Fix using of uninitialized buffer "t"

[svpcom]

Status

READY

Requires Backporting

Yes

Which branch?
All branches

Migrations

NO

Additional comments

Security fix

[AndrzejKurek]

Hello, @svpcom, and thanks for the contribution!
Unfortunately, bzero that you've used is Unix-specific, and we're targeting multiple other platforms. You could consider using mbedtls_platform_zeroize instead, but could you please elaborate first about the details of this fix?
When exactly is t used as an uninitialized buffer? What are the preconditions?
Thanks in advance.

[svpcom]

In mbedtls_hkdf_expand there are unsigned char t[MBEDTLS_MD_MAX_SIZE];
in loop:

/*
     * Compute T = T(1) | T(2) | T(3) | ... | T(N)
     * Where T(N) is defined in RFC 5869 Section 2.3
     */
    for( i = 1; i <= n; i++ )
    {

HMAC read data from uninitialized t at the first iteration of loop

ret = mbedtls_md_hmac_update( &ctx, t, t_len );

So T(1) can be undefined if compiler doesn't zeroize t

[Patater]

Could you please add a regression test? We test with ASan and would expect a failure there if we provide the right test case.

[Patater]

memset() should be fine here, since t doesn't contain secrets at this point.

[Patater]

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Thanks!

[AndrzejKurek]

@svpcom, looking at the first iteration, mbedtls_md_hmac_update is called with t_len equal to 0.
Implementations shouldn't access a buffer of zero length, and if they do, accessing uninitialized data is just the smaller of their problems - accessing data out of bounds is worse (in my opinion).

Thanks to @yanesca for also taking a look at this.