In order to immediately overwrite the old key on the stack, before
servicing a userspace request for bytes, we use the remaining 32 bytes
of block 0 as the key. This means moving indices 8,9,a,b,c,d,e,f ->
4,5,6,7,8,9,a,b. Since 4 < 8, for the kernel implementations of
memcpy(), this doesn't actually appear to be a problem in practice. But
relying on that characteristic seems a bit brittle. So let's change that
to a proper memmove(), which is the by-the-books way of handling
overlapping memory copies.
Cc: Dominik Brodowski <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>
---
drivers/char/random.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 6b01b2be9dd4..3a293f919af9 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -333,7 +333,7 @@ static void crng_fast_key_erasure(u8 key[CHACHA_KEY_SIZE],
chacha20_block(chacha_state, first_block);
memcpy(key, first_block, CHACHA_KEY_SIZE);
- memcpy(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
+ memmove(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
memzero_explicit(first_block, sizeof(first_block));
}
--
2.35.1
Am Thu, Apr 14, 2022 at 01:56:49AM +0200 schrieb Jason A. Donenfeld:
> In order to immediately overwrite the old key on the stack, before
> servicing a userspace request for bytes, we use the remaining 32 bytes
> of block 0 as the key. This means moving indices 8,9,a,b,c,d,e,f ->
> 4,5,6,7,8,9,a,b. Since 4 < 8, for the kernel implementations of
> memcpy(), this doesn't actually appear to be a problem in practice. But
> relying on that characteristic seems a bit brittle. So let's change that
> to a proper memmove(), which is the by-the-books way of handling
> overlapping memory copies.
>
> Cc: Dominik Brodowski <[email protected]>
> Signed-off-by: Jason A. Donenfeld <[email protected]>
Reviewed-by: Dominik Brodowski <[email protected]>
Thanks,
Dominik
Hey Eric,
On Mon, Apr 18, 2022 at 11:43:11AM -0700, Eric Biggers wrote:
> On Thu, Apr 14, 2022 at 01:56:49AM +0200, Jason A. Donenfeld wrote:
> > In order to immediately overwrite the old key on the stack, before
> > servicing a userspace request for bytes, we use the remaining 32 bytes
> > of block 0 as the key. This means moving indices 8,9,a,b,c,d,e,f ->
> > 4,5,6,7,8,9,a,b. Since 4 < 8, for the kernel implementations of
> > memcpy(), this doesn't actually appear to be a problem in practice. But
> > relying on that characteristic seems a bit brittle. So let's change that
> > to a proper memmove(), which is the by-the-books way of handling
> > overlapping memory copies.
> >
> > Cc: Dominik Brodowski <[email protected]>
> > Signed-off-by: Jason A. Donenfeld <[email protected]>
> > ---
> > drivers/char/random.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/char/random.c b/drivers/char/random.c
> > index 6b01b2be9dd4..3a293f919af9 100644
> > --- a/drivers/char/random.c
> > +++ b/drivers/char/random.c
> > @@ -333,7 +333,7 @@ static void crng_fast_key_erasure(u8 key[CHACHA_KEY_SIZE],
> > chacha20_block(chacha_state, first_block);
> >
> > memcpy(key, first_block, CHACHA_KEY_SIZE);
> > - memcpy(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
> > + memmove(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
> > memzero_explicit(first_block, sizeof(first_block));
> > }
>
> first_block is on the stack, so this is never an overlapping copy.
Oh, grrr, yes you're right. I clearly over thought this into non-sense.
Will revert.
>
> It would be more important to document the fact that random_data can point into
> chacha_state, as this is not obvious.
Good idea. I'll do that.
Jason
On Thu, Apr 14, 2022 at 01:56:49AM +0200, Jason A. Donenfeld wrote:
> In order to immediately overwrite the old key on the stack, before
> servicing a userspace request for bytes, we use the remaining 32 bytes
> of block 0 as the key. This means moving indices 8,9,a,b,c,d,e,f ->
> 4,5,6,7,8,9,a,b. Since 4 < 8, for the kernel implementations of
> memcpy(), this doesn't actually appear to be a problem in practice. But
> relying on that characteristic seems a bit brittle. So let's change that
> to a proper memmove(), which is the by-the-books way of handling
> overlapping memory copies.
>
> Cc: Dominik Brodowski <[email protected]>
> Signed-off-by: Jason A. Donenfeld <[email protected]>
> ---
> drivers/char/random.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 6b01b2be9dd4..3a293f919af9 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -333,7 +333,7 @@ static void crng_fast_key_erasure(u8 key[CHACHA_KEY_SIZE],
> chacha20_block(chacha_state, first_block);
>
> memcpy(key, first_block, CHACHA_KEY_SIZE);
> - memcpy(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
> + memmove(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
> memzero_explicit(first_block, sizeof(first_block));
> }
first_block is on the stack, so this is never an overlapping copy.
It would be more important to document the fact that random_data can point into
chacha_state, as this is not obvious.
- Eric