[PATCH] lib/crypto: blake2s: avoid indirect calls to compression function for Clang CFI

From: Jason A. Donenfeld
Date: Wed Jan 19 2022 - 08:55:32 EST

blake2s_compress_generic is weakly aliased to blake2s_generic. The
current harness for function selection uses a function pointer, which is
ordinarily inlined and resolved at compile time. But when Clang's CFI is
enabled, CFI still triggers when making an indirect call via a weak
symbol. This seems like a bug in Clang's CFI, as though it's bucketing
weak symbols and strong symbols differently. It also only seems to
trigger when "full LTO" mode is used, rather than "thin LTO".

[ 0.000000][ T0] Kernel panic - not syncing: CFI failure (target: blake2s_compress_generic+0x0/0x1444)
[ 0.000000][ T0] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.16.0-mainline-06981-g076c855b846e #1
[ 0.000000][ T0] Hardware name: MT6873 (DT)
[ 0.000000][ T0] Call trace:
[ 0.000000][ T0] dump_backtrace+0xfc/0x1dc
[ 0.000000][ T0] dump_stack_lvl+0xa8/0x11c
[ 0.000000][ T0] panic+0x194/0x464
[ 0.000000][ T0] __cfi_check_fail+0x54/0x58
[ 0.000000][ T0] __cfi_slowpath_diag+0x354/0x4b0
[ 0.000000][ T0] blake2s_update+0x14c/0x178
[ 0.000000][ T0] _extract_entropy+0xf4/0x29c
[ 0.000000][ T0] crng_initialize_primary+0x24/0x94
[ 0.000000][ T0] rand_initialize+0x2c/0x6c
[ 0.000000][ T0] start_kernel+0x2f8/0x65c
[ 0.000000][ T0] __primary_switched+0xc4/0x7be4
[ 0.000000][ T0] Rebooting in 5 seconds..

Nonetheless, the function pointer method isn't so terrific anyway, so
this patch replaces it with a simple boolean, which also gets inlined
away. This successfully works around the Clang bug.

In general, I'm not too keen on all of the indirection involved here; it
clearly does more harm than good. Hopefully the whole thing can get
cleaned up down the road when lib/crypto is overhauled more
comprehensively. But for now, we go with a simple bandaid.

Fixes: 6048fdcc5f26 ("lib/crypto: blake2s: include as built-in")
Reported-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
arch/arm/crypto/blake2s-shash.c | 4 ++--
arch/x86/crypto/blake2s-shash.c | 4 ++--
crypto/blake2s_generic.c | 4 ++--
include/crypto/internal/blake2s.h | 36 ++++++++++++++++++-------------
lib/crypto/blake2s.c | 4 ++--
5 files changed, 29 insertions(+), 23 deletions(-)

diff --git a/arch/arm/crypto/blake2s-shash.c b/arch/arm/crypto/blake2s-shash.c
index 17c1c3bfe2f5..763c73beea2d 100644
--- a/arch/arm/crypto/blake2s-shash.c
+++ b/arch/arm/crypto/blake2s-shash.c
@@ -13,12 +13,12 @@
static int crypto_blake2s_update_arm(struct shash_desc *desc,
const u8 *in, unsigned int inlen)
{
- return crypto_blake2s_update(desc, in, inlen, blake2s_compress);
+ return crypto_blake2s_update(desc, in, inlen, false);
}

static int crypto_blake2s_final_arm(struct shash_desc *desc, u8 *out)
{
- return crypto_blake2s_final(desc, out, blake2s_compress);
+ return crypto_blake2s_final(desc, out, false);
}

#define BLAKE2S_ALG(name, driver_name, digest_size) \
diff --git a/arch/x86/crypto/blake2s-shash.c b/arch/x86/crypto/blake2s-shash.c
index f9e2fecdb761..59ae28abe35c 100644
--- a/arch/x86/crypto/blake2s-shash.c
+++ b/arch/x86/crypto/blake2s-shash.c
@@ -18,12 +18,12 @@
static int crypto_blake2s_update_x86(struct shash_desc *desc,
const u8 *in, unsigned int inlen)
{
- return crypto_blake2s_update(desc, in, inlen, blake2s_compress);
+ return crypto_blake2s_update(desc, in, inlen, false);
}

static int crypto_blake2s_final_x86(struct shash_desc *desc, u8 *out)
{
- return crypto_blake2s_final(desc, out, blake2s_compress);
+ return crypto_blake2s_final(desc, out, false);
}

#define BLAKE2S_ALG(name, driver_name, digest_size) \
diff --git a/crypto/blake2s_generic.c b/crypto/blake2s_generic.c
index 72fe480f9bd6..5f96a21f8788 100644
--- a/crypto/blake2s_generic.c
+++ b/crypto/blake2s_generic.c
@@ -15,12 +15,12 @@
static int crypto_blake2s_update_generic(struct shash_desc *desc,
const u8 *in, unsigned int inlen)
{
- return crypto_blake2s_update(desc, in, inlen, blake2s_compress_generic);
+ return crypto_blake2s_update(desc, in, inlen, true);
}

static int crypto_blake2s_final_generic(struct shash_desc *desc, u8 *out)
{
- return crypto_blake2s_final(desc, out, blake2s_compress_generic);
+ return crypto_blake2s_final(desc, out, true);
}

