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authorCesar Eduardo Barros <cesarb@cesarb.eti.br>2013-11-25 22:00:41 -0200
committerHerbert Xu <herbert@gondor.apana.org.au>2013-12-05 21:28:41 +0800
commitfe8c8a126806fea4465c43d62a1f9d273a572bf5 (patch)
treeea40ae4afce934575065383d1de44d04cd6490fc /crypto/Makefile
parentb62ffd8c72c2f04ac737ef3ff0f7ba37d2013633 (diff)
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crypto: more robust crypto_memneq
Disabling compiler optimizations can be fragile, since a new optimization could be added to -O0 or -Os that breaks the assumptions the code is making. Instead of disabling compiler optimizations, use a dummy inline assembly (based on RELOC_HIDE) to block the problematic kinds of optimization, while still allowing other optimizations to be applied to the code. The dummy inline assembly is added after every OR, and has the accumulator variable as its input and output. The compiler is forced to assume that the dummy inline assembly could both depend on the accumulator variable and change the accumulator variable, so it is forced to compute the value correctly before the inline assembly, and cannot assume anything about its value after the inline assembly. This change should be enough to make crypto_memneq work correctly (with data-independent timing) even if it is inlined at its call sites. That can be done later in a followup patch. Compile-tested on x86_64. Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.eti.br> Acked-by: Daniel Borkmann <dborkman@redhat.com> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Diffstat (limited to 'crypto/Makefile')
-rw-r--r--crypto/Makefile5
1 files changed, 0 insertions, 5 deletions
diff --git a/crypto/Makefile b/crypto/Makefile
index 989c510da8cc..b29402a7b9b5 100644
--- a/crypto/Makefile
+++ b/crypto/Makefile
@@ -2,11 +2,6 @@
# Cryptographic API
#
-# memneq MUST be built with -Os or -O0 to prevent early-return optimizations
-# that will defeat memneq's actual purpose to prevent timing attacks.
-CFLAGS_REMOVE_memneq.o := -O1 -O2 -O3
-CFLAGS_memneq.o := -Os
-
obj-$(CONFIG_CRYPTO) += crypto.o
crypto-y := api.o cipher.o compress.o memneq.o