Revert BN_copy() flag copy semantics change #5080
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Commit 9f94429 changed the semantics of BN_copy() to additionally copy the BN_FLG_CONSTTIME flag if it is set. This turns out to be ill advised as it has unintended consequences. For example calling BN_mod_inverse_no_branch() can sometimes return a result with the flag set and sometimes not as a result. This can lead to later failures if we go down code branches that do not support constant time, but check for the presence of the flag. The original commit was made due to an issue in BN_MOD_CTX_set(). The original PR fixed the problem in that function, but it was changed in review to fix it in BN_copy() instead. The solution seems to be to revert the BN_copy() change and go back to the originally proposed way.
paulidale
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Jan 15, 2018
levitte
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Jan 16, 2018
Commit 9f94429 changed the semantics of BN_copy() to additionally copy the BN_FLG_CONSTTIME flag if it is set. This turns out to be ill advised as it has unintended consequences. For example calling BN_mod_inverse_no_branch() can sometimes return a result with the flag set and sometimes not as a result. This can lead to later failures if we go down code branches that do not support constant time, but check for the presence of the flag. The original commit was made due to an issue in BN_MOD_CTX_set(). The original PR fixed the problem in that function, but it was changed in review to fix it in BN_copy() instead. The solution seems to be to revert the BN_copy() change and go back to the originally proposed way. Reviewed-by: Paul Dale <paul.dale@oracle.com> (Merged from #5080) (cherry picked from commit 7d46173)
levitte
pushed a commit
that referenced
this pull request
Jan 16, 2018
Commit 9f94429 changed the semantics of BN_copy() to additionally copy the BN_FLG_CONSTTIME flag if it is set. This turns out to be ill advised as it has unintended consequences. For example calling BN_mod_inverse_no_branch() can sometimes return a result with the flag set and sometimes not as a result. This can lead to later failures if we go down code branches that do not support constant time, but check for the presence of the flag. The original commit was made due to an issue in BN_MOD_CTX_set(). The original PR fixed the problem in that function, but it was changed in review to fix it in BN_copy() instead. The solution seems to be to revert the BN_copy() change and go back to the originally proposed way. Reviewed-by: Paul Dale <paul.dale@oracle.com> (Merged from #5080) (cherry picked from commit 7d46173)
levitte
pushed a commit
that referenced
this pull request
Jan 16, 2018
Commit 9f94429 changed the semantics of BN_copy() to additionally copy the BN_FLG_CONSTTIME flag if it is set. This turns out to be ill advised as it has unintended consequences. For example calling BN_mod_inverse_no_branch() can sometimes return a result with the flag set and sometimes not as a result. This can lead to later failures if we go down code branches that do not support constant time, but check for the presence of the flag. The original commit was made due to an issue in BN_MOD_CTX_set(). The original PR fixed the problem in that function, but it was changed in review to fix it in BN_copy() instead. The solution seems to be to revert the BN_copy() change and go back to the originally proposed way. Reviewed-by: Paul Dale <paul.dale@oracle.com> (Merged from #5080)
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Commit 9f94429 changed the semantics of BN_copy() to additionally
copy the BN_FLG_CONSTTIME flag if it is set. This turns out to be
ill advised as it has unintended consequences. For example calling
BN_mod_inverse_no_branch() can sometimes return a result with the flag
set and sometimes not as a result. This can lead to later failures if we
go down code branches that do not support constant time, but check for
the presence of the flag.
The original commit was made due to an issue in BN_MOD_CTX_set(). The
original PR fixed the problem in that function, but it was changed in
review to fix it in BN_copy() instead. The solution seems to be to revert
the BN_copy() change and go back to the originally proposed way.