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

From: Nathan Chancellor <[email-masked]>
Date: Wed, 19 Jan 2022 15:24:31 -0700

On Wed, Jan 19, 2022 at 02:54:50PM +0100, Jason A. Donenfeld wrote:
> blake2s_compress_generic is weakly aliased to blake2s_generic. The

Isn't it the other way around?

> 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 <[email-masked]>
> Cc: Nick Desaulniers <[email-masked]>
> Cc: Sami Tolvanen <[email-masked]>
> Cc: Ard Biesheuvel <[email-masked]>
> Signed-off-by: Jason A. Donenfeld <[email-masked]>

I am not familiar enough with the inner workings of CFI to comment on
what could be going wrong here in LLVM (perhaps Sami can?) but
eliminating the indirect call resolves the issue for me as well.

Tested-by: Nathan Chancellor <[email-masked]>

> ---
>  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
> 
> 

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