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Ilya Maximets
a6c06fa699
kTLS write is using application buffer always without a memory copy.
And it completely ignores SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER as a
result. If the user frees or re-uses the original buffer and retries
the send on SSL_ERROR_WANT_WRITE, the code will read and send the data
from the original already freed buffer sending whatever happens to be
in that memory now and corrupting the message, potentially crashing
the application as well.
Fix by making a copy if we can't send the whole thing right away and
the moving write buffer is configured.
This preserves the zero-copy semantics for the happy path and avoids
the invalid memory access and data corruption when retry is necessary.
The copy is done in the common code as it is hard to preserve the
zero-copy behavior otherwise.
Test is added that reproduces the issue. It may be possible to modify
the existing kTLS test to conditionally enable the modes and do the
BIO swap, but it feels like the issue deserves a separate one.
The test doesn't rely on any specific cypher or TLS version, so only
one combination is checked, but it should be enough.
There is no TLS_BUFFER_set_len() and the original kTLS code never
sets it, so not setting it on the copy either for now.
Fixes: 50ec750567 "ssl: Linux TLS Tx Offload"
Fixes #21202
Assisted-by: claude-opus-4.6
Reviewed-by: Matt Caswell <matt@openssl.foundation>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
MergeDate: Mon Jun 1 07:29:13 2026
(Merged from https://github.com/openssl/openssl/pull/31146)