crypto: sha512: remove imaginary and mystifying clearing of variables
Subsystems (2)
PATCH 1/1 crypto: sha512: remove imaginary and mystifying clearing of variables
The function sha512_transform() assigns all local variables to 0 before returning to its caller with the intent to erase sensitive data.
However, make clang-analyzer warns that all these assignments are dead stores, and as commit 7a4295f6c9d5 ("crypto: lib/sha256 - Don't clear temporary variables") already points out for sha256_transform():
The assignments to clear a through h and t1/t2 are optimized out by the compiler because they are unused after the assignments.
Clearing individual scalar variables is unlikely to be useful, as they may have been assigned to registers, and even if stack spilling was required, there may be compiler-generated temporaries that are impossible to clear in any case.
This applies here again as well. Drop meaningless clearing of local variables and avoid this way that the code suggests that data is erased, which simply does not happen.
--- crypto/sha512_generic.c | 3 --- 1 file changed, 3 deletions(-)
crypto/sha512_generic.c +0 -3
@@ -143,9 +143,6 @@ sha512_transform(u64 *state, const u8 *input) state[0 ] += a; state[1 ] += b; state[2 ] += c; state[3 ] += d; state[4 ] += e; state[5 ] += f; state[6 ] += g; state[7 ] += h; - -/* erase our data */ - a = b = c = d = e = f = g = h = t1 = t2 =0 ; } static void sha512_generic_block_fn(struct sha512_state *sst, u8 const *src,
Signature · 1 line
2.26.2
Quoted 16 lines · Lukas Bulwahn
The function
sha512_transform()assigns all local variables to 0 before returning to its caller with the intent to erase sensitive data.However, make clang-analyzer warns that all these assignments are dead stores, and as commit
7a4295f6c9d5("crypto: lib/sha256 - Don't clear temporary variables") already points out forsha256_transform():The assignments to clear a through h and t1/t2 are optimized out by the compiler because they are unused after the assignments.
Clearing individual scalar variables is unlikely to be useful, as they may have been assigned to registers, and even if stack spilling was required, there may be compiler-generated temporaries that are impossible to clear in any case.
Quoted 5 lines
This applies here again as well. Drop meaningless clearing of local variables and avoid this way that the code suggests that data is erased, which simply does not happen.
Signed-off-by Lukas Bulwahn <[email protected]>
Thanks for the patch!
Quoted 21 lines
--- crypto/sha512_generic.c | 3 --- 1 file changed, 3 deletions(-)crypto/sha512_generic.c +0
-4@@ -143,9 +143,6 @@ sha512_transform(u64 *state, const u8 *input) state[0 ] += a; state[1 ] += b; state[2 ] += c; state[3 ] += d; state[4 ] += e; state[5 ] += f; state[6 ] += g; state[7 ] += h; - -/* erase our data */ - a = b = c = d = e = f = g = h = t1 = t2 =0 ; } static void sha512_generic_block_fn(struct sha512_state *sst, u8 const *src, --2.26.2
Signature · 2 lines
Thanks, ~Nick Desaulniers
Herbert Xu
maintainerQuoted 15 lines · Lukas Bulwahn
The function
sha512_transform()assigns all local variables to 0 before returning to its caller with the intent to erase sensitive data.However, make clang-analyzer warns that all these assignments are dead stores, and as commit
7a4295f6c9d5("crypto: lib/sha256 - Don't clear temporary variables") already points out forsha256_transform():The assignments to clear a through h and t1/t2 are optimized out by the compiler because they are unused after the assignments.
Clearing individual scalar variables is unlikely to be useful, as they may have been assigned to registers, and even if stack spilling was required, there may be compiler-generated temporaries that are impossible to clear in any case.
Quoted 8 lines
This applies here again as well. Drop meaningless clearing of local variables and avoid this way that the code suggests that data is erased, which simply does not happen.
Signed-off-by Lukas Bulwahn <[email protected]>--- crypto/sha512_generic.c | 3 --- 1 file changed, 3 deletions(-)
Patch applied. Thanks.
Signature · 3 lines
Email: Herbert Xu <[email protected]> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
Quoted 8 lines · Herbert Xu
Quoted 6 lines · Lukas Bulwahn
The function
sha512_transform()assigns all local variables to 0 before returning to its caller with the intent to erase sensitive data.
....The assignments to clear a through h and t1/t2 are optimized out by the compiler because they are unused after the assignments.
Just no.
You are right, there is a problem here. I thank you for pointing it out & I've already fixed it in some of my own code.
However, I think your solution is dead wrong. You are correct that these assignments are useless because the compiler will optimise them out, and that's a problem. However, it is not at all "mistiifying"; they are there for an obvious reason, to avoid leaving state that might be useful to an enemy. That is quite a small risk, but then it is a small mitigation, so worth doing.
The correct solution is not to just remove the assignments, but rather to replace them with code that will not be optimised away, force the compiler to do what we need. We already do that for operations that clear various arrays and structures, using memzero_explicit() rather than memset(). Similarly, we should replace the assignments with calls to this macro:
/*
clear a variable
in a way the compiler will not optimise out
*/
#define clear(x) memzero_explicit( &x, sizeof(x) )
Quoted 29 lines · Sandy Harris
Quoted 8 lines · Herbert Xu
Quoted 6 lines · Lukas Bulwahn
The function
sha512_transform()assigns all local variables to 0 before returning to its caller with the intent to erase sensitive data.
....The assignments to clear a through h and t1/t2 are optimized out by the compiler because they are unused after the assignments.
Just no.
You are right, there is a problem here. I thank you for pointing it out & I've already fixed it in some of my own code.
However, I think your solution is dead wrong. You are correct that these assignments are useless because the compiler will optimise them out, and that's a problem. However, it is not at all "mistiifying"; they are there for an obvious reason, to avoid leaving state that might be useful to an enemy. That is quite a small risk, but then it is a small mitigation, so worth doing.
The correct solution is not to just remove the assignments, but rather to replace them with code that will not be optimised away, force the compiler to do what we need. We already do that for operations that clear various arrays and structures, using
memzero_explicit()rather thanmemset(). Similarly, we should replace the assignments with calls to this macro:
Quoted 5 lines
/*
clear a variable
in a way the compiler will not optimise out
*/
#define clear(x)memzero_explicit( &x, sizeof(x) )
Clearing of local variables is never guaranteed to work properly, as the compiler can create multiple copies and/or put them in registers. It's much more likely to work for arrays than simple variables though (and not cause the variable to be unnecessarily spilled from registers to the stack), so that is the only one the kernel really bothers with.
- Eric