-checks.patch
ubsan-check-panic_on_warn.patch
kasan-unset-panic_on_warn-before-calling-panic.patch
ubsan-include-bug-type-in-report-header.patch