#define BLAKE2S_ALG(name, driver_name, digest_size) \
diff --git a/include/crypto/internal/blake2s.h b/include/crypto/internal/blake2s.h
index d39cfa0d333e..e2da9b413d44 100644
--- a/include/crypto/internal/blake2s.h
+++ b/include/crypto/internal/blake2s.h
@@ -24,14 +24,11 @@ static inline void blake2s_set_lastblock(struct blake2s_state *state)
state->f[0] = -1;
}

-typedef void (*blake2s_compress_t)(struct blake2s_state *state,
- const u8 *block, size_t nblocks, u32 inc);
-
/* Helper functions for BLAKE2s shared by the library and shash APIs */

-static inline void __blake2s_update(struct blake2s_state *state,
- const u8 *in, size_t inlen,
- blake2s_compress_t compress)
+static __always_inline void __blake2s_update(struct blake2s_state *state,
+ const u8 *in, size_t inlen,
+ bool force_generic)
{
const size_t fill = BLAKE2S_BLOCK_SIZE - state->buflen;

@@ -39,7 +36,10 @@ static inline void __blake2s_update(struct blake2s_state *state,
return;
if (inlen > fill) {
memcpy(state->buf + state->buflen, in, fill);
- (*compress)(state, state->buf, 1, BLAKE2S_BLOCK_SIZE);
+ if (force_generic)
+ blake2s_compress_generic(state, state->buf, 1, BLAKE2S_BLOCK_SIZE);
+ else
+ blake2s_compress(state, state->buf, 1, BLAKE2S_BLOCK_SIZE);
state->buflen = 0;
in += fill;
inlen -= fill;
@@ -47,7 +47,10 @@ static inline void __blake2s_update(struct blake2s_state *state,
if (inlen > BLAKE2S_BLOCK_SIZE) {
const size_t nblocks = DIV_ROUND_UP(inlen, BLAKE2S_BLOCK_SIZE);
/* Hash one less (full) block than strictly possible */
- (*compress)(state, in, nblocks - 1, BLAKE2S_BLOCK_SIZE);
+ if (force_generic)
+ blake2s_compress_generic(state, in, nblocks - 1, BLAKE2S_BLOCK_SIZE);
+ else
+ blake2s_compress(state, in, nblocks - 1, BLAKE2S_BLOCK_SIZE);
in += BLAKE2S_BLOCK_SIZE * (nblocks - 1);
inlen -= BLAKE2S_BLOCK_SIZE * (nblocks - 1);
}
@@ -55,13 +58,16 @@ static inline void __blake2s_update(struct blake2s_state *state,
state->buflen += inlen;
}

-static inline void __blake2s_final(struct blake2s_state *state, u8 *out,
- blake2s_compress_t compress)
+static __always_inline void __blake2s_final(struct blake2s_state *state, u8 *out,
+ bool force_generic)
{
blake2s_set_lastblock(state);
memset(state->buf + state->buflen, 0,
BLAKE2S_BLOCK_SIZE - state->buflen); /* Padding */
- (*compress)(state, state->buf, 1, state->buflen);
+ if (force_generic)
+ blake2s_compress_generic(state, state->buf, 1, state->buflen);
+ else
+ blake2s_compress(state, state->buf, 1, state->buflen);
cpu_to_le32_array(state->h, ARRAY_SIZE(state->h));
memcpy(out, state->h, state->outlen);
}
@@ -99,20 +105,20 @@ static inline int crypto_blake2s_init(struct shash_desc *desc)

static inline int crypto_blake2s_update(struct shash_desc *desc,
const u8 *in, unsigned int inlen,
- blake2s_compress_t compress)
+ bool force_generic)
{
struct blake2s_state *state = shash_desc_ctx(desc);

- __blake2s_update(state, in, inlen, compress);
+ __blake2s_update(state, in, inlen, force_generic);
return 0;
}

static inline int crypto_blake2s_final(struct shash_desc *desc, u8 *out,
- blake2s_compress_t compress)
+ bool force_generic)
{
struct blake2s_state *state = shash_desc_ctx(desc);

- __blake2s_final(state, out, compress);
+ __blake2s_final(state, out, force_generic);
return 0;
}

diff --git a/lib/crypto/blake2s.c b/lib/crypto/blake2s.c
index 9364f79937b8..c71c09621c09 100644
--- a/lib/crypto/blake2s.c
+++ b/lib/crypto/blake2s.c
@@ -18,14 +18,14 @@

void blake2s_update(struct blake2s_state *state, const u8 *in, size_t inlen)
{
- __blake2s_update(state, in, inlen, blake2s_compress);
+ __blake2s_update(state, in, inlen, false);
}
EXPORT_SYMBOL(blake2s_update);

void blake2s_final(struct blake2s_state *state, u8 *out)
{
WARN_ON(IS_ENABLED(DEBUG) && !out);
- __blake2s_final(state, out, blake2s_compress);
+ __blake2s_final(state, out, false);
memzero_explicit(state, sizeof(*state));
}
EXPORT_SYMBOL(blake2s_final);
--
2.34.1